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Comment by weaksauce | Hacker News Reader

Thechnically they could have used the thumb switch and regained level flight to override the mcas system temporarily and then shut off the electrical system to the stab and trim manually thereafter. However, they didn’t know about the mcas system and the troubleshooting steps that Boeing has for that mode of failure were insufficient for this precise scenario.

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At 05:43:20, approximately five seconds after the last manual electric trim input, an AND automatic trim command occurred and the stabilizer moved in the AND direction from 2.3 to 1.0 unit in approximately 5 seconds. The aircraft began pitching nose down. Additional simultaneous aft column force was applied, but the nose down pitch continues, eventually reaching 40° nose down. The stabilizer position varied between 1.1 and 0.8 units for the remainder of the recording.

It seems they tried it but MCAS kicked 5 seconds after they turned on the electrical trim. They didn't have enough time to correct the stabilizer. I don't know how fast returning a stabilizer to neutral position can be done, but they probably thought they had more than 5 seconds. They were probably going to turn off electronic trim control afterwards.

I've read that it takes the trim motor 30 seconds to move the jackscrew completely along its path. It was found all the way at one end in the wreck. So naively, it takes up to 15 seconds of continuous motor operation to recover level flight.

The manual electric trim inputs override the MCAS inputs[0], so they could have kept applying the appropriate trim regardless of the MCAS malfunction.

[0] Appendix 1, 05:40:30 (You can see the AND command being canceled by the pilots manual electric ANU command)

It only overrides the MCAS inputs when they're actively adjusting the trim. So, they did try and correct the trim with the manual inputs, and five seconds after the last push of a trim adjust button the malfunctioning MCAS activated and adjusted the trim to something far worse than when they'd started. If I'm reading this right, the plane hit the ground killing everyone on board less than 30 seconds later.

Refer to the graph on Appendix 1, the automatic AND commands begin about 5 minutes before the crash. They counter them partially with the manual ANU electric trim for the first 1.5 minutes before issuing STAB CUTOUT.

this may be a naive question but with all of the advanced technology on these planes, why isn't there a check for altitude before any automated command runs which says "hey we just took off, are 400ft above ground, MCAS is saying nose down, we will crash if that command runs, CANCEL"

It's not that simple. If the AoA is too high that can cause a stall and crash, too.

You'll 100% for sure crash if you stall at 400ft above ground. MCAS is designed to prevent stalls.

Not at take-off. See above comment about flaps.

MCAS system is supposed to stop the plane from stalling.

The faulty AoA sensor is causing the plane to think it’s about to stall and therefore MCAS works as designed to prevent the pilot from putting the plane into a stall.

Of course the design is wrong because the plane is actually pointing toward the ground and MCAS thinks it’s pointing to the sky...

What about a single button to turn MCAS off and return to trim to a neutral position? Or a setting where the pilot is telling the system to ignore the sensor input.

Maybe a bit like a self driving car planes will eventually have a system where they can use visual markers as a check on what sensors are reporting.

That may be where they will go with this but it may require a new certification and training.

Indeed there are many ways to design this better.

The 737 max was designed to not be any different to fly than previous 737s, so adding a button to disable this system was not allowed as I understand it.

As these two tragic accidents have made clear that specification was pretty wrongheaded...

Seriously, a MCAS indicator light (if MCAS is actively issuing commands), a big red button below it called "Disable MCAS" , and a MCAS disabled light (so pilots actually remember when it is disabled) should be more than enough to prevent this.

Besides adding the AoA disagree light AND having MCAS use a triple AoA sensor vote to determine the AoA angle.

I think they're not gonna do that because they'd lose the 737 type.

Until they are forced to. I hope that the FAA doesn't get a say here and other agencies will retract the type rating.

I just took a look at the take-off procedure in the checklist, and it lists that one should extend flaps before and during take-off/ascent. The checklist says that one should start retracting the flaps at 1000'AGL, the altitude where they seem to have started having problems.

MCAS is disabled while flaps are extended. It would have activated the second they retracted the flaps.

Because right after takeoff is the single most common point at which you might stall the aircraft. What's more, at that point it is likely to be a power-on stall, which is the specific situation that MCAS is intended to fix (because the 737 MAX has a tendency to pitch upwards in power-on scenarios).

Really there should just have been a separate cutout for MCAS itself, but that would have triggered retraining requirements, so Boeing wanted to avoid that. But very simply, there should just be an 'off' switch to disable this system if it starts to run away, distinct from the whole runaway trim procedure.

(as well as 2-of-3 redundancy on the sensors, the lack of redundancy is seriously exacerbating the rate of incorrect MCAS system activations)

> is the specific situation that MCAS is intended to fix

Well, not quite: if they wanted it to protect against stalls at very low altitude, it would be operational with flaps extended.

And the AoA disagree needs to be actually used and not just an "option feature pilot warning". AoA disagree on ground = plane doesn't take off. In flight = MCAS cuts out.

You can't have an AoA disagreement when stationary on the ground. It is a wind vane.

You don't know that. If it's malfunctioning it could be in the electronics and nothing to do with how it operates.

And if it's reporting a difference, it could just be because there's no airflow and the vanes aren't at identical positions. You can't rely on the difference meaning anything.

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