jmastron

Isn't one of the key differences that a bad AOA sensor causing an MCAS trim down would also be showing the pilots erroneous AOA information, making it harder/take longer to diagnose, whereas a "normal" runaway stabilizer is going to immediately show as "pitching down" on a correct AOA instrument?

I don't know all of the instruments presented to the pilots, I do know Southwest has added an additional AOA display (and sensor?) option to their new 737MAX deliveries. To this layman, it sounds like at least that shouldn't be an "option", and should be retrofitted in to the current planes.

justatourist

I have some questions for those in the known including the 737 driver above:

- Why so quickly to point fingers at the crew? These "third world countries" and "third rate airlines" and "poorly trained crews" have been flying 737 NGs and A320s for the past decades without any significant accidents, especially during take off. What changed if the 737 MAX is supposed to be equal to the previous generation? (Note i did not use similar because the certification implies the previous generation and the MAX are the same behaviour-wise).

- Everyone knows by now the trim runaway procedure. But do pilots have the time and mindset to even think about it when they are in the take off phase, with incorrect air speed, stall warnings and stick shaker? And if you happen to reduce flaps and not engage autopilot you just add the MCAS to this whole mess.

- Regarding the LionAir accident, it is true and confirmed LionAir has some dubious maintenance practices, including crews not reporting problems with aircraft (as it was shown by the previous crew not reporting some of the issues before the flight that crashed). But as it is known, a single sensor malfunction caused chaos in the cockpit with incorrect air speed and stall warning AND also fed MCAS with incorrect data. Everyone blames LionAir's poor maintenance for the accident but one bird strike is enough to cause the same sensor issues (be it incorrect air speed or incorrect AoA) regardless of the airline or the part of the world you are flying in. And then you'll be in the same position as the LionAirs crew and faulty inputs being fed to MCAS and the system acting on it. Why is everyone overlooking this?

clubord

EWR764

I'm not sure if the Lion Air pilots were concerned with the AOA indicator (different from the sensor) and I don't know how the Lion Air primary flight displays were configured. The sensors were transmitting disagreeing data (20 degree variation) to the flight control computer, but the MCAS, available strictly with autopilot disengaged and flaps up, only took its AOA guidance from the malfunctioning sensor.

My understanding, from reading the report, is that the pilots were struggling with the the nose-down commands the aircraft was independently making, which were countering back pressure on the yoke and nose-up electric trim 'flicks' by the hand-flying pilot. This caused the MCAS to cycle on and off 25+ times, each time processing faulty data suggesting the airplane was flying into a stall and thus re-activating, which ultimately overcame the pilots' efforts to command a climb.

IMO, the biggest issue emerging with the MAX is that the pitch axis augmentation systems (in manual flight only) take data from just one of the two AOA sensors, limiting the fail-safe properties of other automated systems, like yaw damper, which can compare two feeds to diagnose a sensor disagreement. Southwest ordered its airplanes with a AOA sensor disagree annunciation, which is not required, but gives pilots insight and suggests causal factors if an uncommanded nose down pitch trim event is noted.

The forthcoming AD for the 737MAX will likely be a software update to change the architecture of the flight control computer to accept dual-channel AOA input in manual flight, along with a comparison function, for the automated pitch control augmentation systems. The augmentation will automatically disable and annunciate the fault to pilots in the event of an AOA sensor disagree. The capability is there: the autopilot is designed to accept inputs from both sensors and will fault if there is a disagree. The problem is, the AP will fault into manual mode, and manual mode clearly has a shorter progression of events to a catastrophic outcome...

saccoNY

wow. If that is true, that really makes me concerned that one singular faulty control could put an aircraft into an incorrect and unsafe situation.

EWR764

The starting point is a faulty AOA sensor, which, if undetected and not repaired, can lead to an improper response by a flight control safeguard system that mistakenly concludes the aircraft is approaching a hazardous state. That system is only actuated when other limited criteria are met. From that point, there must be a corresponding incorrect action (or inaction) by the crew to lead to a catastrophic outcome. In all likelihood, a cutout of the automatic stab trim at an earlier point in the Lion Air flight (a procedure for which the crew undoubtedly had training) would have resulted in a safe landing.

So, like anything else, there are multiple, independent factors that combine to create a disaster.

I will caution that my entire analysis is strictly in reference to the Lion Air crash. I know the Ethiopian case involves the same type, with some disturbing similarities, but the truth is the public has little to no objective evidence on which to draw a rational finding. It would therefore be incorrect (but perhaps natural) to impute this discussion to ET302 and further, to conclude that a grounding of the fleet is essential. That determination would be rooted in substantially less factual support, but it's gaining momentum nonetheless...

spin88

I discussed this last night with an AA A320 pilot (who has been flying the neo a lot) while waiting to pick up our kids after soccer practice, and he made the same point. Said that every pilot was well trained to deal with an engine failure/compressor stall at take off, so that would not be the root cause in all likelihood, and that something else would have had to come into play given the report the pilot expressed problems with control when he asked to return to the airport and made a turn to return.

Boeing yes, the 737 was launched in 1967 and carried 85 people on missions up to about 1500 miles. One of the issues with the MAX is that the larger engines on the MAX had to be further intergrated into the wing, rather than hanging below the wing. The desire of Boeing to be able to claim that no new training was needed (and anyone trained to fly a NG could just be shoved in the cockpit) caused to them to use fly by wire systems to try to make the MAX fly just like a NG, although the airplanes underlying flying characteristics (in particularly the nose up on the MAX) were different.

OTOH, Airbus - with the A320 being designed in the 80s - does not have the same magnitude of problems that Boeing has. First, the Aircraft is higher off the ground, so adding newer/larger engines did not cause problems with ground clearance, second, the base model (the A320) was two stretches size larger than the base model of the 737 (the 737-100 which seated 85). So the A321neo is a single stretch of the A320, while the MAX9 is three stretches of the original 737 frame, and the new "base" MAX8 is two stretches.

The AA pilot I talked to (who again is an airbus-jock) said that the AA 737 captains he was talking too were beginning to wonder about the fundamental compromises that Boeing appeared to have made and then "fixed" with a pasted over fly-by-wire system... His comment was that for Boeing, depending on what they found this would be (1) bad, (2) horrible, or (3) catastrophic. And the "bad" outcome would be finding that this crash was unrelated to the automated trim system, horrible would be it was related, and catastrophic would be that they could not safely just turn off the automated system and routinely fly planes without it.


+1. Lion Air is sketchy, ET is not. This was expressly mentioned to me by the AA pilot I was talking to last night. He said they were a good airline, and that no one was jumping to the conclusion that the pilot or maintenance should be suspected as being at fault.

yes, better late than never. Does this leave Drunk Dougie as the one outlier on allowing changes?

and p.s. As I said with the first reports, good for UA for allowing changes on the MAX9 w/o any prodding.

LarryJ

There are several systems which can, and frequently do, run the stab trim while you are hand-flying the airplane. This is normal. There is no way to tell from the cockpit which system is running the stab trim at any particular time. If the stab trim runs away then you accomplish the runaway stabilizer procedure. Autopilot (A/P) and Autothrust (A/T) off (MCAS wouldn't operate with A/P on), then, if it hasn't stopped, flip to stab trim cutout switches. That will stop all electric stabilizer trim movement.

MCAS doesn't operate while the A/P is on (or flaps extended). Minimum A/P engagement altitude is 800' AGL on takeoff but we generally handfly much longer than that.

There is only one AoA indication--the one you mention that is optional. If you are hand-flying and the nose is heavy you trim up. If one of several systems trims it back down again you trim it up again. After a couple of these cycles you'll realise that the trim is running away and complete the runaway stabilizer procedure. You don't sit there with a nose-heavy, and getting heavier, airplane and say, "the AoA looks high so the nose really does need to go down". You judge the aircraft's trim by whether the pitch is staying where you need it or if it is taking pressure to keep it there. The AoA indication, if you have it, doesn't help with that.

The Lion Air report isn't final but a good amount of information has been released. Judging by the currently released information, there was nothing wrong with the MCAS system on the accident airplane. The failure was in an AoA vane/sender which fed the MCAS, and other systems, bad data. Every transport jet has similar AoA vanes/senders.

Very little data out on the Ethiopian accident. The flightstats data, which is questionable due to poor coverage in the area, indicates that the flight never reached an altitude where the flaps would normally be retracted (field elevation is 7,657') and the problem started almost immediately. If that holds then this wasn't an MCAS event. The rapid changes in vertical speed almost immediately after liftoff, if correct, might indicate them trying to chase a bad airspeed indication either flying manually or on autoflight. There are three separate airspeed displays and you receive an "IAS DISAGREE" message if they differ by more than 4kts. The problems seem to have started before the flaps would have been retracted or the autopilot would have been engaged.

Realunited

Not trying to argue here but this claim is absurdly inaccurate about no accidents "These "third world countries" and "third rate airlines" and "poorly trained crews" have been flying 737 NGs and A320s for the past decades without any significant accidents"
Point 2 about not having time to react is also wrong and any pilot anywhere knows how to react instantly to an uncommanded pitch down (assuming that's even involved in the Ethiopian crash)
Point 3 is true that bad input data can happen but Lion air knew they had a problem with the sensor and did nothing.

Let me add why a 200hr first officer makes a difference. At least in the US, first officers must have at least 1500 hrs and a type rating in the aircraft which means they have been trained and tested (maneuvers and systems knowledge) to the same standards as a captain so their knowledge and skills are invaluable in an emergency situation. Some situations need 2 fully knowledgeable brains to lead to a successful outcome and 200hrs total time is slightly better than a passenger from 32F coming up to offer advise.

COSPILOT

And now you know why I trust you, but concerned about programming of the airplane. But you are one of a great many pilots in the United States. Still awkward if we are the last hold out.

spin88

I thought I would post this part from the Post's article on why China moved first:

“They have had difficulty making a decision, so we took the lead,” CAAC Deputy Director Li Jian told reporters in Beijing, referring to a lack of stronger FAA measures regarding the 737 Max. Li added that the plane’s software may encounter serious problems when coupled with unreliable sensor readings and suggested that this has been happening to Chinese pilots.“These kinds of situations have already happened many times,” Li said without giving more details, according to state media. China will resume 737 Max flights after it has received sufficient safety guarantees from Boeing, he added, but there was a risk now that Chinese pilots “didn’t dare to fly, and weren’t able to fly” the plane.

https://www.washingtonpost.com/world...=.5b2ad1f14174 This suggests a systemic problem. Unless CAAC is lying, this is a big problem. And I might add that at this point I have little faith in Boeing and even less in a politicized FAA.

ExplorerWannabe

Of course it does. Two incidents like this in such a short time, especially on a new aircraft, should give anyone pause. Pause, however, does not have to mean panic. The thing that greatly disturbs me so far is that Boeing's latest flight control software appears to override the pilots in certain circumstances and it sounds like that isn't limited to the -8. One of the reasons I have a distinct preference for flying on Boeing aircraft over Airbus is that I understood traditionally, Boeing built aircraft for pilots -- that is to say the autopilot and flight control software was designed to assist rather than override the pilot -- while Airbus designed their planes for the pilots to assist the autopilot/flight control software. I may have that information wrong but I detest the latter philosophy and want my aircraft commanded by pilots, not software.

Having said that, I would like the aircraft safety professionals to have a chance to dig in and obtain facts before I panic -- and hope others will do as well. If anything, I expect the increased attention over this will make pilots particularly attentive on the 737MAX going forward.

JimInOhio

Maybe I was only guessing but I was under the impression the ET FO had 200 hrs flying for ET, not 200 total hours flying. Has anyone said they think the FO was flying the aircraft?

blueman2

I am no expert by any means, but this is exactly what I have been thinking based on info so far. With multiple reports of debris coming from the plane before the crash, and the tracking data that shows the planes air speed and altitude shifting up and down at time intervals that roughly match MCAS interactions, it makes me think something bad happened which was made worse by the MCAS.

FlyngSvyr

No one is saying that the FO was flying the aircraft, but it usually takes the teamwork of both pilots to successfully navigate an emergency situation. I don't think anyone is denigrating the pilots in this situation, just pointing out that the it would have been better to have both pilots "seasoned", which might have led to a different outcome regardless if the airplanes control systems were a contributing cause.