Comment by sseth | Hacker News Reader

Comment by sseth | Hacker News Reader

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Agreed.

After the Lion Air crash, Boeing basically asserted the plane was perfectly safe, and that the pilots should have followed the runaway trim process - this means turning off the electric motor which moves the trim, and having the pilots move it manually using a wheel provided for the purpose. Boeing has tried to imply that in the midst of an emergency which looks very different from a runaway trim, pilots should have followed this procedure and this was in some way their fault.

The Ethiopian Airlines crash has shown that Boeing was being disingenous. The problem is that turning off the trim motors at this stage of the flight leaves the pilots unable to move the trim fast enough manually before they crash and they are forced to switch on the electric trim - which again allows the MCAS to kick in. When pilots who are fully informed of MCAS still were unable to control the plane following Boeing's procedures then imagine the plight of the Lion Air pilots who recovered from the fault 26 times before crashing but had no idea anything called MCAS even existed. An honest admission by Boeing after the first crash would have prevented the second crash and the loss of further lives.

There's a major difference between the Lion Air and Ethiopian Airlines crashes though - altitude and incident duration. Things that are recoverable from 5000' AGL might not be from 1000' AGL. Speed is life, and altitude is life insurance after all.

From my reading, the Lion Air pilots could have disabled electronic trim anytime after those 26 recoveries and been fine. But the Ethiopian Airlines pilots only had 1000' of altitude to recover in, and that wasn't enough. Which leads to the question of why MCAS was triggering at 1000' AGL, since it isn't supposed to activate when flaps are deployed... I don't think MCAS is going to end up being the sole hardware issue on that flight.

Could have disable it, but didn't? We don't actually know yet whether they did or when they did, and if they did if it was already too late. Critically we don't know their mental state, as we don't have transcripts yet.

If the pilots are reactive or even panicked at the unexpected uncommanded pitch behavior, they may have been doomed right from that moment. The previous flight, Lion Air 34, the pilots likewise did not immediately cutoff autotrim. We have no transcript from that flight either. None of these pilots have been told of, or simulator trained, an MCAS upset scenario. Startle factor is relevant and can cause total cockpit breakdown. The 737 PCU design flaw and subsequent accidents show this. Can problem specific training mitigate crashes? Sure. How would these pilots have received MCAS upset training when they don't even know MCAS exists and the simulators didn't have such an failure option? And should they now be required to have such training? Including simulator training?

As for why MCAS is triggered below 1500' AGL in the Ethiopian Airlines case, that is a very good question. I have not seen the altitude above which MCAS is supposed to apply, and I haven't discovered how that trigger altitude is determined, radar altimeter?

Most of the facts we still don't know.

> Could have disable it, but didn't? We don't actually know yet whether they did or when they did, and if they did if it was already too late. Critically we don't know their mental state, as we don't have transcripts yet.

'Could have' in the purely mechanical "we think this would have been possible and allowed the aircraft to survive" sense, not the "the pilots knew how to do this" sense.

> As for why MCAS is triggered below 1500' AGL in the Ethiopian Airlines case, that is a very good question. I have not seen the altitude above which MCAS is supposed to apply, and I haven't discovered how that trigger altitude is determined, radar altimeter?

It's not altitude limited, but flaps limited - so would have engaged when flaps were retracted sometime after reaching 1000' AGL. (at least, 1000' seems to be the altitude mentioned in the procedure summaries I've seen for other 737 models - things might be different for high altitude takeoffs) And as it turns out, ET-302 reached FL 140 ~5 minutes into the flight, so there goes that theory.

> Most of the facts we still don't know.

And hopefully we'll eventually learn them, and write some new rules with them.

The report is now available at https://www.documentcloud.org/documents/5793877-Preliminary-...

Flaps-up was commanded about a minute after take-off, which seems about right. The AGL altitude of the plane is hard to determine from the graphs as they only list pressure altitude. It's a high airport with, I believe, rising terrain. I'm not sure if the 1000' AGL cited is supposed to be at first MCAS engagement, or the last one, or at some other point in between.

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