[danjayh]

You clearly have absolutely no background in aerospace engineering. The 737 airframe is actually relatively stable compared to many other things that fly ... for instance, many high-performance jets will literally begin to oscillate and tear themselves apart without help from their flight computers. The flight computer is a part of the airframe, and it is perfectly valid for the flight computer to contribute to the airframe's handling characteristics. Boeing's mistake was in underestimating the burden placed on the pilots in a runaway trim situation, and assuming that pilots could execute a recovery procedure correctly that remained unchanged from prior iterations but that had also been rarely needed on prior iterations (vs. the MAX, where MCAS failures were common enough that an average pilot might actually need to execute the procedure).

[bobowzki]

People like to bring up the instability of high performance jets, but I'd like to point out that they generally have ejection seats and don't carry passengers.

[varjag]

They also crash incredibly often by civil aviation standards.

[danjayh]

Airbus jets carry passengers and have been fly-by-wire for decades. Cables and hydraulics have been replaced with software and computers. Like it or not, the software and hardware that make up the flight computer are an integral part of the airframe on most newly produced passenger jets, regardless of who makes them.

[VBprogrammer]

It's an interesting point. One of the reversion modes on Airbus is "direct law". Under that control mode the stick displacement is proportional to the control movements.

If, hypothetically, an Airbus had a problem in the same region of flight would it even be detected in flight testing? As far as I'm aware it wouldn't be as direct mode is a reversionary mode intended to get the aircraft safely on the ground.

[ethbro]

One of the things that is regulated is stick control forces.

They cannot be greater than a set level (essentially, an average strength pilot), particularly when performing critical manuevers.

This was actually the whole reason MCAS was engineered in the first place: to lower the effective stick force required to within the acceptable limits in certain scenarios.

So presumably Airbus in direct mode could still have similar issues flagged in direct mode, if the plane behaved in such a way as to require unacceptably high stick force to move control elements (even if it was a 1:1 mapping).

[VBprogrammer]

Technically I'd say they introduced MCAS to increase the control force in certain scenarios (high power high angle of attack). But yes I mostly agree with you.

One thing I'm not sure about is whether Airbus would have to demonstrate proper controllability (i.e. adherence to control force regulations) in all phases of flight and corner conditions.

You could have a scenario where by they have a complete control reversal on the approach to stall but under normal law the pilot would be oblivious. This would obviously show up in direct law.

[ethbro]

I'm tracking what you're saying now.

I'd hope they would have to demonstrate both things: (1) that a given mode can or cannot be active in a certain scenario, and (2) how the plane behaves in all tested scenarios under all modes potentially active.

Is there a reason this necessarily wouldn't be the case?

[XorNot]

Fly-by-wire is not about airframe stability. High-performance jets are fly-by-wire because high-performance generally means you need to build an unstable airframe, and computer control is the only way to compensate for it.

But that's not commerical aviation - that's combat aviation. That's "you fly faster or the missile catches you and you die anyway" aviation.

Building intentionally unstable airframes for civilian aviation is a very different proposition. And unrelated to the use of fly-by-wire.

[mehrdadn]

How is

> Airbus jets carry passengers and have been fly-by-wire for decades

a response to

> instability of high performance jets

?

[tigershark]

Your comment is absolutely deceptive. Fly by wire doesn’t mean that airbus planes are as unstable as jet fighters.

[windexh8er]

> The 737 airframe is actually relatively stable compared to many other things that fly.

I agree, so where is your comparison of 737 Max and non-Max airframe? Because we have real data, including death toll, that shows one airframe is not like the other.

> Boeing's mistake was in underestimating the burden placed on the pilots in a runaway trim situation, and assuming that pilots could execute a recovery procedure correctly that remained unchanged from prior iterations but that had also been rarely needed on prior iterations (vs. the MAX, where MCAS failures were common enough that an average pilot might actually need to execute the procedure).

This is not the whole truth as we know in this situation MCAS 1.0 was a critical flight system with a non-redundant data source. The "burden" placed on pilots was that the system did not have sufficient and trustworthy information to prevent a faulty and inadequately designed system (MCAS) from crashing the airplane. Furthermore since MCAS is not required, again, on non-Max airframe it seems your assertion that you have background in aerospace engineering is egregious because you should understand all 737 Max are currently grounded because the 737 Max is not a 737 airframe and should be recertified as such.

[yardie]

If passenger planes had the failure rate of high performance jets we’d all be using boats and trains. The fact that high performance jets have ejection seats while passenger planes do not should clue you in that failure is anticipated.

[ulfw]

Please list all passenger airframes that would “tear themselves apart”. Should be easy for you to do considering your background in aerospace engineering.

[xchaotic]

High speed jet fighters flying above Mach speed doesn’t really compare with a chubby airliner flying 900km/h?

[tus88]

> The 737 airframe is actually relatively stable compared to many other things that fly

Nice strawman. We are talking about the MAX not the 737 in general.

Why don't you explain why previous variants of the 737 didn't require MCAS?

[VBprogrammer]

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.

[tus88]

For the same reasons it has control surfaces at all. Planes need to adjust their aerodynamic properties in order to adapt to various external and intrinsic factors. Trim in particular maintains aerodynamic stability in the presence of some conflux of the two.

The trim equivalent of MCAS would be some elevator trim that ensures the excess AOA potential of the MAX could not occur, if set correctly. That is how a stable aircraft behaves. MCAS is a computer override to compensate for a condition that should not really occur in the first place.

[bwilli123]

..."relatively stable compared to many other things that fly"...

Balloons, paper jets...