I haven't checked but I'd bet STS is speed trim system (Aka Mach trim). Interesting that he came up with a reasonable mental model of what was wrong even though Boeing hadn't given them any details.
Oh yeah, that was interesting. They were renaming it to be part of the Speed Trim System, with the MCAS name used only internally. After doing some looking, apparently the STS is a system on all 737s that adjusts the stabilizer in order to increase the stick force. This is necessary to ensure the plane meets the FAR speed stability requirement for the minimum increase in stick force as the plane moves away from the current speed. It does this by moving the stabilizer in a way that opposes the flight stick input, and is active 5 seconds after the last manual trim input when autopilot is off. If you've been following the discussion of MCAS, all of those details are probably feeling awfully familiar...
Except that it doesn't manifest like a classic Stabilizer Trim Runaway.
The classic Stabilizer Trim Runaway is a continuous uncommanded actuation of the trim mechanism on a particular direction.
Catastrophic MCAS failure manifests as a series of discrete, amplifying activations, which are far easier for a pilot to dismiss as the normal operation of the Auto-Trim, and speed trim systems.
The is is a manifestly different pattern to look for, and was only caught by one air crew (on the penultimate flight of the Lion Air aircraft) by the assistance of a third, uninvolved pilot along for the ride.
I understand how one could see Boeing's internal test pilot as being reasonable, but keep in mind who signs his paycheck, and future prospects for him if the company gets found liable.
Perspective, and understanding of what different parties have at stake is essential in evaluating what is actually being communicated, and why. Words have long tails in the aftermath of a crisis like this.
I'm not sure why you've edited your response rather than responding.
It's a minor system. There is no firm evidence yet that it was responsible for this crash other than some ADS-B data which only really shows that the aircraft was having control difficulties.
Pilots are trained not to get near the stall. Unless something has gone seriously wrong an Airliner will never go near that region of flight in normal service. Most airliners have stall behaviour which is likely to enter a spin or otherwise difficult to recover from, that is why most are fitted with stick pushers which prevent a stall ever occurring.
There are lots of other systems which can control trim on an airliner (the Mach trim system, the auto autopilot etc). Each of these have failure modes that can lead to a runaway trim situation.
The MCAS system is trivial. It may have a flaw which led to both of these accidents but it isn't the world is falling and Boeing tried to patch it with different software.
The discovery that there's a gulf between what the Boeing engineers (many of whom are pilots) and the FAA previously thought of as runaway trim, uncommanded trim movement, and what they now know about this behavior not being recognized, means that many systems risk factors will get reclassified and thusly will have to be redesigned to be more robust. In this case, one of the main modes for the electric trim running away has been shorts in that system's wiring harness, so it now has to be redesigned because Boeing can no longer rely on the pilots recognizing it.
Yeah I don't disagree. I don't fly, but have sort of been following this out of curiosity. One comment I've seen from some pilots is basically "ehh, runaway trim isn't a new thing, we train for it in the sim, and there's a standard way to deal with it (disengage the automatic system and trim manually)."
So perhaps Boeing felt that this didn't really change anything in that regard. They seem to have been proven wrong.
In theory, nothing here is obscure. Trim is a control that airplanes have that cars don't. It's a very basic control, it's right there on the control column. A mechanical or computer failure could cause the plane to mistrim itself, so one of the most basic emergency responses that every airliner pilot knows is how to disable the auto-trim and fix it manually. MCAS is a new way for mistrim to happen, but there were already many others before.
It sounds like the main issue is that Boeing didn't predict that MCAS would mistrim at such low altitude that you don't have time to disable the trim motor and perform the recovery before the ground intervenes.
> You and other keeps parroting this line, I don't believe it to be true.
You are free to believe anything you want to believe. Of course if you where involved in the 737 Max development and you have inside information to the contrary then I'll accept that.
It's the explanation which best tallies with the official description of the system, the circumstances surrounding its introduction and the certification requirements.
Many aircraft have systems implemented to prevent stalls (see stick pusher); and systems to prevent undamped yawing movements becoming uncontrollable (see yaw damper). And to prevent mach tuck (see mach trim system); and to prevent changes in pitch trim due to speed changes (see Speed trim system).
I'll need to check for relevant sources, but I'm fairly sure that the idea that boeing removed the "trim brake" is wrong. It's simply that the MCAS doesn't respect it. I think this is consistent with the Speed trim system which may be trimming opposite the pilot in normal operations.
Why don't you explain why previous 737 versions have a Mach trim to prevent Mach tuck, or why it had a speed trim system to control the pitch force response to speed changes.
> The pilots did not diagnose it as stab trim runaway because it's not stab trim runaway
Runaway stab trim is uncommanded movement of the trim. MCAS failure exhibits as exactly this. The most likely cause of uncommanded movement is an electrical fault of some sort. Electrical faults are often intermittent. When you don't know why an intermittent fault is happening, you turn it off. This is not rocket science. In the first LA crash, the crew restored normal trim 25 times. The uncommanded trim happened twenty five times. In what universe is this not runaway trim?
From what I've seen the definition you're using is one invented after the fact by parsing word meanings like a lawyer. If it's your butt in the pilot's seat, I recommend thinking like a pilot, not like a lawyer.
From your link:
"in at least one simulator session, Boeing pilots took more than 10 sec. to react to a runaway stabilizer"
First off, they don't seem confused about what a "runaway" is. Secondly, 10 seconds was plenty of time, as in both incidences the crew battled it for several minutes.
"Aerodynamic loads on the mis-trimmed aircraft made the trim wheel hard to turn"
The loads were too high because the pilots were at full throttle and ignored the overspeed warning horn.
> It's very easy as a pilot to say that you would do a better job and something like this would never happen to you.
And in the first incident, that's what the crew did. They just turned it off and continued to their destination.
> the wheels going clack clack is a normal thing at that phase of flight
When the wheels are clack clacking and the nose suddenly pitches sharply down, there's the clue that the stab trim is running away.
> The earth is pockmarked with the smoking holes of these people's airplanes
It sure is, and that's why airline pilots are supposed to know things like what the switches in the cockpit are for and what stabilizer trim runaway is and read and understand all Emergency Airworthiness Directives. Flying an airplane is not a joke and you'd better pay attention in training. My dad flew 23 years in the AF, including combat) and anything less than 100% proficiency was unacceptable.
Yes, from everything that I've researched about this (not working in this industry anymore), MCAS was to adjust the feel of the controls when at the edge of the flight envelope. The plane, during normal flight, would behave very similarly to existing 737s.
What I read that shocked me was that Boeing was relying on a single angle of attack sensor at a time, with software rules that should have never flown. Such as: not tossing out obviously impossible angle of attack readings, doubling down on nose-down stall corrections, and not limiting MCAS to an input that ensured pilot controllability if the electric trim had to be disabled. (this may not have been possible, actually, in certain flight regimes)
The system pushed the plane into a regime where, from everything I have read, the aerodynamic forces were strong enough that manual pilot trimming (via a physical wheel near the control column) was not possible for a normal human, especially while the pilots were experiencing negative G's from such a strong nose-down trim. The flight log information I could find showed a very difficult cockpit situation. Enabling electric trim (and MCAS) shoved the nose downward, but also allowed some level of trim correction by the pilots that they could not achieve with the trim wheel. Stick forces were also enormous on the elevator, having to pull back over 50lbs if my memory serves.
Imagine yanking back with all your might while fiddling with secondary controls that also required enormous force to move, while the plane is forcing you up and out of your seat, against the seatbelt. This is where any cockpit communication issues between the two pilots would be severely complicating.
That Boeing released this with such flaws, and the FAA accepted it, seems quite damning to Boeing and the FAA. This is something that I can imagine technical leadership and management at Boeing would know about and should have caught and stopped much earlier in the process.
No, it's not, and most of the regulatory agencies around the world agree. Pilots that Boeing handpicked to test out MCAS fixes (and thus knew what it was, what to do, and expected a problem) agree. And that whole runaway stab trim checklist is a red herring especially after Boeing has come out and said MCAS is a part of STS — itself a system that trims the stabilizer in a counterintuitive manner on the NG. The stabilizer operates in a counterintuitive manner on the NG and MCAS runaway doesn't meet Boeing's definition of runaway trim. Full. Stop.
Those involved in the testing hadn’t fully understood just how powerful the system was until they flew the plane on a 737 Max simulator, according to the two people.
> had some serious design problems that they had tried to fix in software
The problem was not the aerodynamic design. It was the software that granted too much authority to the MCAS system and the MCAS system relying on a single sensor.
Both those problems have been fixed.
A further problem was two crews not remembering how to deal with runaway stabilizer trim (it's supposed to be a "memory item").
Generally what is in the popular press about the MAX is wrong.
> Boeing has a point about MCAS issues manifesting exactly like another well-known event in other models and having the exact same solution.
No, they don't, because (a) the MCAS issue does not manifest exactly the same as another well-known event (runaway trim), and (b) the solution to the MCAS issue is not the same as the solution to runaway trim.
Re (a), normal runaway trim on a 737 manifests as a continuous automatic adjustment of trim. The trim starts moving and doesn't stop until the pilot disables the automatic trim system entirely.
The MCAS issue, however, manifests as an intermittent automatic adjustment of trim, at times that seem completely random to the pilot. It is not at all the same as normal runaway trim.
Re (b), the procedure for normal runaway trim on previous 737s is to shut off the automatic trim system. You can then use the manual electric trim system to put the trim back where it belongs. The critical fact here is that shutting off the automatic trim system does not shut off the manual electric trim system.
But on the 737 MAX, shutting off the automatic trim system, which is what you need to do to disable MCAS, also shuts off the manual electric trim system. So now the only way to get the trim back where it is supposed to be is to use the mechanical trim wheel; and MCAS has enough control authority to put the trim in a place where it is impossible for the pilot to exert enough mechanical force on the trim wheel to move the trim back to where it belongs. So the normal runaway trim response procedure does not work for an MCAS failure.
There is a way to recover from an MCAS failure, which is to wait until you are in between intermittent MCAS adjustments of the trim, use the manual electric trim system to put the trim where it is supposed to be, and then shut off the automatic trim system (which, as noted, also disables the manual electric trim system in the 737 MAX). Then you have MCAS disabled and you have the trim in a place where you can use the mechanical trim wheel if needed. But that procedure is not the standard procedure that 737 pilots are trained to do.
The entire concept is nuts. You can't fix a primary control characteristic with a slow trim system driven by a screw jack. If Boeing wants to fix their wonky flight characteristic with software then great, but they actually have to do that.
Everything I wrote is true. The LA crew restored normal trim 25 times, but never thought to turn off the stab trim system. The trim cutoff switch is right there on the center console within easy reach for just that purpose.
The EA crew oversped the airplane (you can hear the overspeed warning horn on the CVR) and did nothing to correct it. This made things worse. They were also given an Emergency Airworthiness Directive which said to restore normal trim switches, then turn off the trim system. They did not.
That's it.
I'd say half the fault was Boeing's, the other half the flight crews'.
The MCAS is not a bad concept, note that MCAS is still there in the MAX.
Pilots are a brotherhood, and they don't care to criticize other pilots in public. But they will in private.
I'll also point this out "Unlike the EFS system, MCAS can make huge nose down trim changes."
That's a potential reason why an unaware pilot wouldn't disengage the auto trim system. Because he wouldn't think the trim system as being capable of causing an extreme pitch down.
Also this response.
> Interesting. So how did Boeing get a plane without redundancy in a critical sub system approved?
Answered own question, by hiding it in what is normally a non-critical subsystem.
> They should have been able to get back to a neutral stabilizer position, disabled electronic control, and manually trim from there.
Yes. But that's not what the checklist in Boeing's MCAS bulletin said.
It said, disable the electric trim motor then trim manually.
In hindsight, prehaps it should have said "return stabilizer to neutral then disable electric trim motor".
I wonder if Boeing did that deliberately, because a bulletin with a modified checklist would have undercut Boeing's implied narrative that a procedure already existed and it was the Lion Air pilots fault for not using the existing runaway stab trim checklist. Or did Boeing simply not test the procedure in a simulator?
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