Holy crap: the 737 fiasco is worse than we thought (Lion Air crash)

[Recursion]

The investigation into this is absolutely astounding.

I'm in an aviation tech company and we just got emailed this advisory from the Air Line Pilots' Assocation:

MCAS (Maneuvering Characteristics Augmentation System) is implemented on the 737 MAX to enhance pitch characteristics with flaps UP and at elevated angles of attack. The MCAS function commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors and during flaps up flight at airspeeds approaching stall. MCAS is activated without pilot input and only operates in manual, flaps up flight. The system is designed to allow the flight crew to use column trim switch or stabilizer aislestand cutout switches to override MCAS input. The function is commanded by the Flight Control computer using input data from sensors and other airplane systems.

The MCAS function becomes active when the airplane Angle of Attack exceeds a threshold based on airspeed and altitude. Stabilizer incremental commands are limited to 2.5 degrees and are provided at a rate of 0.27 degrees per second. The magnitude of the stabilizer input is lower at high Mach number and greater at low Mach numbers. The function is reset once angle of attack falls below the Angle of Attack threshold or if manual stabilizer commands are provided by the flight crew. If the original elevated AOA condition persists, the MCAS function commands another incremental stabilizer nose down command according to current aircraft Mach number at actuation.

This is the first description you, as 737 pilots, have seen. It is not in the AA 737 Flight Manual Part 2, nor is there a description in the Boeing FCOM. It will be soon.

(Emphasis mine)

Holy shit.

[Cousin Dupree]

1. Recursion, can you translate this to us laypeople?

[lagomorph777]

3. Seriously. This probably looks shocking to professional pilots. Looks like Greek to me.

[Baitball Blogger]

5. Seriously. My mental stabilizers are not synchronized to the neuronic pelican fans,

causing a massive dose of WTF?

And, shit. It's raining outside so now I have to wait to start my errands.

[Recursion]

4. Boeing added a new, undocumented control subsystem to the 737-MAX series

This control subsystem points the nose of the airplane down if the plane starts pointing "too far" up, by its own definition.

This control subsystem was introduced without any documentation or warning to pilots, and it seems to be responsible for the Lion Air crash.

[Baitball Blogger]

7. That computes, captain.

Thank you.

[Cattledog]

9. Yep, the pilots of Lion Air were trying to pull the nose up but the computer pulled the nose down.

[joshcryer]

10. Wasn't there also a problem with the sensor that controls this?

[Adrahil]

12. I have read that, but....

it's tricky. There should generally be a manual override in any circumstance that control inputs are automatically generated. If pilots KNEW that the MCAS could generate a nose down command, despite manual inputs, then they could have attempted a manual override. As it was, it seems the MCAS thought the pilots were commanding the aircraft into a stall condition. A lay person might say, "yeah, but the aircraft was descending! Rapidly!" Yes. But the instinct of many (though not usually trained pilots) would be to pull back the yoke and try to get the nose up. But if the aircraft is in, or close to a stall, that just makes things worse. The automatic down command is intended to prevent that from happening. If the AOA sensor had failed and there was no way to manually override the MCAS, then the aircraft was not savable. If there WAS a way to override the MCAS (likely, but I don't know), then pilots probably should have attempted such an override, but if they were not trained in this specific scenario, they may not have had enough time to think through the problem.

[joshcryer]

16. Absolutely, I agree.

And I agree that the pilots should've known about this feature. When I read about the part being installed incorrectly or something to that effect, I figured there had to also be a pilot issue (in this case lack of information about an undocumented technology). There was an airspeed sensor that froze up in one flight and newer pilots were in control of the plane because the captain was asleep. He comes back from sleep after the new pilots struggled with the plane for a few minutes (causing it to stall), and immediately tells them to stop climbing. Was too late though as it'd been falling like a rock for a few minutes at that point, cockpit transcript reading is haunting.

[Adrahil]

20. Yeah.... Super sad when that happens....

If one realizes the paradox too late, then nothing can be done.

[lagomorph777]

11. Thanks!

[FreepFryer]

14. Yup. Yikes. (n/t)

[uponit7771]

15. +1, looks like some kind of auto leveling gyro takes over. I'd be pissed if I were a pilot Boeing HA

... HAD to have said something to someone

[Cousin Dupree]

21. Thank you, and OMG. That is horrendous.

[Delphinus]

22. Thanks for that.

This does not sound safe.

[Fuzzpope]

2. Eh?

Engrish, please!

[Recursion]

8. ALPA

http://www.alpa.org/

I can't link to a mailing list email. When they get it up on their website I'll be happy to link to that.

[FreepFryer]

13. I believe the "holy shit" part is that no one knew about the MCAS or its operation for the 737...

its presence was undcumented... and so when it failed, there were no procedures to adjust to the condition.

A grievous error, indeed.

Is that correct, OP?

[Recursion]

19. Right

Boeing adds new subsystems all the time, but if they affect control characteristics they have always documented that and told simulator manufacturers about them.

[WhiskeyGrinder]

17. JFC. This is a bombshell.

[0rganism]

18. Skynet is trying to kill us

with help from corporate idiots who don't furnish this information to the pilots who need it

[Xolodno]

23. So to use an analogy for me to get this right...

...its like Microsoft updating my PC quietly in the background and suddenly the programs I use most of the time are inoperable.

[Recursion]

24. Well, changing what one function key does, but not telling you

In unusual circumstances, the same control inputs that can produce a climb in the old 737s produce a nosedive in the new ones, and they didn't mention that in the documentation.