Comment by FabHK | Hacker News Reader

Comment by FabHK | Hacker News Reader

One question to ponder: Would the two crashes still have happened without this particular mistake, that is, if the planes had had a working AoA disagree indicator?

It depends, I think:

1. Did the AoA sensor malfunction occur before the takeoff roll?

2. Would the AoA disagree indicator come on early enough to abandon takeoff?

3. Would the crew reject takeoff once the AoA disagree indication comes on?

If it's a "yes" on all three questions, then presumably the accidents would not have happened.

Once you're in the air, and all hell breaks loose (stick shaker etc.) while MCAS tries to dive you in the ground, I doubt that the indication would have made any difference: before the Lion Air crash, crews didn't even know about MCAS. After, they did know, and the Ethiopian Airlines crew, from what I gather, diagnosed the problem, though maybe too late - but again, I doubt whether the AoA disagree indication would have helped.

That leaves above questions.

1. For the Lion Air crash, the AoA malfunctioning was there before takeoff. For Ethiopian, I'm not sure.

2. I'd think yes.

3. Don't know. Anyone an idea? (Today, presumably you'd abort with a MAX and AoA issues. Back then?)

I think you're too quick to dismiss the importance of the disagree indicator, and doing so on a shaky basis. A significant amount of time was spent trying to figure out what the hell was happening. I think it's safe to assume that if the pilots saw an error warning pop up warning that there was a disagreement between two sensors, that would focus their attention on that system—which quite conceivably could have caused them to shut it off manually sooner.

> that would focus their attention on that system—which quite conceivably could have caused them to shut it off manually sooner.

But you can't shut off the AoA measurement "system", AFAIK. And the system causing the trouble (under wrong AoA information), the MCAS, can be shut off by setting the Stab Trim switch to Cut Out, but how should the crew know the connection between AoA and MCAS when they weren't even briefed about MCAS in the first place?

iirc the Malaysian flight had in the cockpit a pilot that was hitching a ride, who figured out what was going on but too late. He knew about the MCAS. Had they vocally stated something about the AoA sensor early on, or had he caught a glimpse of the message, perhaps he could have intervened sooner.

For the Ethiopian flight, you're right. They likely wouldn't have known what to do with that information. But it might have prompted them to get on the radio and ask someone, or consult a certain part of the manual (assuming there was any relevant information there.. I don't know).

It certainly seems like crucial information. And even in the small chance it could have made a difference, the absence of it is critical

For Lion Air, he remedied the symptom; he realized there was some sort of periodic trim runaway, and guided them to remedy it with Stab Trim cutout without having any intrinsic understanding of what the cause or underlying principle of the runaway was.

Big difference. Still didn't do anything for the fellows on the next flight.

Re:2. The article says the alert will only come once the aircraft is above 400ft. Originally Boeing told pilots it would activate on the ground.

True, it does say that... Why would that be the case, though?

At any rate, having had that AoA disagree warning (limited to above 400 ft) would not have saved these planes, it seems to me. Having had an AoA disagree warning on the ground might have.

It's because you have to have moving air for an AoA sensor to work. Unmatched sensors when there isn't enough air movement for proper operation doesn't mean anything.

But the air certainly moves fast enough at the moment of taking off, and if the sensors are sensibly sensitive, they should register AoA well before V_1.

EDIT to add:

Hm, Boeing states:

"During rotation, pitch angle is the critical parameter that ensures tail clearance. Once the airplane is airborne and at a sufficient altitude where ground effect and crosswinds do not affect the sensor reading, AOA will provide valid information."

https://www.boeing.com/commercial/aeromagazine/aero_12/attac...

Looking at the FDR for Ethiopia [1], it looks like the AOA sensors disagreed 2 seconds before they left the runway. The sensor disagreement happens after 05:38.43, they started climbing at 05:38:45 and were at 1000ft at 05:39:06 ish if I am reading the PDF image correctly.

[1]http://www.ecaa.gov.et/documents/20435/0/Preliminary+Report+...

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