Tuesday, March 12, 2019

Is the Boeing 737 Max 8 Safe?

737 Max 8

This past Sunday, an Ethiopian Boeing 737 Max 8 aircraft crashed shortly after takeoff from Addis Ababa to Nairobi with 157 passengers and crew. There were no survivors. This is the second crash of a Max 8 variant of the 737 in five months after the crash of a Lion Air Max 8 last October.

An undocumented system was brought under scrutiny in the Lion Air crash and now questions are being raised as to whether this same system, known as maneuvering characteristics augmentation system (MCAS), might have played a roll in this latest crash. If that is found to be the case, the safety of the aircraft itself will be called into question.

What We Know

At the current time, the cause of both accidents is unknown as the accident investigation is still underway on the Lion Air crash and the Ethiopian Air crash investigation is just getting under way. The flight data recorders and cockpit voice recorders have been recovered from both accidents and are being analyzed.

The flight recorder data from the Lion Air crash suggests that the pilots were having control difficulty due to erroneous inputs from the aircraft's MCAS system which itself received faulty inputs from a malfunctioning angle of attack (AOA) sensor. It is this errant sensor and its maintenance history that investigators are focusing on.

Initial reports from the Ethiopian Air crash suggest that the aircraft experienced control difficulties shortly after takeoff. Data from a flight tracking and reporting system known as ADS-B show highly unstable vertical velocity and airspeed readouts which were similar to the airspeed and altitude excursions of the Lion Air mishap aircraft.

Unconfirmed reports from listeners on the frequency reported that the Ethiopian pilots stated that they had unreliable airspeed indications and were declaring an emergency.

And right now, that's it. There are similarities, but no confirmation that the same system brought down both aircraft.

Is It Safe?

Given that we know little about the cause of the first accident and nothing about the cause of the second, a grounding of this model aircraft is premature. I am qualified and current in this model aircraft and am confident that it is as safe as any aircraft flying. Airplanes sometimes crash. It is always a tragedy when they do, but barring a definitive indictment of the design, there is no reason to overreact.

Even should the MCAS system be found primarily at fault, the system can be completely deactivated by two easily reached switches on the center console of the cockpit. Why the Lion Air pilots didn't take this action is unknown, but the investigation should eventually reveal the cause. A similar malfunction occurred on a previous flight of the mishap aircraft, and those pilots took the correct action and landed uneventfully. Questions as to why the aircraft flew again without being properly repaired should be asked.

In the event of unreliable airspeed, which can happen to any aircraft independent of model, routine practice of this malfunction in the simulator should make it a non-event. I recently underwent this training myself, but the basics of pitch and power date back to Wilbur and Orville. Recognition is the toughest part, but after that, known pitch and power settings will keep the aircraft from stalling and in control.

I don't mention these questions to cast blame, but rather to answer critics who don't understand aviation or engage in magical thinking. The Max is still a 737 at heart and flies nearly identically to the other four models of the aircraft that I have flown. So yes, it is safe, and I'd gladly put my family on one and fly it myself with no reservations.

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 robertgraves.com. He also writes for Avgeekery.com. 


  1. Capt Graves, you refer to the MCAS as an "undocumented system". I'm wondering what you mean by that?

    1. It was not included in the flight manual for the MAX. Boeing chose to not include documentation. That has since been rectified.

  2. I believe he means it is supposed to be transparent to the pilots, and that the only control they have over it is to remove the automatic trim motors from the loop via the stabilizer trim cutout switches.

  3. Captain Graves, so you know Captain Marvel?

  4. The MCAS system was originally not included in the Boeing flight manual. The reason for this was that Boeing engineers felt that since the system was an emergency backup and only would activate during a manually flown stall, it didn't need to be documented in the flight manual. The Lion crash highlighted the existence of the system as a possible cause of the accident. The system has since been included in the flight manual.

  5. Captain Marvel? She's ok. Maybe a bit full of herself.

  6. Hi Captain,

    Is the downward pitch trim protection new to the 737 Max only? Were they in the Classic and NG series?

    But I can only imagine how difficult it must have been when the Lion Air pilots encountered unreliable airspeed, which adds to the workload.

    Then they get blasted by the stick shaker.

    And while they're getting rattle by that thing, suddenly the aircraft decides to pitch down because it thinks its a stall.

    And the pilots weren't told about this feature.

    So you're dealing with 3 things now:

    Unreliable Airspeed.
    Stick shake.
    And finally runaway trim because of the MCAS.

    The Aircraft I fly (Avro RJ) has the same protection, but the difference is that the aircraft doesn't use the trim to pitch nose down. Instead, the control column pushes forward (stick push), but you can fight it if you don't want it to.

    Just curious about this. But if that is what happened, it would take a super pilot to recover from that.

  7. Worst case for Boeing is if there is nothing wrong with the Ethopian 737 and it crashed due to pilot error. Specifically, pilot error in disabling MCAS and stalling out. That's say that MCAS can cause a crash if a sensor is bad and not using MCAS can cause a crash because the aircraft is tricky to fly without it. Either way you are screwed. There will be no "out" for Boeing.

  8. Hi Captain,

    Something has not worked well.
    The pilots ? the technology of the plane? The maintenance? The formation ? MCAS, AOA, ADS-B?

    I do not know. I do not know if it's safe. Yes, planes sometimes fail, and it's very bold to say bluntly that it's safe.

    Until you know what has failed, I urge you not to put your family on one. Por favor. Love your family, protect it.

    My little brother can not fly anymore, he's still in Ethiopia. I could not protect it.


  9. Hi captain Oh, this is totally off topic but it came up again and I'm looking for somebody to ask about it. I'm referring to the Gimli glider. I'm an old FE, 22nd MAS among other things, and I can't figure out why those two guys didn't notice that their airplane was performing a lot better than expected. Especially when they reach their cruising altitude and had all that power left. Thoughts please?

  10. It occurred to me you might not know what I'm talkin about:
    Basically their fuel quantity indicators were inop and they and the ground crew got mixed up between the metric and English systems and they took off with less than half the gas they should have had.

  11. Probably simple complacency...they might have attributed the increased performance (had they noticed it) to a low temp dev or perhaps a light cargo/pax load. Inexperience can also play a part. It may have looked good to them. Or maybe they were just reading the morning paper up there.

  12. I agree with The Captain - I have flown the Max 8 several times, and wouldn't hesitate to fly it again today. It was grossly premature to ground them with what little data we have on the crashes, and with data for tens of thousands of uneventful flights showing no problems. I'd much sooner question the pilots' training than the aircraft. I also support Boeing in not adding MCAS to their manual. It functions almost identically (though for different reasons) to the speed trim system, which has been in use for decades. If either system we're causing uncommanded trim, I doubt a pilot could discern which system was causing the erroneous trim, but the solution is the same - "runaway stabilizer" checklist, which has the pilot turn off the two switches already mentioned. A properly trained pilot would have no issues coming to the proper conclusion to disable the trim motor. Grounding an entire fleet over this is absurd.

  13. Its really unfortunate that the 737 Max was grounded. But the question would be, knowing that investigations take months or even years it would be really premature to conclude that the pilots did not try to disable MCAS. And if they actually did that as you claim since its a prety straightforward ptocedure, if another one crashed before conclusion of the investigations would you still be this confident in the Max

    1. The system can be completely deactivated using two switches. The question is why would the pilots not do this. Airplanes occasionally crash for many different reasons. The Lion pilots had no knowledge of the MCAS system, but presumably the ET302 pilots did. It now appears as though the MCAS system is implicated in the ET302 crash, and all operators now are aware of the potential problem. Boeing is also offering software fixes to the system, so I believe the aircraft will be flying again soon.

  14. Hi Capt Rob. I followed your blog and find it extremely educational and valuable. Thus far, it appears that MCAS remains, in fact, an essential safety asset, but for that fact that it lacks AoA input redundancy which leaves it vulnerable to erroneous activation in critical phases of fight. That shouldn't be hard to fix so I expect the grounding might be over by April.

    With regard to the erroneous MCAS activation that bought down Lion Air Flt 610, while I realize the crew CAN use the cutout switches to disengage the pitch anomaly, do you think this alone (while still awaiting a promised software fix) makes it sufficiently safe? What I mean is, a trim runaway has a certain 'fingerprint' to it that pilots can quickly identify, whereas the sequence of alerts observed on the Lion Air flight are markedly different. The AD issued in the wake of the crash describes a new 'fingerprint' to recognize, involving loss of air data and the failure of a system not typically associated to a trim runaway event. I'm speaking about situational awareness, and it seems that when this unfamiliar system fails, pilot SA can become confused and become subject to tunnelling and false assessments of the situation, possibly even to the point of missing the trim wheel movements. For some of the same reasons you listed in your recent post about the dangers of go-arounds (low altitude, workload, distractions, pitch vector changes), and the added factors of startle, stress and confusion, the MAX would seem currently vulnerable to a condition where correct pilot situational awareness and actions are the ONLY redundancy. Could you possibly post your opinion on this aspect from a veteran pilot point-of-view?

    1. Agree completely...it would not present as a "classic" runaway trim. That plus the distraction of the stick shaker and other warnings would make this a very difficult situation.


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Capt Rob