Sunday, February 06, 2022

Aviation vs 5G Telecoms: Bring Popcorn for this Food Fight

There has been a fair amount of ink spilt documenting the battle between the aviation and telecom industries over the issue of 5G deployment and the threat of electronic interference with aviation safety systems. There are literally billions of dollars of telecom investment lined up against the threat of massive disruptions in the national airspace system, the airlines, and cargo airline supply chains. Fronting for the telecom and airline industries are two old growth government alphabet agencies, the FCC and FAA respectively.

The FCC is an independent agency charged with regulating the nation's electronic spectrum. The FAA, organized under the Department of Transportation, regulates the nation's airspace and airlines. Also lined up in their respective corners are two industry trade groups, 5G Americas representing the telecom companies, and Airlines for America (A4A) representing the airlines. It is in this arena that this drama is being played.

At issue is whether newly implemented 5G transmissions will adversely affect radio altimeters on commercial aircraft. Radio altimeters use radio transmissions to determine precise aircraft altitude when an aircraft is close to the ground. They are much more accurate than classic barometric altimeters and are therefore used during low visibility approaches where precise measurement is needed. A number of other systems including thrust reversers, terrain avoidance systems, and windshear avoidance systems use the data from the radio altimeters to function properly. The loss, therefore, of the radio altimeter will invalidate all low visibility approaches which in effect takes us back to the 1970s for landing in fog, snow, or rain.

The electronic details are mundane. 5G transmissions are in the C-band spectrum which is adjacent to that which radio altimeters use. The fear is that the high power 5G transmissions will bleed over and disrupt the altimeters. Partisans who point out that the Europeans and others haven't experienced any 5G interference issues neglect to note that the European C-band deployment is different than US C-band, and has a larger frequency buffer. The question remains though of how much interference actually exists and whether safety would be potentially compromised by 5G interference.

Airline passengers should rest easy. The FAA, for all its faults, will not let airline safety be compromised. Yes, I hear you laughing after the 737 MAX debacle, but consider that the aircraft certification and operations branches are effectively separate organizations. But also, as a result of that incident, the FAA is now hyper-sensitive to any perceived laxity towards safety. The operations side is obsessively conservative and has already issued multiple airworthiness directives forbidding the use of any low visibility approaches requiring the use of the radio altimeter in the presence of 5G towers. One might even surmise that the new directives are overly conservative in order to compel diversions which would bolster the political case the FAA is attempting to make.

How Did We Get Here?

Cock-ups of this magnitude, pitting US industrial sectors along with their regulatory watchdogs against one another, are rare. The FAA has published a timeline of events from their perspective stating that the alarm had been sounded multiple times dating back to 2015. For their part, 5G Americas, in a white paper, states that a working group studying the issue took these concerns into account and yet did not reach any consensus and therefore did not submit any technical reports nor recommendations to the FCC. My personal guess is that even if the senior leaders in either the FCC or the telecoms had even been briefed about a potential problem, which seems unlikely, the thought of billions of dollars of revenue from spectrum sales or 5G deployment easily diverted their attention.

Claiming the Moral High Ground

At this point the lines have been drawn and each side is playing for keeps. The telecoms are unlikely to back down from their 5G deployment after spending billions for the spectrum, hardware, and marketing costs. The AT&T girl doesn't work for free after all. The telecoms claim that the aviation sector botched their chance at any input and are now whining after the fact.

The aviation sector will of course roll out the safety card. They will also try to make the case that it is the telecoms who are the interlopers in an already stable and established system. And as day follows night, this entire fight will likely end up revolving around money. This is where it gets interesting.

One need only take a peek at the market caps of the telecoms versus the airlines to see where the 800 pound gorillas reside. The combined market caps of AT&T, Verizon, and T-Mobile, the big players in this episode, come to about $549B. The big four airlines (AA, UA, DL, SW) tote up to a whopping $75B. UPS and FedEx add another $244B but only a fraction of their operations are impacted. My guess is that the telecoms are better positioned to make more "friends" in the administration and Congress.

The current administration would love nothing better than for this mess to go away as both of the agencies in question belong to the executive branch. It is just not a good look; though each party will waste no time in blaming the other for the mess.

Pay the Man and Move On

The denouement of this melodrama will undoubtedly consist of part engineering solutions, such as adjusting the power output or orientation of the 5G towers. However, it may well include more costly fixes such as replacing current radar altimeters with updated models employing filtering to reject 5G interference. The airlines will likely attempt, and fail, to get the telecoms to pay for these upgrades. In any event, the entertainment will be worth the price of admission.

Captain Rob Graves is a veteran airline pilot and retired Air Force officer. He currently flies a Boeing 737 for a major American airline where he has over 30 years of experience. His Air Force career included instructing future USAF pilots in the T-37 primary jet trainer, aerial refueling in the KC-135 Stratotanker, and conducting worldwide logistics in the C-5 Galaxy cargo aircraft. He is the author of This is Your Captain Speaking, an aviation blog. It can be found at

Thursday, July 01, 2021

The Lost Christmas Trees of the Strategic Air Command

The Strategic Air Command (SAC) was a major command within the US Air Force from its inception in the aftermath of World War II until it was disbanded at the end of the Cold War. SAC was charged with conducting long range offensive air operations world wide. During the height of the Cold War, SAC consumed a large share of the Air Force budget in support of two of three legs of our nuclear response triad: bombers and intercontinental missiles. The Navy had responsibility for the third leg which consisted of nuclear missile armed submarines.

The manned bomber leg of the nuclear triad eventually came to be dominated by the Boeing B-52 Stratofortress. The B-52 was a large aircraft with a wingspan of about 185ft. With aerial refueling, these aircraft were intended to bomb targets in Russia after launching from their US and allied bases. Given the large fuel and armament loads, long runways had to be constructed to accommodate this intercontinental mission. These runways were often as long as 12,000 feet and as wide as 300 feet to accommodate the B-52's wingtip gear.

Dozens of these gargantuan runways were built in both the United States and some allied nations during the 50s and 60s. Accompanying these runways were parking spots for the bombers and their mated tankers to allow a quick takeoff in the case of incoming Russian missiles. These "alert" parking areas came to be known as "Christmas trees" due to the shape of parking aprons which empty out on to a central taxiway leading to the runway. Aircrews would spend up to a week living in a special dormitory known as an "alert facility", ready to launch their aircraft to fight WWIII on the several minutes notice of an incoming attack.

SAC was disbanded in 1992 at the termination of the cold war and many of the bases that hosted B-52s have been either closed or repurposed. Those Christmas Tree ramps and huge runways live on however. Some have been overtaken by weeds and others are now commercial airports. They can be readily seen from the air if you know where to look. Here are a few of my favorites though this is nowhere near an exhaustive list. 

Bergstrom AFB,  Austin, TX

Bergstrom AFB was established in 1942 and ping ponged several times between SAC and the Tactical Air Command (TAC) during its life as an Air Force base. B-52s were stationed there in the early 60s until they were replaced by RF-4 reconnaissance aircraft. A pair of F-4s on approach into Bergstrom were famously captured in a music video by the Clash of their hit Rock the Casbah in the early 80s. The base was closed in 1993 and repurposed as Austin-Bergstrom International Airport. The alert parking aprons were incorporated into the cargo ramp and can still be seen. The airport it replaced, known as Austin Mueller Airport, was redeveloped into residential and commercial space, but the historic control tower still stands in that neighborhood.


Grissom AFB, IN

Grissom AFB started out in 1942 as a Navy base. The base was transferred to the Air Force in 1951 and reopened in 1954 as Bunker Hill AFB. While the base never hosted B-52s, it did host both B-47, B-58 and interceptor aircraft which would also have used the alert pads. In 1968 Bunker Hill was renamed Grissom AFB (three letter identifier GUS) in honor of Astronaut Gus Grissom who died in a Nasa space capsule fire. The base is now an Air Reserve base hosting KC-135 aircraft and several other reserve units. The runway is 12,500 ft long.

Larson AFB, Moses Lake, WA

Larson AFB like many other bases was activated during the war as a training base. The close location to Boeing Aircraft Company in Seattle meant that Larson would see quite a bit of test aircraft operations. The XB-47 made its first flight from Moses Lake as did the YB-52. The base hosted B-52 and KC-135 operations from 1960 to 1966 when the base was closed. The main runway is 13,500 ft long. I also have some personal experience here as this is where I got my 737 type rating in a rented 737. This was before the time when type ratings could be attained with simulator time alone. After getting my bounces, our destination of SEA went below minimums and we ended up holding for an hour on my nickel. 737s rented for something like $25/minute if I recall. Expensive, but it all worked out in the end.

Glasgow AFB, MT

Many SAC bases were located in northern states as it reduced the flying distance to the Soviet Union. Known as "northern tier" bases, these were considered less than desirable assignments by many as the locations were isolated and the weather was cold. Glasgow was operational as a SAC base with their own assigned aircraft from 1960 through 1968. It was used for dispersed operations of aircraft from 1971 until the base closure in 1976. Since that time the runway and buildings have been left abandoned with occasional use by the Boeing Company for noise testing of airliners.

Clinton-Sherman AFB, OK

Clinton Sherman had its start in 1942 as a Navy base. After the war, it reverted to civilian use and was used to salvage and dispose of surplus military aircraft. Reactivated in 1954, the Air Force then went on to build a 13,500 ft runway for SAC operations. The base hosted B-52 and KC-135 operations from 1959 to the closure of the base in 1969. This base illustrates some of the problems with commercial redevelopment of old SAC bases. These bases were intentionally placed well away from cities and commercial areas for the reason that it was believed that in the event of nuclear war, they would attract missile attacks. When the bases closed, attempts were made to repurpose the facilities with limited success.

McCoy AFB, Orlando, FL

Anyone who's flown down to see the Mouse in Orlando may have wondered why they purchased a ticket to "MCO", the three letter identifier for Orlando International Airport and not something like "MOU" or "DIS". That's because before there was Orlando International, there was McCoy AFB. Named for Colonel Michael McCoy who was killed in an aircraft accident at the base, it was a front line SAC base from 1951 to its closure and conversion to civilian use in 1975. The primary SAC runway and alert ramps are still in existence.

Amarillo AFB, TX

The airfield now known as the Rick Husband Amarillo International airport dates back to 1918 when two Army Signal Corps aircraft landed in a field near Amarillo. The airfield was a SAC base from 1959 to its closure in 1968. During that time it hosted both B-52 bombers and KC-135 tankers. Lore has it that it was President Lyndon Johnson who ordered the base closed after 16 panhandle counties voted Republican in his reelection contest. Also of personal interest to me is that Amarillo was the origin of the 909th Air Refueling Squadron. Now based in Okinawa, Japan, it was the squadron where I flew the KC-135 back in the 80s. The alert parking and long runway are still in existence.

Dow AFB, Bangor, ME

Dow AFB was established in 1947 and sold to the city of Bangor, Maine in 1968 to become Bangor International Airport. B-52s and KC-135 tankers were stationed there beginning in 1960. The base was closed in 1968 with reasons given as the maturation of the ICBM fleet which meant fewer bombers were needed as well as budgetary pressures from the Viet Nam war. The long runway and alert parking are still visible and in civilian use.


In the span of about 20 years from about 1950 to 1970, tens of millions of dollars were spent and hundreds of thousands of yards of concrete were poured to create amazing runways which in some cases were only in use for a decade or less. The era saw amazing aircraft such as the B-58 Hustler be designed, deployed, and retired in as little as a decade. It was a time of possibilities. Much of what was built is now gone except for the B-52s of course, and the weed covered Christmas Trees of the Strategic Air Command.

Saturday, January 16, 2021

Pilot Report: 737 MAX Return to Service Simulator Ride

It has been nearly two years since the 737 MAX was grounded in the aftermath of several crashes. After the redesign of a flight control system, the aircraft has been re-certified by the US FAA and EU EASA and is returning to the skies. Several US and international airlines have already returned their MAX aircraft to scheduled service with others to quickly follow.

One of the requirements for the return to service of this aircraft is that pilots undergo a training session in a simulator. The purpose of the sim ride is to familiarize pilots with the behavior and possible failure modes of various flight control and indication systems. The ride included demonstrations of the normal function of the speed trim system (STS) and Maneuvering Characteristics Augmentation System (MCAS) during a stall and failure scenarios of angle of attack (AOA) and airspeed indicators. Runaway stabilizer trim malfunctions and flight with manual trim were also included. Failures of the MCAS subsystem, which is unique to the MAX, were implicated as contributing causes in two crashes prior to the aircraft being grounded.

Preparation for the ride included an extensive computer based refresher course on MAX systems and procedures and a detailed pre-brief on the planned simulator training events. Flight in high fidelity flight simulators is considered equivalent to flight in an actual aircraft, but superior as a training device as many maneuvers and failure scenarios can not be safely accomplished in an aircraft. That said, the devices cost tens of millions of dollars and training time is scarce and valuable. My airline has procured nine MAX simulators for the purpose of re-qualifying all their pilots in as short a time as possible.

Stabilizer Trim: What Is It?

The events surrounding the grounding of the MAX center on the stabilizer trim system. I am going to attempt to keep my explanations in reach of a general audience with some basic understanding of the dynamics of flight. There are many online resources available for those who wish for a more in depth explanation of aerodynamics.

The horizontal wing on the tail of a conventional aircraft is known as the horizontal stabilizer (stab). The elevator is attached to the back of the horizontal stabilizer and is a primary flight control. It moves to change the pitch of the aircraft. The horizontal stabilizer itself also moves a bit to “trim” the aircraft for a particular airspeed. Change the speed of the aircraft, and the trim will need to be changed to prevent the pilot (or autopilot) from having to hold constant force on the controls. A pilot can release the controls of a well-trimmed aircraft without it wanting to climb or descend. This condition is also the most aerodynamically efficient configuration resulting in a smaller fuel burn.

On the 737, stab trim is normally controlled electrically by switches on the control column which are activated with the thumb. The switches (two for redundancy) control an electric motor which spins a large wheel next to the pilot’s knee on the center stand. This wheel is mechanically connected to a jack screw which physically moves the stab. The motor has two speeds determined by flap position. The slow speed is used for flaps up and the fast speed activates when the flaps are extended. This wheel also serves as a manual crank to be used if the electric motor fails. Accidentally leaving the crank handle extended is a self-critiquing error as the handle hurts like heck when it hits your knee (so I’ve been told).

It is impossible to miss this wheel turning as it has stripes painted on it. While flying manually (without the autopilot), the pilot will use the thumb switches to activate the electric trim, but when the autopilot is engaged, the autopilot keeps the aircraft in trim using the same system. Therefore, the trim wheel will be seen moving on occasion in automatic flight as the autopilot adjusts the trim.

Starting in the 80s on the “Classic” version of the 737 (models 300 through 500), Boeing introduced a trim subsystem known as “speed trim”. Speed trim would operate in manual flight under certain conditions should the aircraft deviate from the trimmed airspeed. As mentioned above, trim correlates to airspeed. This system would make trim inputs in opposition to any speed deviation to encourage a return to the originally trimmed airspeed. The important thing to note here is that the trim wheel might now be seen moving in manual flight un-commanded by the pilot.

MCAS: What Is It?

MCAS or Maneuvering Characteristics Augmentation System, is another trim subsystem which was introduced on the MAX aircraft. It was found during certification of the MAX that the aircraft had some unwanted handling characteristics when approaching a stall. Specifically, just before stall entry, and well below any normally encountered airspeed, control column pitch forces became lighter when they are required by certification rules to become heavier. MCAS was designed to run the trim forward under these specific conditions to counter this tendency. It uses the high speed rate of the trim motor regardless of flap position. 

Why did the MAX handle differently than its predecessors? The aerodynamics is complex, but the larger engines on the MAX had to be placed further forward on the wing to ensure ground clearance. This and other design factors likely caused the handling differences. This is the source of some controversy about whether the MAX should have been given a separate type certificate, but reviewing that subject is not the purpose of this report.

One point I’d like to make about flight control augmentation systems in general is that they are ubiquitous and date back to the 1950s. The existence of an augmentation system does not ipso facto indict the underlying design, but rather is an engineering solution that enhances the flight characteristics of nearly all modern high performance aircraft. I have experience flying aircraft which were virtually un-flyable without augmentation. Even fly-by-wire aircraft flight control systems, which are common today, can be thought of as augmentation systems with 100% control authority.

Lastly, the most important part of the entire electric stab trim system is that it can be deactivated at any time through the use of two stab trim cutout switches located on the center stand directly behind the throttles. These switches remove power from the electric trim system and all subsystems including speed trim, autopilot trim, and MCAS. Following deactivation, the aircraft can still be trimmed by manually cranking the trim wheel.

The MAX Return to Service (RTS) Simulator Ride

The simulator session was scheduled for a two hour training event preceded by a one hour pre-brief. The session was designed to cover both normal and non-normal flight profiles. The normal profile included a demonstration of the speed trim system on a routine departure and the expected annunciations and flight control behavior during an approach to stall. 

The non-normal profiles demonstrated trim system failures and angle of attack (AOA) and airspeed indicator failures. The trim system failures included the use of the Runaway Stabilizer non-normal checklist and immediate action items, and subsequent flight using only manual trim. The AOA and airspeed failure profiles were designed to replicate the startle effect and confusion that can manifest from multiple annunciations and aural warnings during this type of malfunction. Subsequent use of the Airspeed Unreliable non-normal checklist and known pitch and power settings were required.

The simulator used was a CAE 7000XR series high fidelity simulator with full motion and daylight wrap-around visuals. All of the training events were flown from SeaTac airport in VFR conditions. The sim was initialized for takeoff on RWY 16L with all preflight items having been accomplished. I was paired with a line first officer for the training event.

Speed Trim Demonstration

A normal takeoff and RNAV departure to 10,000 ft were accomplished. During the climb in manual flight, deviations from trimmed flight were purposely introduced through the use of increased or decreased pitch inputs on the control column. The speed trim system was then observed to make trim inputs opposite of the speed deviations to encourage the aircraft to return to its originally trimmed airspeed. Once the originally trimmed airspeed was re-achieved, the speed trim inputs were automatically removed by the system.

The speed trim inputs were accomplished by the slow rate of the trim motor as the flaps were already up. The effect of these inputs was subtle and easily overridden if needed. 

Approach to Stall/Stall Demonstration

After level off, we each were directed to pull the power to idle but to maintain altitude in manual flight through pitch control to observe the annunciations and flight control behavior during a stall. The approach to stall maneuver has been a staple of airline training for many years, but typically the maneuver would terminate with the activation of the stick shaker stall warning followed by a recovery. It was felt that exploring actual stall characteristics was unneeded and possibly negative training as this situation would never theoretically materialize in actual line operations. A recovery would always be made upon the activation of the stick shaker.

In the aftermath of the Colgan and some other crashes which served to highlight concern about deficiencies in manual flight skills, the FAA introduced extended envelope training (EET). This training went beyond traditional airline flight training to explore flight handling characteristics in areas of the flight envelope that would never be expected to be seen in line flying. The new thinking was that having some experience in these unusual situations might be of use in case one ever developed.

We were asked to make nose up trim inputs down to the lowest flaps up maneuver speed and afterward to continue to maintain altitude through control column pressure alone. A number of visual and aural alerts displayed and sounded as airspeed continued to decrease. The aural "Airspeed Low" alert sounded followed by the "Buffet Alert" FMC advisory message. The pitch limit indication appeared showing that we were within 5 degrees angle of attack to stick shaker activation. The stick shaker activated upon reaching the airspeed where natural stall warning buffet is computed to commence by the stall warning yaw damper (SWYD) computer.

During this demonstration, rearward control column forces continued to increase. As airspeed decreased below minimum maneuver speed, the speed trim High AOA mode activated thereby adding nose down trim at the slow rate of speed (because the flaps were retracted). This served to increase the necessary force to maintain altitude. Note that the high AOA speed trim feature is not unique to the MAX, but is included on older 737 models.

Eventually, the trim wheel made an abrupt twitch forward at the high rate, but only for a fraction of a turn. This, our instructor told us, was the MCAS system becoming active or "waking up". What was happening behind the scene was the MCAS logic took a "snapshot" of the existing trim position when its threshold AOA was reached. It then calculated a maximum amount of trim that could be added. Should the trim ever meet this computed limit, the speed trim and MCAS system becomes inhibited for the remainder of the flight. A short time later, MCAS activated adding additional nose down trim at the high rate of trim motor speed. The control forces were now heavier, but still manageable. 

Finally, the Elevator Feel Shift (EFS) module increased the system 'A' hydraulic pressure to the elevator feel and centering unit as the AOA approached its stall value. The elevator feel and centering unit is how any elevator force is transmitted to the pilots through the control column. Changes in trim go through this unit before they are felt by the pilot. This hydraulic pressure increase dramatically increases forward pressure on the controls and felt like someone was trying to jerk the controls out of my hands. Again, it should be noted that the EFS module is not unique to the MAX but is also included on earlier 737 models.

At this point the aircraft was in a full stall with strong buffet being felt. The controls could still be held aft, but only with two hands on the yoke and significant effort. We were then instructed to release back pressure and to let the aircraft recover and accelerate. The aircraft recovered quickly. The inputs previously made by MCAS and speed trim were automatically removed as airspeed increased and AOA decreased.

This was the end of the demonstration. Both of us ran through this event several times so that we were familiar with the sequence of alerts and flight control inputs. 737 stall behavior is benign with no significant roll or wing drop being noted. Recovery was prompt with back pressure release and flying airspeed was quickly reestablished.

Runaway Trim Demonstration

All aircraft with a powered trim system are subject to a condition known as runaway trim. Recall that elevator trim repositions the horizontal stabilizer in order to "trim" the aircraft to a particular airspeed. When properly trimmed, elevator stick forces are minimized. "Trim to relieve stick pressure" was the mantra when I was in USAF pilot training in 1982. It is still true.

The converse that a badly untrimmed aircraft is difficult or impossible to fly is also true. An inoperative trim system is sub-optimal, but one that continues to trim after the trim switch is released, or trims on its own can quickly create a dangerous situation. Stick forces will quickly become so unmanageable that continued controlled flight is not possible. This may manifest as either nose down or nose up trim. Neither is good.

Fortunately, Boeing has always included a non-normal checklist (NNC) to address runaway trim. If correctly followed, this checklist will result in the runaway trim malfunction being corrected, or the electric trim system being deactivated. Recall that the electric system, including speed trim, autopilot trim, and MCAS, has always been able to be deactivated through the use of the stab trim cutout switches located on the center stand.

Our demonstration started with the instructor introducing a runaway nose down rapid rate trim malfunction. The most important step of any non-normal event in an aircraft is identifying the malfunction correctly and then applying the correct non-normal checklist. Many an accident has been the result of a wrong assessment of the problem or the application of the wrong checklist. 

Recall that it is now normal for the trim wheel to spin un-commanded by the pilot in manual flight due to inputs by either the speed trim system or MCAS. Since the flaps were up and the aircraft was not in a stall situation, the fast rate trim activation immediately telegraphed a malfunction.This is how an MCAS malfunction might present along with some nuisance alerts. At this point, accomplishing the immediate action items (IAIs) of the Runaway Trim checklist on the Quick Reference Card (QRC) followed by the remaining steps listed in the Quick Reference Handbook (QRH) left us in a situation with the aircraft in manual flight with the electric trim disconnected by the stab trim cutout switches.

Immediate action items are steps on a non-normal checklist that must be performed from memory due to their urgent nature. The Runaway Trim checklist contains several of these steps which include disengaging the autopilot and autothrottles, controlling aircraft pitch and airspeed, and disconnecting the stab trim cutout switches if necessary. This checklist has remained largely unchanged over the many different models of the 737.

At this point, the aircraft had to be flown and trimmed manually using the trim wheel. There is a note in the checklist which emphasizes that reducing airspeed helps to relieve the air loads on the stabilizer which reduces the efforts needed to manually trim. Our malfunction was introduced at about 250 knots. Manual trim at this speed took some effort, but was easily achieved. Slowing to 210 kts allowed the flying pilot to easily fly and trim without assistance from the non-flying pilot.

Close crew coordination is of course required to split the duties of trimming and flying depending upon the situation. Flight in instrument conditions might require the flying pilot to direct the non-flying pilot to manipulate the trim. Instructive to me was the ease with which trim changes through the configuration process were able to be made. We were directed to go around on short final to see the trim changes needed for that maneuver. Using less than full power for the go-around made the maneuver very smooth and controllable.

Unreliable Airspeed Demonstration

One of the most disconcerting and dangerous malfunctions any pilot can face is the loss of reliable airspeed. Airspeed is the oxygen of controlled flight, and loss of reliable airspeed must be quickly recognized and corrected or ameliorated for a successful outcome. The importance of this instrument is why there is a lot of redundancy built in. The 737 has two primary and one auxiliary pitot probes used to measure dynamic air pressure which is then converted to airspeed measurement for the pilot's primary airspeed indicators along with an auxiliary airspeed indicator.

In addition to the direct measurement of airspeed, the 737 has displays of groundspeed derived from the air data inertial reference unit (ADIRU). While airspeed and groundspeed are not the same, at low altitudes, they are close enough to be useful.

Our demonstration simulated a bird strike or similar damage on takeoff which disabled the captain's alpha vane AOA transmitter though we were not informed of this beforehand. Immediately after rotation, a cacophony of alerts sounded accompanied by numerous messages on the displays. The indications included AOA Disagree, ALT Disagree, IAS Disagree, Speed Trim Fail, Feel Diff Press, along with erroneous airspeed, altitude, and flight director indications. The stick shaker sounded and did not cease for the entire demonstration.

As I mentioned above, the most important thing in any non-normal situation is to recognize what has failed and more importantly, what has not failed. A quick scan of the first officer's and auxiliary airspeed indicators told us that it was my instruments that had failed as the other two instruments were in agreement. I transferred control of the aircraft to the first officer who continued the climbout as I then referenced the Unreliable Airspeed checklist.

This checklist is fairly straightforward directing the autopilot and autothrottles to be disengaged if engaged. The flight directors are not to be used as they may also give erroneous information, and lastly for complete airspeed failure, some known pitch and power settings are given which are calculated to keep the aircraft from stalling or overspeeding.

We explored setting these values to see the performance of the aircraft with flaps both retracted and extended. The checklist values will keep the aircraft safe until a more detailed chart in the quick reference handbook, which uses aircraft weight, altitude, and phase of flight to set pitch and power, can be referenced.

Since our situation resulted in useable airspeed indications for the first officer, returning the aircraft to the airport was a matter of accounting for the nuisance stick shaker and other alerts, accomplishing the appropriate checklists and landing. We had been advised to bring ear protection for this segment, and it was worthwhile advice.

Changes Made to the MAX

The changes made to the MAX center around added redundancy to the Speed Trim System (including MCAS), and the Flight Control Computer. Input is now used from both AOA vanes and compared before being routed to the MCAS system. Previously, MCAS received input from only one AOA vane. A difference between input values from the two sensors will inhibit the system. New logic has been added which limits the amount of trim that the MCAS system can add. An exceedance of this limit also inhibits the system. 

Additional safeguards, redundancies and self monitors have been added to the flight control computers themselves to prevent erroneous stabilizer trim commands. The odds of a runaway trim scenario are now effectively nil, but the runaway trim procedures and checklists will remain as immediate action items on the quick reference cards and handbooks.


I published my impressions of flying the MAX back in 2018 after I first flew the aircraft. I thought it was a great flying machine back then and I think it will be better than ever after its return. Was the aircraft as well designed as it could have been? Perhaps not, but then in no human endeavor is perfection ever achieved. I do not mean to denigrate the seriousness of the accidents that occurred nor the memory of those lost. Airline crashes are nightmares for all involved.

That said, underlying causes of any accident are complex and many differing narratives develop, some with agendas of their own. Causal chains behind any accident must be considered in total. A focus upon one aspect of an accident in isolation will inevitably lead to a missed or wrong conclusion. 

Having now flown both the old and newer versions of the MAX, I am more convinced than ever that this aircraft is rock solid, whatever discrepancies there were have been corrected, and that it has a bright future as the preeminent narrow body airliner.

Captain Rob Graves is a veteran airline pilot and retired Air Force officer. He currently flies a Boeing 737 for a major American airline where he has over 30 years of experience. His Air Force career included instructing future USAF pilots in the T-37 primary jet trainer, aerial refueling in the KC-135 Stratotanker, and conducting worldwide logistics in the C-5 Galaxy cargo aircraft. He is the author of This is Your Captain Speaking, an aviation blog. It can be found at

Thursday, March 19, 2020

Dispatch from the Front: Flying in a Pandemic

Yesterday I finished my eighth day of flying out of the past eleven, and to say it's been a bit crazy would be an understatement. The travel industry, having endured the lost decade of the 2000s following 9/11 and finally regaining its footing, is being thrown back into disarray. Entire fleets of aircraft are being grounded and aircrews are being asked to take leave without pay or are being furloughed outright. Several airlines have already ceased operations.

To those of us who lived through 9/11 and its aftermath, this all seems eerily familiar. In a few short weeks we've gone from celebrating a new widebody captain bid (my wife) to investigating how to secure a new home equity line of credit. We'll be fine, but to the new kids who are experiencing their first major industry disruption, I say welcome to the lifeboat party! You will find that an airline career is really just a game of Chutes and Ladders writ large.

The first change I noted back on March 8th was that my commute flight to work was wide open. On a plane which usually has less than ten open seats, there were over a hundred empties. In fact, for those hardy souls who are still out there commuting to or from work by plane, social distancing will be a breeze on empty planes. My flight home from Chicago last night had perhaps a half a dozen passengers and my good friends over at American gave me a first class seat. I felt the thrill of an adrenaline junkie by ordering a glass of water.

Once at work, things seemed more normal. Our first two legs, a Cancun roundtrip, actually had pretty good loads. At the earliest stages of the crisis, it seems that bargaining or denial held sway, making vacationers reluctant to abandon already paid for accommodations. This view rapidly gave way to a desire to not be stuck at a vacation destination should airline service be curtailed, or the fear of becoming sick while away from home. This was evidenced by our last trip, to the Dominican Republic, which carried only a few dozen intrepid souls down, but was full coming home.

Resigned to Illness

Pilots, by their nature, routinely employ a certain insouciance, or gallows humor, when referencing the inherent risks in aviation. Failing to check the terrain charts could "ruin your whole day", or a statement like "it's better to die than to screw up on the radio" has been known to be overheard in a ready room or two. Tied up in this sentiment is a certain fatalism, but also confidence in one's ability to avoid the fate of someone who "bought the farm", even though an outcome might have little to do with ability and more with just lady luck.

These sentiments are in some ways a simple defense mechanism used to ease the knowledge of being at risk. Now that aircrew are being stalked by an unseen menace by virtue of being at work, this defense mechanism has been repurposed from mitigating aviation risks to those of catching the virus. It seemed that most of the aircrew I've spoken to over the past several weeks are resigned to the idea of coming down with the virus regardless of their actions to stay healthy.

Oh, we're all still washing our hands and making herculean efforts to not touch our faces, but we also realize that commercial aircraft, especially with dozens of switches in the cockpit, are flying Petri dishes. From waiting in line at security, to the jet bridge, to sitting in proximity to other people for hours on end, opportunities to pick up a viral hitch hiker seem manifest. Reports that the virus can be spread by simple breathing near an infected individual do little to allay this fatalism.

And why do the TSA agents need to touch everyone's ID? I used to tell my kiddos to look with their eyes, not their hands. Yuck.

Ten Cities in Eleven Days

My last eleven days of flying included seven domestic cities, three international destinations, and overnights in six hotels. I feel perfectly fine, but let's assume that the virus can be contracted and spread for a few days before symptoms appear. Should this have happened, then I've probably left quite a wide wake of disease behind. Could I have just called in sick and stayed home? Sure, but someone else would've been tapped to fly the trip. Agree or not with whether airlines should be shut down, at least some commercial flights will continue to operate.

The economic pain imposed by this event is going to be far reaching and deep. Unknown is how long lasting it will be. One analogy I overheard is that the economy is not sick per se, but rather has been put into a medical coma until the crisis passes. I'm hopeful that this is true, though it is also possible that after being forced by the virus to conduct more business online and through teleconferencing, business travel may never recover to pre-pandemic levels.

Airlines, of course, are large corporations and do have resources and credit lines to weather the storm, unlike many smaller businesses such as restaurants and hotels. I've been reliably informed that aircrew are the only guests in some of our crew hotels and the only business keeping the doors open. Food venues have been ordered closed which is a problem for overnighting aircrew who have no other ability to eat as my airline serves no meals.

9/11 Redux?

The terrorist attacks of 9/11 were a gut punch to the airline industry which didn't fully get back on its feet until ten years later. Career progression was on hold for many for what is now called the "lost decade". This pandemic has already caused a deeper loss of revenue for the industry than did the brief industry shutdown in the wake of the attacks. When and how a recovery will unfold is an open question.

Life changing or life delaying setbacks are emblematic of a career in commercial aviation. My first officer this trip was lamenting the unfolding events, and while he maintained a great attitude, I could sense his frustration. Let's call him Fred.

Fred has a less conventional background than many first officers with whom I fly. Somewhat of a bon vivant, he grew up on St Thomas and splits his time between the islands, a condo in Chicago, and a farm in the Carolinas. He spent some time on commercial fishing boats where he picked up enough Spanish to be useful in flirting with the waitresses in the Dominican Republic and Mexico. A story he tells of bartering with the crew of a Japanese fishing boat in the south Pacific for some soy sauce for the Korean sailors on his boat was quite entertaining.

He was a captain at his previous airline before it was merged with my airline and as a result he was bounced back to the right seat. Due to several career detours, he is older than I am and is close to having seniority to move back to the left seat for his few remaining years before mandatory retirement. This will now likely be delayed. The fallout for him is real.

Our flight attendants on the trip seemed to have varying degrees of stress due to the crisis. One of them, Bev, seemed to take it all in stride. She has a knack for poker apparently and has played semi-professionally. She was in a good mood having won about $400 in the resort casino, about half being Fred's. I don't gamble and was safely in bed when all this transpired. Tracey, on the other hand, was much more junior and had just purchased a condo. She was quite concerned about financial events though not as much about the virus itself, which makes sense as she is young and hale.

Call Dispatch

Upon arrival at the airport on our last leg home, I received a message to call our dispatch before departing. Given the choice of using the gate agent's dirty and broken screen mobile phone or activating international calling on my phone, I chose the latter.

The control tower at Chicago's Midway airport had been shut down due to three workers there being diagnosed with the virus. The airport was still operating but without a control tower. Think of an intersection where the traffic light goes out. You treat it like a four-way stop sign, but not nearly as much traffic can pass. It's just as safe. I had to get a briefing by a chief pilot concerning the different procedures.

The flight and landing were uneventful, but traffic had been severely restricted due to the closed tower. We were the only aircraft moving on the entire airport after landing. This also meant that my flight home had been cancelled. A quick check on FlightView revealed both an American and United flight were still operating from O'Hare to my hometown.

A Useful Prophylactic

James was my Uber driver from Midway to O'Hare. As I was in uniform, the subject of aviation came up. It turns out that he had been a flight attendant with ATA airlines for 20 years before that airline ceased operations. Family obligations forced his departure from the industry, but he remembered his years fondly. His income from driving has recently fallen drastically as a result of the pandemic. He has applied for a position with Target and has an upcoming interview. I wished him luck.

James was an older gentleman, and I asked if the virus concerned him. He assured me that drinking hot water would serve as an internal cleanse to remove any virus infection. Furthermore, using a hair dryer on the face and nostrils would then remove any offending virus thus ensuring safe passage in our newly infectious landscape. 

The more you know...

Sunday, October 20, 2019

Chemtrails: A Little Truth Goes a Long Way

Proof of chemtrail's existence [Source: the internet]

Running a modest (~3600 follower) aviation themed Facebook page for the past four years has been quite an education in human online behavior. I have a following of fellow professional aviators, air traffic controllers, aviation industry employees and general lovers of aviation. Most everyone abides by the unwritten laws of "netiquette" but there are of course always a few of the usual characters who don't play well with others.

Pedants will pick out a minor mistake or unimportant nuance to trumpet superiority. Last worders must always finish any argument, and reading comprehension aficionados will repeat your point back to you as if it was their idea. Sometimes just not responding to a provocative comment will make the person doing the commenting explode. I don't use the ban hammer often but rather try to talk excitable respondents down off the ledge. But occasionally I get a live conspiracy theorist to happen by. These guys are the most fun of all.

Pick Your Conspiracy

There are many flavors of conspiracies out there from 9/11 "truthers" to flat Earthers to the ur-conspiracy surrounding the Kennedy assassination. Rather than the dissipation of pockets of unknowing as you might expect now that we have the sum total of human science and discovery in our phones, access to the internet only seems to have injected steroids into many conspiracy theories. Confirmation bias kicks into overdrive as whole communities spring up around crazy ideas.

Joseph Pierre, a psychiatrist writing in Psychology Today, makes the point that conspiracy believers venture from a healthy scientific skepticism into nihilistic denialism. Any and all facts are subject to question regardless of any evidence, no matter how convincing, as a matter of principle. I suppose there's an attraction to walking around believing that you have secret knowledge to which few others are privy.


In aviation circles, the conspiracy of choice is known as the "chemtrail" conspiracy. A portmanteau of chemical and contrails, itself a combination of "condensation trails", the chemtrail conspiracy posits that the lines in the sky which originate behind high flying aircraft are not condensation from the moisture in aircraft exhaust, but actually a chemical spray of nefarious origin designed for a sinister end.

These chemicals can be either psychotropic substances which are designed to keep a restive population compliant, or a melange of metallic particles which are used to control the weather or "geoengineer" the planetary climate by blocking solar radiation. This particular conspiracy dates back to the late 90s and appears to have been sparked by a military research paper speculating about a future method of warfare that might include chemical dispersion from aircraft.

A Kernel of Truth

The use of aircraft to spray chemicals, usually fertilizers and pesticides, has a history dating back to the 1920s. The first aerial application of agricultural chemicals took place in 1921 from McCook Field in Dayton, Ohio. More recently, the US military undertook a large scale defoliation effort from 1962 to 1971 during the Vietnam conflict. Dubbed Operation Ranch Hand, Air Force C-123 Provider aircraft dispensed an estimated total of 20 million gallons of herbicides over Vietnam jungles.

Given this history, it isn't too great of a jump for the conspiracy minded to believe that the government has just upped their game and is now using commercial aircraft to poison the population or control the climate. Adherents will often conflate high altitude contrails which are a product of jet exhaust with low altitude wing top mist generation which is a result of the generation of lift in high humidity environments. Entire websites complete with well produced video content exist to promulgate chemtrail theories.

My Very Own Conspiracist

As I'd had thoughts about addressing this subject for awhile, I had the good fortune to have a true believer find his way onto my page. Let's call him Rob (name changed). Rob started out sending me private messages asking how it was that an attitude indicator (artificial horizon) could stay erect to the horizon if an aircraft is actually travelling over a globe.

It's actually a fair question and a real issue. He was unknowingly describing an actual phenomenon called "Earth rate" or "Earth transport" precession. An uncorrected gyro actually would slowly become inaccurate if it stayed true to its starting location as an aircraft travelled the globe. Both of these effects are accounted and corrected for in modern flight management computers. 

This explanation was answered by an angry response that he'd taken an attitude indicator apart and saw no such correction mechanism. This was probably true in that general aviation aircraft don't need such mechanisms due to their high cost and short range. A link to a Wikipedia article was met with a scoff. I didn't really expect him to believe in anything from Wikipedia, did I? How stupid did I think he was after all?

The conversation continued in this fashion. He'd ask why an aircraft travelling in a straight line over a globe wouldn't simply leave the atmosphere as if on a tangent line. The reply that aircraft don't fly in straight lines but use barometric pressure to maintain altitude in the atmosphere which is curved over the planet was also scoffed at. I hadn't "proved" anything.

It then occurred to me that he was a flat Earth believer. Rather than chase him off, I asked how he had come to his views. He mentioned that a ride in a helicopter some years past had provided him with his epiphany. He never did say exactly how that ride convinced him of the flatness of the Earth though. 

I then asked for some reference material. A trove of internet memes and links to videos followed. These memes would show things like graphic representations of an aircraft flying off into space on a straight line course, or a picture of drain or fuel dump vents on airliners as proof of the conspiracy.

Concerning chemtrails, he eventually conceded that rather than a conscious conspiracy of many thousands of commercial pilots and aircraft mechanics, the chemtrail chemicals might actually be added into the fuel directly, thereby limiting the conspiracy to fuel handlers and refiners.

The Pendulum

Looking for a simple way to demonstrate the Earth was not flat which would not rely on technology which he mistrusted, I recalled seeing a heavy four story pendulum hung in a stairwell in the Franklin Institute science museum in Philadelphia. Every morning employees would set up a circle of chess pieces under the pendulum and set it swinging. The pendulum would  knock down the chess pieces as the day progressed thereby demonstrating the rotation of the Earth. There was no way to ignore that!

The answer came back that Franklin was a Freemason and, well, nothing that he touched could be trusted. Interestingly, that was followed by several videos showing Masonic iconography which depicts the sun the same size as the moon with a flat horizon in the background, so it was difficult to know whether the Masons were with him or against him. What was clear was that a thread of mystical religiosity informed his world view and he was not going to let anyone pop the bubble.

What was also clear was that he wasn't going to attempt to address any incongruities that I raised such as how I could video call my wife from China where it was clearly dark at noon. This was just fascinating to me. I clearly didn't want to know the "truth" of his "research" which consisted of internet memes and videos posted by like-minded conspiracists.

Is It Harmless?

By all accounts my interlocutor seemed like a nice guy who got on well enough in society to hold a job and function normally. He mentioned that his job involved some sort of mechanical proficiency, and he did spend quite a bit of time on Facebook, so he wasn't obviously a technophobe. I even admired his inquisitiveness about the world. 

He was thinking about things that many people never consider, but at some point he wouldn't make the leap to the rational conclusion. Many of his positions started with a bit of truth which was then extrapolated to fantasy. The Psychology Today article referenced above noted that a confusion of the notions of  "believe in" versus "believe that" might be part of the cause of these belief systems.

I can't say that walking around believing in a flat Earth or chemtrails is an unalloyed tragedy. If it works for him, then great. In the words of that great 20th century philosopher, John Lennon: "Whatever gets you through the night is alright."

In Conclusion

One of the attributes which is emblematic of all conspiracies is that they're unfalsifiable. Any time an explanation is offered, there will be a counter-explanation which can't be verified. One of the best depictions of this was the scene from the first Terminator movie where Michael Biehn tries to explain to an incredulous prison doctor how robots from the future are coming to kill Sarah Connor.

Of course the joke here is that there actually were robots coming from the future to commit mayhem. But then, it was only a movie. Or was it?

Sunday, October 13, 2019

Did Bad Grammar Doom the Max?

emergency checklist to be used for a runaway stabilizer trim malfunction

As of this writing, the 737 MAX remains grounded with projected return dates now stretching into the first quarter of 2020. Boeing has not as of yet submitted the software fix for the controversial MCAS system to the FAA for evaluation. The European Aviation Safety Agency (EASA) recently indicated that they will seek their own additional testing of the software fix possibly resulting in a staggered return of the aircraft to service.

The post-mortem examinations of what went wrong at Boeing and the assumptions that were made concerning the flawed MCAS software continue. At issue is one assumption made early on that any malfunction in the MCAS system would be immediately recognized by average pilots as a malfunction known as "runaway stabilizer" for which a checklist already exists.

The stabilizer trim system is used by the pilots or the autopilot to keep the horizontal stabilizer in "trim" which means keeping the stabilizer aligned with the slipstream of air. It does this by actually moving the entire stabilizer a through a range of angles which change with airspeed. An "out of trim" stabilizer means the stabilizer is not perfectly aligned with the passing wind. This results in the need to hold force on the control column to maintain altitude. Letting go of the controls in such a condition would result in an undesired climb or descent. A well trimmed aircraft will stay where you put it.

In the 737, the stabilizer trim is normally controlled electrically through a motor, but can also be adjusted manually through a wheel and handle on the center stand. This system has a failure mode known as "runaway trim" wherein the motor runs after the control column electric trim switch has been released most likely due to a sticky or failed switch. This malfunction can result in an unflyable condition if not quickly corrected. It is this failure mode which is addressed by the "runaway stabilizer" checklist reproduced above.

Continuously or Continually?

Boeing engineers were also counting on pilots using this same runaway stabilizer checklist in the event that the MCAS system, which also uses the stabilizer trim, malfunctioned. The problem with this assumption is that the two malfunctions can appear to be very different things. During a classic stuck switch runaway trim, the trim wheel in the cockpit starts spinning and does not stop. That's the definition of "continuously" and is correctly annotated as one of the conditions on the top of the checklist.

An MCAS malfunction, however, presented quite differently. During that malfunction, the MCAS system would spin the trim wheel forward for a specified amount and then stop. If the pilot then used the trim switches to adjust the trim in a nose up direction, a malfunctioning MCAS would wait five seconds and trim forward again after each input by the pilot. This "very often; at regular or frequent intervals"  behavior of the MCAS system is the definition of "continually", not "continuously". 

This is exactly what happened to Lion 610. After reversing the MCAS inputs multiple times, the captain passed control of the aircraft to his first officer who was apparently unaware of the inputs the captain had been making. He never countered the next MCAS input which doomed them.

From Dictionary [dot] com:

In formal contexts, continually should be used to mean “very often; at regular or frequent intervals,” and continuously to mean “unceasingly; constantly; without interruption.”

Is this a minor and pedantic point? Perhaps, but perhaps not. English is the international language of aviation, and all pilots are expected to be proficient in English to be qualified to fly in international airspace. The pilots of both Lion 610 and Ethiopian 302 were likely not native English speakers and were highly unlikely to be aware of such a nuance as the difference in meaning of these two words.

They were, however trained in the various failure modes of their aircraft, and were not likely to be expecting the intermittent behavior of the failed MCAS system. The pilots of Lion 610 had no knowledge of the existence of the MCAS system as it was not included in their flight manuals. The pilots of Ethiopian 302 did have the Emergency Airworthiness Directive (EAD) published by Boeing describing the MCAS system, but were still slow to recognize that their problem originated from a bad MCAS system until too late.

In Conclusion

Aircraft flight manuals should contain all the information needed by pilots to safely operate their aircraft. This information should include accurate descriptions of possible failures, the recognizance of such failures, and best practices on how to solve or mitigate problems that arise. The omission of the existence and description of MCAS from the MAX airplane flight manual only compounded the problems faced by the two mishap aircrews. Faced with a fusillade of warnings and distractions which served to conceal the real nature of their problem, they were defenseless against a poorly designed and undocumented but deadly adversary.

Sunday, May 12, 2019

737 MAX Update

I haven't written about the saga of the MAX lately because there hasn't been much change in the situation concerning the grounded airliner. Progress is being made in fixing the MCAS system implicated in the two 737 MAX crashes, and estimates for the ungrounding of the aircraft range into the July-August timeframe.

The FAA recently convened a multi-agency Technical Advisory Board to review Boeing's proposed software fix for the MCAS system. The results of that review will be needed prior to FAA approval of the design changes.

That said, there have been a number of stories brought to light as to how the MCAS system came to be designed, and some more disturbing revelations about Boeing failing to disclose an inoperative warning feature to its customer airlines.

MCAS: What It Is and What It Is Not

The Maneuvering Characteristics Augmentation System (MCAS) has been routinely described in popular media stories as a stall prevention or mitigation device. It is really neither, but rather is a system designed to make the MAX "handle" just like the older Boeing NG series aircraft it replaces.

During flight testing of the MAX, test pilots and engineers noticed that in a very small corner of the flight envelope: lightweight, aft center of gravity (CG) and approaching a stall, the forces on the stick varied from the NG version of the aircraft. MCAS was introduced in order to counter this divergence in longitudinal stability between the two models to make them "feel" the same. The genesis for the difference in handling is due to the MAX having larger, heavier engines which are set further forward on the wing for ground clearance.

The need for identical handling between the two aircraft was to maintain a common "type rating" on both aircraft thereby allowing pilots qualified on earlier versions of the 737 to fly the new aircraft without extensive training. Airline pilots, unlike, say, flight attendants, cannot fly separate types of aircraft but are generally only qualified on one "type" of aircraft (at a time).

The word "type" has a very specific technical definition in that the FAA designates which aircraft fall under the specific "type rating". For instance, being "type rated" in the 737 allows pilots to operate all the various sub-models of that series (-200, -300, -400, etc) without an extensive course of study for each sub-model. The Boeing 757 and 767 were also given a shared type rating as those aircraft were considered similar enough that pilots could fly both of them under a single "type rating". These ratings are annotated on all pilots' licenses. The same is true for the Airbus A320 series of aircraft.

As we now know, the MCAS system was flawed in its design due to being able to be triggered by a single angle of attack (AOA) indicator, and also by the ability of the system to reset itself and re-engage multiple times without limit. The question of how and why this design flaw happened is the subject of multiple investigations into the certification process.

Inoperative Angle of Attack Warnings

The WSJ has done some excellent investigative reporting on the MAX story and revealed recently that not only did the MAX aircraft not have a specific AOA warning indication which had been included on the earlier NG, models but that Boeing engineers were unaware that the warning on the MAX was inoperative. Furthermore, Boeing delayed notifying their customer airlines of the situation for nearly a year.

Angle of attack cockpit indicators are not known as what are "primary flight instruments" such as airspeed, altitude, and attitude. Pilots use primary flight instruments to directly fly the aircraft. An angle of attack indicator, however, is not required to safely operate most aircraft and is usually not included in cockpit displays. 

An analogy might be to a tachometer in your car. Nice to have but not needed. Much outrage has been vented over Boeing's not including this cockpit indicator as standard equipment, but I don't see it that way. AOA cockpit indicators are simply not needed for safe flight.

Angle of attack sensors, small vanes on the exterior of the aircraft, are traditionally used to send AOA information to an airliner's flight control computers and are used to provide "stick shaker" stall warnings. There are two installed on the 737 and the cockpit warning "AOA Disagree" would display should the two indicators return different readings, indicating a malfunction in one or both.

It was this "AOA Disagree" warning which was inadvertently deactivated on the MAX aircraft. Had the AOA only served its previous function of activating stall warning, this would be no big deal. But because the MCAS system was triggered by a single AOA sensor, not having this warning quickly became a very big deal. 

A single malfunctioning AOA indicator has been implicated as a possible cause for the inadvertent activation of the MCAS system on both the Lion and Ethiopian crashes. Having this alert enabled might have aided the pilots of those aircraft to figure out what was going wrong.

Pinto, Tylenol, MAX?

It is becoming apparent that the MAX will be back flying at some point, but the question now arises as to how well Boeing will weather the ongoing tsunami of negative PR. Even President Trump weighed in on this question recommending that Boeing rebrand the aircraft.

If you recall, the Ford Pinto never rose above safety concerns after several accident caused fires and the model was eventually terminated. The response to the Tylenol poisonings, however, is now considered a textbook example of how to manage a public relations crisis. One thing learned is that transparency and being forthcoming in light of a tragedy is essential. Boeing has only made matters worse by their perceived lack of candor.

My guess is that once the MAX is back in the air, the crisis will be quickly forgotten. My reasoning is that the public has a notoriously short memory for these sorts of things. Other aircraft have had spotty beginnings and went on to become successful. The MAX, which is still a 737 at heart, has a long and enviable safety record. As the parable states, the dogs may bark, but the caravan moves on.

Sunday, March 31, 2019

Flight Data Results from Ethiopia 302 and the MCAS System - Smoking Gun or False Lead?

737 MAX throttle quadrant showing trim wheel and stab trim cutout switches (lower right).

The Wall Street Journal is reporting that, after preliminary analysis of flight data from the downed Ethiopian 737 (ET302), investigators now believe the controversial Maneuvering Characteristics Augmentation System (MCAS) activated and may have played a part in the accident.  This finding is significant because the MCAS system has now been implicated in both this crash and the Lion Air crash which occurred last year.

MCAS - What is It?

To recap, the MCAS system was installed on the 737 Max aircraft to mitigate some unique handling characteristics of the new model which differed from older 737s. Boeing originally chose not to document this new system in the aircraft flight manual, but has since briefed all Max operators on the existence and function of the system in the aftermath of the Lion Air crash.

Among the reasons Boeing engineers may have had for not including the system in the flight manual are that the system was only supposed to ever activate during aerodynamic stall conditions in manually controlled flight, which in normal operations would never be seen. Entire careers are flown without ever seeing an actual stall, so this rationale might have been thought sound.

The problem for the MCAS system wasn't necessarily its intended operation, which was to be rarely if ever seen, but rather any potential failure modes. Unintended activation of the system due to a mechanical fault has now been suggested as a factor in both Max crashes. Flight data from the Lion Air crash show the pilots repeatedly fighting the inputs from a misfiring MCAS system, and according to latest reports, the MCAS system also activated on the mishap Ethiopian airliner.

Adding to the controversy of the existence of an undocumented system is the revelation that the system can be activated by a single angle of attack (AOA) sensor. Angle of attack sensors measure the angle of the relative wind over the wings. Too great of an angle between the wing and the airflow over it will result in an aerodynamic stall wherein the wing stops producing lift.

The questions being asked involve the engineering decision to use the input of a single AOA sensor to trigger the MCAS system to operate. There are two (or more) AOA sensors installed on all airliners which among other things are used to provide "stick shaker" stall warning to pilots if they get too slow or approach a stall. Again, a stall is something that most airline pilots will never see outside of a training simulator where stall recovery is practiced routinely.

What Did the ET302 Pilots Know about the MCAS System?

Lion Air JT-610 crashed on October 29, 2018. The investigation of that crash first brought the MCAS system and a malfunctioning AOA sensor to light. On November 7th, Boeing released an Operations Manual Bulletin (OMB) to all 737 Max operators. This bulletin mentioned that erroneous AOA signals can cause the trim to run uncommanded by the pilot. The directed remedy is to apply the runaway stabilizer trim checklist which directs the use of the center pedestal mounted stabilizer trim cutout switches. The text of the bulletin is as follows:

The Indonesian National Transportation Safety Committee has indicated that Lion Air flight 610 experienced erroneous AOA data. Boeing would like to call attention to an AOA failure condition that can occur during manual flight only.

This bulletin directs flight crews to existing procedures to address this condition. In the event of erroneous AOA data, the pitch trim system can trim the stabilizer nose down in increments lasting up to 10 seconds. The nose down stabilizer trim movement can be stopped and reversed with the use of the electric stabilizer trim switches but may restart 5 seconds after the electric stabilizer trim switches are released. Repetitive cycles of uncommanded nose down stabilizer continue to occur unless the stabilizer trim system is deactivated through use of both STAB TRIM CUTOUT switches in accordance with the existing procedures in the Runaway Stabilizer NNC. It is possible for the stabilizer to reach the nose down limit unless the system inputs are counteracted completely by pilot trim inputs and both STAB TRIM CUTOUT switches are moved to CUTOUT.
Additionally, pilots are reminded that an erroneous AOA can cause some or all of the following indications and effects:

- Continuous or intermittent stick shaker on the affected side only.
- Minimum speed bar (red and black) on the affected side only.
- Increasing nose down control forces.
- Inability to engage autopilot.
- Automatic disengagement of autopilot.
- AOA DISAGREE alert (if the AOA indicator option is installed)

In the event an uncommanded nose down stabilizer trim is experienced on the 737 - 8 / - 9, in conjunction with one or more of the above indications or effects, do the Runaway Stabilizer NNC ensuring that the STAB TRIM CUTOUT switches are set to CUTOUT and stay in the CUTOUT position for the remainder of the flight.

A subsequent Emergency Airworthiness Directive (EAD) directed this information to be included in the flight manual of all Max aircraft within three days.

In my view, it is reasonable to assume that the ET302 pilots were well aware of the MCAS system, its possible failure mode due to an erroneous AOA sensor, and the steps to be taken to remedy the malfunction.

Why Didn't They Just Turn it Off?

The investigation of the Lion Air crash has revealed that on the flight immediately preceding the mishap flight, an off-duty 737 qualified pilot was occupying the jumpseat. That aircraft also suffered the same malfunction of the AOA sensor resulting in uncommanded nose down trim. On that flight, however, the guest pilot recommended that the operating pilots use the stabilizer trim cutout switches, which they did. That flight landed uneventfully.

The pilots on the subsequent Lion Air flight fought against the nose down trim commands continually, but never did deactivate the electric stabilizer trim with the cutout switches. The errant automated trim commands eventually trimmed the aircraft into an unflyable condition.

It would seem easy to Monday morning QB the actions of the Lion Air mishap pilots, but it must be remembered that there were many other things happening at the same time. One important thing to note is that the stick shaker activated right at liftoff and continued for the entire flight. The stick shaker is a device that literally vibrates the control yoke when an aircraft approaches an actual stall. It is loud and disconcerting when activated. The pilots were no doubt startled and distracted.

Another point to note is that the MCAS inputs would not "present" like a traditional runaway trim situation. Typically, a runaway trim malfunction in a simulator would simulate a stuck switch where the trim wheel would run continuously in one direction. During the mishap Lion flight, the flight data recorder showed the pilot actively trimming back against the MCAS inputs followed by a few seconds delay when the MCAS system would reactivate and start trimming forward again.

Another system called "speed trim" installed on earlier and subsequent 737 models can also run the electric trim with the autopilot disengaged, so it is not completely unusual to see the trim wheel spinning by itself with the autopilot off. This "negative training" may have contributed to the pilots not focusing on the uncommanded movement of the trim wheel even though speed trim only functions with flaps extended while the MCAS system only functions with the flaps retracted.

What Happened Then on ET302?

The flight data recorder and cockpit voice recorders from ET302 have been recovered and sent to France where they were downloaded and decoded by the BEA, the French equivalent of the NTSB. The data from the recorders have not been released to the public, however investigators have an "emerging consensus" that the MCAS system activated and contributed to the accident. The story also noted that this preliminary finding is subject to revision.

The pilots of ET302, however, had something that the Lion pilots did not, and that is a detailed description and knowledge of the MCAS system and the procedure to disable it by throwing two easily reached switches. Without more information from the accident investigation, it is simply too early to reach any definitive conclusions about the fate of that airliner.

Thursday, March 14, 2019

What's Next for the Max?

As an aviation blogger, the past few days have been simultaneously hope inspiring and depressing. Hope inspiring as many people understand, or make a good faith attempt to understand, the underlying issues surrounding the 737 Max. But also depressing as random fanbois, trolls, and low information, yet self proclaimed experts, happen by my comments section to disgorge their dubious wisdom on things about which they know little or nothing.

Mencken was Right: No One Ever went Broke Underestimating the Intelligence of the Public

One commenter offered, based on no information other than two 737s had crashed, that all of them should be grounded. I pointed out that by that logic, it would be even more beneficial to ground all airplanes everywhere as it would be safer still. The response was "I didn't say all airplanes should be grounded" displaying an ironclad grip on logical fallacies.

When I noted that the MCAS system could be completely deactivated using two switches mounted on the center console, a commenter replied that well, "maybe the switches reconnected themselves". Other than the testing of those switches being a mandatory preflight item, this commenter has obviously confused the Boeing 737 with the SkyNet model T-1000 Terminator which can rewire itself automatically.

Lastly, when one commenter [Hi Scott!] boldly opined that the 737 was the worst airplane he'd ever flown on, I replied that my passenger experience is usually more dependent upon the particular airline and class of service rather than the aircraft type. This big brained person assured me, however, that no, none of that mattered. He apparently would rather sit in a non reclining 28 inch pitch economy seat on a Spirit A320 than a first class seat on a JAL 737.  [Sigh]

Public Relations and Marketing Wins

So the FAA bowed to international and media pressure and grounded all Max aircraft, which is proving to be a minor inconvenience to most operators of the aircraft. I was personally walking out to a Max to fly to Phoenix when the announcement came. Someone somewhere had done some preparations and an -800 was towed to the gate by maintenance about 10 minutes later for a slightly delayed departure.

We of course are now treated to the circular logic of all the "I told you so" stories. The process starts as media sensationalism whips up a gullible and credulous public followed by outraged calls for the aircraft to be grounded. After weather-vaning politicians cave into public pressure, preening media talking heads then get to state that something must have been really been wrong. And so it goes.

Make no mistake: this grounding has more to do with public relations and marketing than safety. As of yet, there is very little evidence that the two Max crashes are in any way related other than the most superficial of circumstances. But the tsunami of media scare stories and sensationalism showed no signs of abatement, so this was the correct decision. 

The FAA cited "newly" discovered satellite data which finally swayed their decision.They are referring to the ADS-B tracking system which relays flight parameters to air traffic control through satellite. This information, however, was publicly available shortly after the crash and it does show some minor altitude excursions, though nothing is conclusive.

The cockpit voice recorder and flight data recorder from the Ethiopian crash have been recovered and sent to France for analysis. Again, prescient commenters noted that this was a good thing because, of course, had they been sent to the US, American investigators would falsify any result finding the US producer of the aircraft at fault. I actually agree with this decision in spite of the slander against the integrity of the NTSB and other US investigators. Having French investigators analyze the data will deflect the inevitable cries of bias should the investigation find fault in anything except the aircraft itself.

What Next?

What happens next is we wait for the data from ET302 to be downloaded and released. When that happens and a likely cause of the accident can be discerned, the Max will be cleared to fly. Notice that I didn't say that this clearance will in any way be dependent on the outcome of the investigation. The aircraft will be flying again in a matter of weeks regardless of the findings.

Why you ask? Should the MCAS system be implicated in this crash (unlikely in my opinion), there will be software fixes and training updates offered. As I've noted many times, the system can be deactivated completely through the use of two center console mounted switches. Even then, the system should only activate in the case of gross pilot negligence resulting in an aerodynamic stall or, as in the case of the Lion crash, an errant sensor input due to a mechanical malfunction. 

The software fixes will preclude the activation of the system due to the failure of a single sensor. The training updates will reemphasize to all operators that undesirable electric trim inputs can be inhibited through the use of the center console mounted stab cutout switches. 

Should the MCAS system not be implicated in the ET302 crash, the Max will be back in the air that much sooner. Make no mistake, all airline crashes are tragedies of the highest order for everyone involved. The object of any investigation is to find out what happened and to take measures to prevent any future recurrence. Commercial aviation is one of the safest, if not the safest means of transportation available. 

What will be left is a mopping up by the lawyers.

Captain Rob Graves is a veteran airline pilot and retired Air Force officer. He currently flies a Boeing 737 for a major American airline where he has over 25 years of experience. His Air Force career included instructing future USAF pilots in the T-37 primary jet trainer, aerial refueling in the KC-135 Stratotanker, and conducting worldwide logistics in the C-5 Galaxy cargo aircraft. He is the author of This is Your Captain Speaking, an aviation blog. It can be found at He also writes for