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Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

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Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#101

Post by jcoby »

petilon wrote: post #88
ams6110 wrote: post #81

Earlier quoted context omitted.

MCAS only operates at high angles of attack, higher than normal flight would encounter. The reason it was even a factor in these crashes was the bad AoA sensor readings. If the AoA data had been correct, MCAS would not have been changing the trim.

When an aircraft pitches up, it increases the angle of attack, right? (If not MCAS would not be a fix for the pitching up problem.) And the pitching up can happen during normal flights, correct?

> When an aircraft pitches up, it increases the angle of attack, right?

Yes, but not what you may be thinking. The AoA is the angle of the wing vs relative wind (the angle the wing is attacking the air). You can have high relative pitch (attitude) to the horizon but a low AoA, such as during climb. That same attitude is a full stall at slower speeds/power.

The wing doesn't care where the nose points. All it cares about is its relative angle to the wind. Once it diverges past a critical angle the wing stalls.

What appears to have happened with these MCAS issues is that the MCAS senses that the AoA is too high when it's completely normal and safe, so it auto-trims down.

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#102

Post by jboles »

petilon wrote: post #88
ams6110 wrote: post #81

Earlier quoted context omitted.

MCAS only operates at high angles of attack, higher than normal flight would encounter. The reason it was even a factor in these crashes was the bad AoA sensor readings. If the AoA data had been correct, MCAS would not have been changing the trim.

When an aircraft pitches up, it increases the angle of attack, right? (If not MCAS would not be a fix for the pitching up problem.) And the pitching up can happen during normal flights, correct?

They can be related but not necessarily. Pitch is where the nose is pointed, while angle of attack is the angle at which the passing air hits the wings.

The nose can be pointed up with a low angle of attack (e.g. a climb during cruise) and conversely the aircraft can be pitched down with a high angle of attack (e.g. descent approach with flaps extended)

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#103

Post by salawat »

MockObject wrote: post #99
phkahler wrote: post #28

Earlier quoted context omitted.

>> This is something almost everyone is missing: A normal flight should not encounter this. Nobody said there was anything abnormal about the two flights that crashed. It really doesn't matter since the root problem is the plane doing things without telling anyone and then doing it wrongly. Without MCAS these accidents would not have happened.

> Without MCAS these accidents would not have happened. Without MCAS, wouldn't there be accidents caused by the nonlinear pitch though?

No, because the air frame would not have been certified for airworthyness.

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#104

Post by sokoloff »

chopin wrote: post #46
dba7dba wrote: post #23

Earlier quoted context omitted.

Main stream media made it sound like 40 seconds is way too short of a time to troubleshoot the issue. But now that you state it that way, I gotta agree with you it's not Only Boeings fault. Sure they are big significant part of the issue. But not the only part.

40s is only enough if you know what to do right away from the start of the situation. A failing sensor may confront the pilots with many (unrelated to a trim runaway) warnings (like unreliable airspeed, stick shaker, alt disagree). That has been the case for Lion Air at least.

Take a car analogy. Suppose your car starts drifting into oncoming traffic. Maybe the road crown changed, maybe your alignment changed from a debris strike, maybe you have a tire going down, maybe you have a crosswind.

In all those cases, your reaction is the same and instinctive regardless of whether or not you know the root cause.

Flying an airplane manually on instruments, the pilot sees the airplane undershooting the pitch target and pulls back on the control column every bit as naturally as the car driver above. When the pitch target is correct and the control force high, use the stab trim to take the force away. This is every bit as much a continual process as the driver lane keeping.

If MCAS is trimming against you every time you let go of the trim, you get feedback that this is likely a trim issue. Keep flying the airplane and trimming as needed.

This is not a case where the airplane automation fails and an airplane suddenly gets “dumped” onto a pilot who has to regain control and figure things out.

It’s obvious that the time and space available was not enough for that Lion Air or that Ethiopian Air flight, so you have real world evidence to support your point. I acknowledge that fact, of course.

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#105

Post by pdonis »

zaxomi wrote: post #22
sergiosgc wrote: post #14

Earlier quoted context omitted.

Worse. Why don't they do it with three sensors and a voting system? By using two sensors, when they disagree, MCAS will have to be disabled. It seems the plane is unstable without MCAS, due to the forward positioning of the engines, requiring specific training and abilities from pilots.

No, it is not unstable without MCAS. MCAS is only designed to change the trim if the airplane is hand flown in flaps-retracted, low-speed, nose-up flight. This is something almost everyone is missing: A normal flight should not encounter this.

> A normal flight should not encounter this.

Plenty of normal flights fly manually for a significant portion of the climb-out after takeoff, which is exactly the portion of the flight when MCAS issues happened for the Lion Air and Ethiopian Air flights.

Also, you will have higher AoA in a bank, which can be encountered in manual flight on normal flights.

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#106

Post by pdonis »

ams6110 wrote: post #81
petilon wrote: post #78

Earlier quoted context omitted.

>> A normal flight should not encounter this. What is your source? This is not what I have been reading. What I have been reading is that the MAX has a bigger engine and this engine has been positioned forward on the wing in order to preserve ground clearance. As a result of this unusual engine position, the MAX has a tendency to pitch up during acceleration. Nowhere did I read that this doesn't happen during normal…

MCAS only operates at high angles of attack, higher than normal flight would encounter. The reason it was even a factor in these crashes was the bad AoA sensor readings. If the AoA data had been correct, MCAS would not have been changing the trim.

> MCAS only operates at high angles of attack, higher than normal flight would encounter.

This is true for climb and cruise, but not necessarily true for a bank.

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#107

Post by sergiosgc »

zaxomi wrote: post #22
sergiosgc wrote: post #14

Earlier quoted context omitted.

Worse. Why don't they do it with three sensors and a voting system? By using two sensors, when they disagree, MCAS will have to be disabled. It seems the plane is unstable without MCAS, due to the forward positioning of the engines, requiring specific training and abilities from pilots.

No, it is not unstable without MCAS. MCAS is only designed to change the trim if the airplane is hand flown in flaps-retracted, low-speed, nose-up flight. This is something almost everyone is missing: A normal flight should not encounter this.

As far as I understand, the forward positioning of the engines makes it so that the plane tends to pitch up when it is already pitching up, and tends to pitch down when it is already pitching down. This is the definition of an unstable system.

MCAS compensates this by trimming elevators. Without MCAS, it is up to the pilot to handle the unstable system.

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#108

Post by SilasX »

Svip wrote: post #19
isolli wrote: post #13

Earlier quoted context omitted.

Related question: how does regulatory approval of planes work, worldwide? I read a few times that the FAA had approved the new plane too quickly, but don't other regulators have their word?

FAA have (or had) such a good record, that other regulators, like EASA, simply trusted a FAA certification. In the same way, FAA trusts an EASA certification for an Airbus plane.

Well, they're working hard to change that, I guess...

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#109

Post by bobowzki »

wp381640 wrote: post #98
bobowzki wrote: post #95

Earlier quoted context omitted.

While it's true that fighter jets have been aerodynamically unstable for decades, they're not built for transportation and have ejection seats. The main reason they are unstable is to increase agility. Is this necessary or even wanted in passenger aviation? I don't know but I'd guess not. Probably safety, comfort and fuel economy are far more important and can (should?) be achieved with a stable airframe.

fighter jets are the more extreme and established example, it also applies to most modern airliners which have software systems in place to prevent human controls from exceeding the aircraft flight envelope fuel efficiency in these modern aircraft has been gained with swept back wings, larger intake engines etc. which with direct control and no software would be almost impossible to keep flying discarding all of thes…

Swept back wings and larger intake/bypass engines does not make the airframe inherently unstable.

The fly-by-wire system has multiple redundancies and layered protection, including direct law (at least for Airbus). Fly by wire is, like you say, of course a great innovation for improved control and safety, but it's nice when the plane continues flying even during a failure, however unlikely.

I guess in the case of MCAS the software was activity working against the pilots so maybe it's more of a problem with the design of this particular system and training.

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#110

Post by oflannabhra »

wp381640 wrote: post #96
just_steve_h wrote: post #92

Here's the design decision that puzzles me the most: MCAS commands nose-down trim. The pilot uses the trim button on the yoke to command nose-up trim. Why isn't that enough to disable MCAS and cause an audible alert? Think of an analogy: when the autopilot commands a nose-down elevator input, and the pilot pulls back on the control column, that's enough to disengage the autopilot. Or think of cruise control in a car:…

> Why isn't that enough to disable MCAS From what I understand it did but it would reengage again. It's the reason why the Lion Air flight lasted longer in the air than Ethiopian, because the pilots in that case kept hitting the trim and buying themselves 40-60 seconds each time

Yep, and on top of that the 737 MAX did not require retraining for pilots (cost savings for Boeing and airlines) and Boeing did not provide documentation to pilots that MCAS would re-engage after a short timeout.

Pilots use the term "ahead of the plane" to describe being in a state where they understand exactly how a plane and its systems will respond to inputs (including theirs). MCAS (and lack of training) resulted in pilots being "behind the plane", which meant they could not regain control over the plane and ultimately crashed.

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