Originally Posted by
ecto1
I think there is a reasonable consensus (lest's say 50% chances) that vane shaft broke, keeping AOA vane attached or not, and subsequent alarms and checklists (both useful and useless) threw the crew out of balance. They didn't manage airspeed and lost ability to trim. They reengaged electric trim, was not strong enough either, forgot to CUTOUT, mcas trimmed down, EOF.
I think one of the biggest problems here is the compounding of stupidities. Each stupidity on its own is very much survivable, but all of them is a huge mess.
0. Of course MCAS MUST NOT BE operative with AOA disagree. Minimal software mod.
1. one AOA clearly fails, why not use a switch to transfer everything to the other (manually or automatically). It's a 3 way switch (AOA input L/NORM/R). In the event of stick shaker on, AOA disagree, check if any AOA is stupid (75 is quite stupid), switch to the other side, no more alarms in the cabin, crisis over in 10s tops. minimal wiring loom mod.
Even if you don't do it:
2. We have now perfect data about the influence of AOA over airspeed. 30 knots tops over the full AOA range and airspeed. Probably 15 knots 0 to 15 degrees 0 to 300 knts, probably less than 5 knots in the really tricky areas (slow). Upon AOA disagree, both airspeeds should use a default AOA value (4 deg maybe) instead of throwing UAS. and offer a reading with a possible +-7 knot deviation. But keep autothrottle and autopilot, maybe a caution message (airspeed calculation inacurate, stay 20 knots away from limits). Not a really disturbing unreliable airspeed, just because of a few knots. Minimal software mod.
3. Same with altitude. (altitude calculation inaccurate, stay 1000 feet clear from limits). Minimal software mod.
So that the only remaining alarm would be a stick shaker plus AOA disagree, and you still have autopilots. Much, much easier to handle. But if this is still enough for you to have the aircraft out of trim and miss speed management,
4. If speed goes over 280, message: reduce speed to regain trim ability). Minimal software mod.
My point is: most probably ANY of those mods would have saved the day, and all of them are pretty evident.
To me the problem is simply a huge lack of effort at design level to 1) Imagine 2) prepare for failures.
Chances are that the very same secuence of events, without MCAS final strike, has happened more than once before in other 737 variants (AOA fails, unreliable air data, stick shaker, big confusion, lack of proper aviation and or navigation and possibly all the way to overspeed and uneffective trim). Only without MCAS the aircraft would have been more or less in trim and therefore not nosediving and making the news.
Completely agree, great post, I have been thinkig along the same lines since the first crash, this one only reinforces that.
0) I think Boeing is working on that belatedly.
1). I flew the F50, and you could silence all/most alarms with a dedicated switch on one panel. I have read several accident reports that made me wonder if the crew would have done better without distracting and conflicting (stick shaker and over-speed simultaneously on Aeroperu flight PL603) error messages. Having a switch to deselect a system that gives an erroneous indication is an established concept in certifying aircraft (, and I think it should be a mandatory re-fit to all B737). Having one stick shaker go of on the failed AOA side with no other way to switch it off than going through the CB-panel isn't good enough.
2) + 3) Seems sensible, there might be areas in the envelope where this wouldn't work, so I can't positively endorse without more info.
4) Yes, UAS/AOA failure, max speed should be kept lower, so advise the pilots to lower their speed by
reducing power is good, however, just lowering the limit so the clacker goes of brings me back to item 1)
Definitely Boeing needs to:
-Limit MCAS authority, and preferably by adding aerodynamic fixes like tail strakes.
-Disable MCAS in the case of AOA disagree.
-Install the AOA indicator and AOA-disagree indicator, preferably better than the current way.
-Install an AOA left/norm/right switch that limits error messages/stall warnings for those cases where it is clear to the crew which one is wrong.
-Bring back the option to disable auto trim (AP, STS ,MCAS, LAM), without losing the thumb-trim.
-Improve the NNC, to differentiate between MCAS and "regular" runaway trim.
-Verify it is possible to trim the aircraft back to in-trim from a runaway trim, aircraft full AND situation at low altitude, and see if there is any speed restrictions required for this, for all B737.
Obviously al of this should be paid by Boeing.
Maybe not all are required, but at this point I think Boeing better go the extra mile to make sure this doesn't happen again.
Also I do believe there should be more required training for pilots and more push in the SOPs to manually handle the aircraft, so people will not try to switch on the AP to resolve a problem requiring manual flight.