Monday, October 17, 2016

NTSB Attributes Design Flaw and Pilot Input for Crash at LaGuardia

Back in March of last year, a Delta MD-88 slid off the runway in a snow storm at LaGuardia. The aircraft was heavily damaged but there were no injuries. I wrote about that incident here.

To recap, the aircraft landed in really lousy visibility during a snowstorm and slid off the side of the runway and almost into Flushing Bay. While the approach and landing were normal, the aircraft drifted off the left side of the runway after landing, eventually hitting the airport perimeter fence and coming to a rest on the berm that borders the airport and the water.

Since the winds were not particularly strong, my first guess was that the problem might have been a braking problem, but that was not the case. The NTSB recently concluded their investigation of the incident and have blamed the accident on an obscure directional control characteristic of aircraft with tail mounted engines known as "rudder blanking" along with the pilot's reaction to that effect. The full accident report can be found here.

Design Flaw

Inherent in the design of turbine powered aircraft with tail mounted engines and thrust reversers is an effect known as rudder blanking. Shortly after touchdown, pilots command the thrust reversers open using levers located on the throttles. On the MD-88, the aircraft involved, baffles actually close over the exhaust of the engines and redirect the thrust to the sides and forward of the engine. This redirected thrust helps to slow the aircraft.

The problem with this configuration is the location of the engines on the rear fuselage near the tail of the aircraft. The redirected thrust also has the effect of reducing the relative wind over the vertical stabilizer and rudder. Reducing the air over the rudder reduces its effectiveness. The reduced rudder effectiveness combined with the crosswind allowed the aircraft to depart the left side of the runway and to hit the boundary fence.

Boeing (which purchased McDonnell Douglas, the manufacturer of the MD-88) was aware of the flaw and recommended that reverse thrust was not to be used at full power during landing. Boeing recommended a further restricted use of reverse thrust when landing on a runway "contaminated by clutter" which is aviation-speak to mean a buildup of snow or slush.

Here's an excerpt from the Flight Operations Bulletin published by Boeing:

Due to the geometry of the MD-80 thrust reversers, the exhaust gas efflux pattern will, at certain rollout speeds and EPR settings, interfere with the free-stream airflow across the rudder surfaces. This will result in partial “rudder blanking”; with a resultant reduction in directional control authority. As rudder effectiveness is more critical on wet or slippery surfaces, “rudder blanking” becomes a concern above a reverse thrust level of 1.3 EPR. Normal dry runway maximum reverse thrust power is 1.6 EPR [emphasis in original].

Will the Brakes Stop the Airplane on the Runway?

Another concern of the crew was the "braking action" on the runway. This is also aviation-speak to mean the slipperiness of the pavement. The crew had heard reports that the braking had been reported as "fair" which meant that they would not have been able to stop the aircraft on the runway and would have needed to divert. Later on in the flight, the braking action had been reported as "good" by another airliner and the crew made the decision to continue.

The problem with braking action reports by other aircraft is that they are highly subjective. Some pilots make the determination by how many times their anti-skid cycles during a landing. Others use different criteria and each aircraft can respond differently to the same conditions. One pilot's "fair" report can easily be another's "good".

Weather and runway reports from the airport itself indicated that the runway was covered with 1/4 inch of wet snow. The runway had recently been swept, but by the time Delta 1086 landed, it was already white in appearance from new snowfall.

So you can see that as a minimum, this crew was concerned that they didn't have much room for error on this landing.

The Friction Measuring Trucks Were Parked

As far as the actual condition of the pavement, airport authorities have been using friction measuring equipment for decades. So what was the actual friction measurement at the time Delta 1086 landed? No one knows. Due to bureaucratic ambiguity and confusion between FAA directives and the New York Port Authority, the Port Authority elected to not use either of the two trucks it had available to measure runway surface friction.

From a bureaucratic point of view this makes perfect sense as you can be held liable for inaccurate or missing reports if your policy was to collect them. Change your policy to keep the trucks parked and you're off the hook. This works best for bureaucrats sitting in heated offices, but not so well for passengers landing in a snowstorm. But it's completely legal.

On the Edge of Safe

So in essence, the story so far is that the aircraft landed in 1/4 mile of visibility in a snowstorm on an extremely short runway bounded on three sides by water with an essentially unknown slickness of the pavement.

As a reminder, a quarter mile of visibility is 1320 feet and a touchdown speed of 140 kts is about 236 feet per second. This gave the pilot about five and a half seconds of time between seeing the runway and landing on it. And according to the flight recorder, the touchdown was well within the landing zone and on speed.

What happened next was measured in seconds. Here's the synopsis from the NTSB report:

During a postaccident interview, the captain stated that, as he was lowering the airplane’s
nose to the ground after main gear touchdown, he moved the thrust reversers to idle and then “one knob width on the reverser handle” to obtain Delta’s target setting of 1.3 engine pressure ratio (EPR).16 FDR data showed that engine reverse thrust exceeded 1.3 EPR between 3 and 4 seconds after main gear touchdown (with the left engine exceeding 1.3 EPR before the right engine) and was advancing through 1.6 EPR immediately after the nose gear touched down. FDR data showed that the EPR value exceeded 1.6 for 5 seconds, reaching maximum EPR values of 2.07 on the left engine and 1.91 on the right engine between 6 and 7 seconds after main gear touchdown. Engine power decreased after this point, and the thrust reversers were stowed at 1102:25 (7.5 seconds after deployment, 9 seconds after main gear touchdown, and 2,500 feet from the runway threshold) at an EPR value of 1.8 on the left engine and 1.6 on the right engine. At that time, the airplane’s groundspeed was 93 knots. 

In plain-speak, the pilot used reverse thrust in excess of the recommended amount for a total of five seconds and then stowed the reversers. The aircraft started to drift left at six seconds after touchdown, or three seconds before the reversers were stowed. This meant that it was in the three seconds between the time the aircraft started to drift and the time the reversers were stowed that the problem occurred.

To further complicate the landing, the aircraft by this time had slowed to 93 knots. At this speed the rudder itself loses effectiveness as there is not enough air moving over the surface to keep the aircraft on the runway. Other than the rudder, directional control of an aircraft on the ground is obtained through the use of nosewheel steering and differential braking. Both of these were used but were not effective enough to get the aircraft under control.

No Win Situation

Pilots are goal oriented people. We like to get the job done. But we are also called upon to get the job done correctly and are tasked with being the final arbiters of safety. To this end, we are given the tools and the criteria to use those tools correctly. But when some of the supposedly objective information we have turns out to be highly subjective and incomplete, the process becomes a crapshoot.

In my view, these guys were set up. They were told that if the braking was "good" they could land, but if it was "fair" they'd go for a swim. So for a few seconds the pilot overcompensates with too much reverse thrust and they nearly go swimming anyway. Another MD-88 had landed minutes before and had no problem while also exceeding reverse thrust limits. These guys just got hit with an unlucky gust of wind.

The whole idea of risk management is to make sure that the operation does not become a crapshoot. Had these pilots diverted when the "system" said they could make it safely and other airplanes were landing, they'd be questioned by their chief pilot and ridiculed by their passengers. Now they're probably wishing they had diverted. A three second and immediately corrected deviation from SOP during extreme conditions should not result in a major accident.

In the appendix to the accident report, board member Robert Sumwalt, himself a retired airline pilot, had this to say about the situation facing the captain:

As a former airline pilot for over 20 years, I’m confident saying that having to limit reverse thrust on a relatively short, slippery runway is counter-intuitive: When you need it the most, you have to use it the least.

So the next time your pilot diverts or goes around when others are landing, you will be frustrated or angry that you don't get to your meeting or home on time. Don't be. The guy or gal up front is trying to get you where you're going, but also trying to keep you alive.

Addendum: A Few Words about the The NTSB

I'm going to take a moment to say a few words about the NTSB. Good words. The NTSB in my view is a national treasure. They are staffed with a group of smart and thoughtful professionals who take the time to get to the bottom of the accidents they investigate. And while they don't have any real regulatory power to force changes, nor do they make economic cost-benefit assessments of their various proposals, their recommendations often serve as signposts to be disregarded by industry and regulators at their peril.

Delta is the Best!

Here's an amusing comparison from a Delta captain of block hours, revenue, and profits at the big four carriers that I found on a pilot forum...

Delta is THE BEST...

...Just ask them! A Delta Pilot's perspective on how the big 4 measure up.


Hello all, me again, with more numbers to share. As of late we've been all up in arms
about the status of negotiations. And somewhat displeased and confused by the rumors of
what's happening inside the MEC and perhaps even in a panic by the NMB parking us. No
doubt these are all depressing things for the average line pilots.

Well, relax a second. I'd like to take a moment of your time to give you some numbers
that will lift up your spirits and make you proud to be a DELTA pilot working for a truly
good company. Ironically, my numbers will also expose a number of lies our boss, the
company and DALPA are trying to sell us.

The Steve Dickson LIE - If you have attended a meeting recently in which our boss, a good
guy by most accounts, has briefed us about the status of the company - you will have
heard him say that the company needs improved productivity from the pilots. He has said
to me directly, that we are the least productive pilots in the industry. That our average
block hours per pilot per month are well below our competitors and that we must make
improvements to remain competitive. If you look at the data from MIT, at first, his
statement appears to have some validity.


Let's look at the big three and half airlines for comparison. They are American, United,
Delta and Southwest. The first three are similar in size and SW is about 60 percent of
the other's size. According to MTI, the average pilot flies...

> 46.9 hours per month at American
> 40.2 hours per month at United
> 43.1 hours per month at Delta

And - those energizer bunny guys at SWA...

> they fly 57.8 hours per month (Whooah!)

[data based on 2015 total annual block hours flown, divided by total pilots, divided by
twelve months]

Okay - first little lie. We're not the least productive, by the above data we are number
three and United is dead last. Clearly lazy bastards with too much quality of life
(remember this statement). But - geeze, those SWA boys and girls are kicking everyone
else's asses in productivity. How do they do it? I can see Management is dying to ramp us
to that level of efficiency... but is it really?

Block hours flown is not a complete picture. A better picture is how much revenue each
company pulls in with their annual block hours. So - let's compare annual block hour
generation first. In 2015...

> American with 12,363 pilots flew 3,478,489 total block hours
> Delta with 11,476 pilots flew 2,966,095 total block block hours
> United with 11,128 pilots flew 2,687,068 total block hours

Again - American, averaging 46.9 per month seems to have a clear lead among the big
three, but fucking SWA, wow...

> Southwest with a mere 7,457 pilots flew 2,587,191 block hours (almost equal to
united) - [could it be their taxi speeds?]

Clearly the Managements of the bigger three want to know SWA's secret and salivate over
the thought of driving us to such high levels of productivity. Look at SWA, you guys can
do just as good!

On a side note, Dickson's least productive guys delivered the second highest amount of
block hours to their company last year.
But - this is a rather meaningless number without looking at the revenue the block hours
generated. Revenue generated per block hour is important because it is an excellent
indicator of comparative fleet capacity effects on money generation. And of the big
three, Delta has the least wide bodies, and SWA has none.


In 2015 the big three and half generated the following total revenues:

> American's 3.478 million block hours delivered 41.084 billion $.
> Delta's 2.966 million block hours delivered 40.704 billion $.
> United's 2.687 million block hours delivered 37.864 billion $.
> And SWA's 2.587 million block hours only got 19.820 billion $.

Stated another way - as in dollars per block hour, a measure of efficiency of the
operation, it looks like this.

> SWA earned $7,661 per block hour flown.
> American earned $11,811 per block hour flown.
> United earned a whopping $14,091 per block hour flown.

Wait a minute, weren't those United guys the lazy goofs who only flew 40.2 hours per
month? Aren't they less productive than everyone else? Why isn't this lining up with
Steve Dickson's accusations? Liar... Oh, and what about Delta.

> Delta earned $13,723 per block hour flown.

Hey! Don't we have the second lowest average block hours flown per month per pilot? Yet
we produced the second highest amount of revenue per block hour of all the airlines.
Could that mean that there is less a correlation of block hours flown to revenue than
fleet capacity per hour flown to revenue? And doesn't more hours mean less quality of
life? Those poor rabbits at SWA are running hither and dither working way harder than
anyone else yet barely generating half the total revenue. Could there be a problem with
an all narrow-body fleet mix. Could there be a correlation with United having the most
wide bodies being able to produce the most revenue with the least effort, perhaps even
having a better QOL than us Delta guys? I'd say yes.

The good news fellow Delta pilots, by this measure we are in second place among the
airlines. But wait...there's more. Revenue is just one measure of a company's
productivity. Each airline is unique for its expenses, so let's look at how good they are
making money, not in terms of revenue, but in terms of profits.


You have revenue and then expenses and what's left is profit. This is the REAL measure of
pilot productivity. Not total work generated but rather total profits generated by equal
work. Since each company is different in size and make up, and in it's total block hours
flown, we have to go one step further and look at how much profit each pilot generates
for its company per year, all things being equal, that means per block hour. Alright -
let's see who has the best business model going.

In 2015...

> SWA generated 3.479 billion in profits. That works out to $1,344 and 70 cents per
block hour flown. As it takes two pilots to fly a block hour, each pilot at SWA generates
$672.35 in profits per block hour flown.

> American generated 4.668 billion in profits. That is $1,341.96 per block hour and
$670.98 in profits per pilot per block hour.

> United generated 4.219 billion in profits. That is $1,570.12 per block hour and
$785.06 per United pilot per block hour.

And what about the home team? Dickson's "least productive" pilots in the
industry? The ones who are third in average block hours flown and only second in revenue
generated? The one's Management MUST have become more productive lest the competition
bury us? The one's DALPA is bending over backwards to rob QOL from in order to meet the
company's needs???

Yeah, what about us Delta line guys and gals?

> Delta generated an amazing 7.157 billion in profits last year.

That is $2,412 and 94 cents per block hour, which is 53.7 percent better than the
powerhouse United gang and almost double the others.

THAT - is a whopping $1,206.47 in profits per pilot per block hour flown.


Ladies and gentlemen, fellow Delta pilots, esteemed comrades, despite our in fighting,
our fears, and management's lies and distortions...


Take pride - wear your orange, fight for what you deserve!

(Mr. Dickson - take your lack of productivity message and ditch it. DALPA, negotiate us
what we are truly worth).

J.D. Webster
7ER Capt. C4