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Comment by na85 | Hacker News Reader

>The author makes the bizarre assertion that runaway trim is only caused by short circuits

That's not the only problem with TFA. I worked on the Hercules, not any of the Boeing airliners, but this "30-year pilot" has some pretty fundamental misunderstandings of how MCAS works, what the effects of moving the engines are, and what actually went wrong with the 737 Max. Color me surprised.

Regurgitating the same "MCAS prevents the aircraft from stalling because it's unstable" shlock betrays the author's surface level (at best) understanding of the issue.

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As a surface-level-understanding layman myself, what about this is wrong? The 737 MAX is unstable, no?

>The 737 MAX is unstable, no?

No, it's not unstable. The 737 Max exhibits stable pitch behavior along the entire envelope. A lot of people think "unstable" means "it crashes" but these terms have precise meanings.

The 737 Max is a stable airplane. One of the problems it exhibits is that with increasing angle of attack, the stick force does not increase (linearly is the requirement, I think, I'm writing from memory here).

The way you know this is because MCAS moves the horizontal stabilizer directly, and not the control column that the pilot is holding. An anti-stall device like a stick pusher or a stick shaker provides tactile feedback to the pilot via the control column. Moving the horizontal stab directly is (basically) altering the trim of the aircraft. Thus it is a handling-qualities system.

I see. So calling it "unstable" isn't really fair. It's more that it just flies differently enough that it would feel unfamiliar to typical 737-trained pilots.

Not getting pilots retrained was the cost saving strategy for companies buying the new area aircrafts.

Not needing retraining was a good goal because it improves safety. Sometimes airplanes have crashed because in a crisis the pilot did the right thing for another airplane he was trained on, but was the wrong thing for what he was piloting at the moment.

It's not just a cost saving measure.

MCAS was a sound concept. It's implementation, however, was not sound at all.

A typical pilot switches aircraft type multiple times in their career. It's all about the cost and convenience ultimately, allowing same pilots to fly both 737MAX and the older 737NG as most 737MAX customers operate both.

As far as MCAS goes, in my opinion Airbus got it right already in the 80's in their A320 with their flight envelope protection.

Do you understand how delusional it sounds in this specific context to say that not needing retraining improves safety? Not that it might improve safety or should improve safety, but that it does improve safety?

Rightly said!

"Not needing retraining" is another way of saying "works as expected".

Yes, it would likely have been safer if the 737 Max worked as expected by the pilots flying it.

> Not needing retraining was a good goal because it improves safety.

That only works if retraining is not actually needed. Boeing's own test program gave adequate (but undisclosed, at the time) information for an independent observer to conclude that this was not the case here.

I'm not sure I could agree that MCAS is a sound concept. Moving the trim automatically to account for the pitch up at high angles of attack feels like a hack, like restarting a dodgy service every couple of hours rather than fixing the underlying issues.

I've never understood why the fix wasn't implemented in the elevator feel computer. Except perhaps that it doesn't have AoA inputs and it's an old hydroelectrical system for which there may be a lack detailed knowledge.

Correct, but it's more subtle than that. FAR's are prescriptive. To carry passengers a plane from the pilot's perspective must have that linear increase in stick pressure required to pull into a stall, otherwise it can't be certified for carrying passengers.

This is part of why the MCAS could not be decoupled from the flight computer as I recall. If you turned off MCAS, you'd be non-compliant in the wind-up turn flight regime, and when coming in on final if I recall correctly.

So in aerodynamic parlance it is stable, but with disqualifying flying characteristics that had to be mitigated via a gadget whose nature, failure modes, and existence was largely hidden from the pilot, and downplayed in the FMEA/FMECA. (Failure Mode, Effects, and Criticality Analysis)

This downplaying was specifically intended to keep regulators from stipulating enhanced training for pilots. Thereby saving money.

This is a good summary.

No, it's always stable. The issue is that pulling back further on the control stick wasn't causing an increase in force feedback at high angles of attack, making it more likely for the pilot to inadvertently stall the aircraft.

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