Antonio,
The 737 saga goes much further than “just” the Max accidents. In the end, the Max accidents were the consequence of a generally unacceptable design flaw, which allows the trim to be frozen in a irrecoverable position. Yes, if you have enough altitude, you can perform a maneuver where you can get the trim out of there using manual trim but at low altitude, your chance is practically zero.
Now, why has that not mattered so far? Well, Boeing has not had any runaways in this situation, those which are recorded were recovered due to much higher onset altitude. And there was no system in place, which would initiate runaways as part of their job to keep the plane “safe”. Yet, it will remain to be seen if this has not been at least a contributing reason to some other crashes which have been attributed to pilot error. Such as the crashes at Kazan, Perm and Rostok. These were -500 and NG models.
The Max has had several massive design problems which, imho, might prevent it from ever getting recertified. MCAS was the attempt to correct this, but it disregarded the dangers of it employing it’s capabilities at low altitude.
- You got an airplane which is extremely sensitive to power changes due to the actually impossibly far forward position of the engines. This alone should have been a no-go due to the trim changes required by every power change.
- You then put a system in place to deal with the trim changes but don’t document it properly, there are people who say that even Boeing did not really realize all the ramifications this system would have.
- You base this system of the input of one sensor.
Now you get the situation that you have an airplane which inherently from Series one on has the problem that at high speed you can not move the trim manually as it’s frozen due to insufficient power available.
You add engines where power changes are not controllable without the excessive use of trim. So you rely on the very system that you know has a critical flaw.
The emergency procedure to stop a runaway (or rather a MCAS induced trim down) is to take off the electric trim altogether. Which, upwards of a certain speed, does not allow you to manually trim the airplane as the airloads freeze the trim.
So now you get a situation where during initial climb at flap retraction, you get an immediate trim down to full position. You can hold barely attitude with holding the control column fully aft. You trim back to normal with electrical trim, but MCAS immediately trims nose down again. By that time, speed has reached the limit of the manual trim system. The emergency procedure for that now is to switch electrical trim off altogether.
Now you have a plane which is in a pitch attitude which has to be held with enormous force to stop it from going into a nose dive, you can not trim manually and if you reduce engine power it will take the nose down significantly and you have nothing to counter that. In order to operate the manual trim, you need to unload the forces of the elevator, which means you have to let it nosedive. In that situation, the accident is a fact.
The Crews in both accidents were not aware that MCAS is causing this and consequently, just setting flaps would have inhibited MCAS. Because nobody told them that.
The first accident, they actually did that initially and regained control, then raised the flaps again and crashed. In the 2nd accident, they never got that far.
Combine that with what we know now about the certification issues (FAA vs Boeing) and all the other issues involved, including recent findings that the FAA employed inspectors not properly trained to certify pilots or assess pilot training on procedures and maneuvers, the FAA has not lost it’s credibility as a certifying agency it has upheld for almost a century. The blow is so massive, I wonder if the FAA will survive as a agency without massive changes. Also, it questions the FAA as the so far undisputed world leader in certification and rulemaking, where many claimed other countries would just have to copy the FAA or take what the FAA sais as biblical truth. That is gone for now.
IMHO anything done to the Max now to make it safer will be a fix on a badly designed airplane.Add to that, the FAA is in a position where they can not allow themselfs any other doubt. So what will happen is,that the FAA will do a CYA on this to an extent nobody has ever seen. Which in the end would mean, the Max can not regain certification as the changes would be such which are impossible to implement in the fleet. The fix proposed added a sensor, changed the authority of MCAS and some other stuff which generally would have made sense and likely gained approval, but in the political climate the FAA finds itself now, is not sufficient. The proper solution would be to correct the manual trim problem on ALL 737’s from the 100 series upwards, to correct the problems caused by the engine position on the Max and to inhibit MCAS below say 10’000 ft altogether which rises other problems.
Yes, in afterthought, there could have been other solutions to prevent a grounding, such as ordering Max operators to fly with flaps extended below 10’000 ft, such as to implement procedures known to stop the effect e.t.c. but I think the signal to Boeing was necessary. Otherwise we end up with more and more such blunders in design. It will not be enough from now on to take a 50 year old design and murx something to make it bigger.
And that is good.
I think the Airbus control law changes are a bit of a red herring. AIUI, an A320 in its most “fully degraded” control law state will still fly as well as any swept wing jet, and the normal control laws about envelope protection.
The 737-MAX on the other hand needs MCAS because in certain flight regimes the aircraft is inherently unstable due to the size and positioning of the engines – specifically, the tendency not merely to pitch up when power is applied, but to get into a positive feedback loop – the pitch up slows the plane and further increases the tendency to pitch up (basically, become divergent) to the point it cannot meet certification standards for stability (AIUI it’s not merely because the MAX wouldn’t fly enough like a classic B737 as Langewiesche said in his article, but because it wouldn’t meet stability requirements to be certified). This seems to be because Boeing has taken a very old design with parts dating back to the 1950s and “graunched” it to do what the 757 used to do (the MAX 10 which looks – like a 757 – a bit like a stick insect). Airbus hasn’t had to do this with the Neo because the A320 was designed from the outset for high bypass turbofans and so has much longer landing gear.
Perhaps they would have been better off dumping the 737 design altogether and coming up with a re-engined 757 “shrink” to replace the shorter 737s…
the aircraft is inherently unstable
I find that hard to believe, because it would not be human-flyable with electric trim switched off.
My take on this is that MCAS was put in to make it flyable in the stall regime. It was very badly implemented, and the 737 TR was badly mismanaged*, but it probably explains why no competent crews crashed them, or indeed had any (reported) issues with them.
All airliners need lots of trimming. You can see the trim wheel moving as the cabin crew wheel the food trolleys up and down the isle…
* One could also argue the airlines are complicit in this, by demanding the same TR for everything. Boeing gave them what they wanted. Everybody is playing the same game… the CJ type rating covers the CJ4 as well.
Peter wrote:
I find that hard to believe, because it would not be human-flyable with electric trim switched off.
I don’t think it inherently unstable across the “normal” part of the envelope. But is does NOT meet the stability requirements within the “required” envelope around the stall regime. i.e. without MCAS. AFAIK In the past aircraft with this problem have had a stick pusher, and the stick pusher becomes the “stall” rather than the stall itself, e.g. 727. This of course would be a change, requiring training etc etc.
IMHO this whole problem comes down to trying to pretend nothing has changed when of course it really has, rather than a engineering problem that is easily solved.
Peter wrote:
I find that hard to believe, because it would not be human-flyable with electric trim switched off.
That’s why they added MCAS. It becomes inherently unstable in pitch at low speeds but before the stall, with a pitch up positive feedback loop that would mean it can not be certified without MCAS providing automatic nose down trim to counter this. Not “inherently unstable” to the degree that (say) an F117 or even an F16 is unherently unstable, but with a highly undesirable positive feedback loop (in nearly any system that is supposed to be stable, whether financial systems, op-amp feedback networks, vehicle suspension – you want negative feedback loops and a positive feedback loop is a disaster)
Don’t forget helicopters are inherently unstable, and allegedly humans can fly them without electric trim :-)
Peter wrote:
because it would not be human-flyable with electric trim switched off
Yes, view it as take-off with GC in a well back position under full power and slow speeds, most pilots will tell you you will not be able to recover from stall or nose up attitudes if things go-fast but hey you get a different answer if you ask a low level C-programmer with some mechanical engineer
In theory, you can “dynamically” control the pitch up moment instability/divergence by going very really fast forward on the trim, obviously you need 3 human hands to do that and more than seat of your pants skills, if the algorithm blow-up then alternative is to cut power and take low Gs while resetting the trim in a flash moment, that also need 3 hands and good stomach 
Peter wrote:
I find that hard to believe, because it would not be human-flyable with electric trim switched off.
That is exactly the point, it isn’t in a large extent of the envelope because the manual trim blocks at higher speeds.
And yes, it is very unstable due to the location of the engines. Any power change will produce a massive pitch up or down force. Flying it by hand means constant electrical trim input, as with manual trim you are either too slow or it will block over a certain airspeed.
And as alioth and Ibra wrote, MCAS has to assist with the trim in lower airspeed regimes to help counter this effect, but apparently only does so with flaps retracted and AP off. Both of which would help in the case of MCAS misbehaving, but nobody knew that at the time.
So to make sure MCAS won’t try to kill you by making a rather large hole in the ground, you leave flaps extended up to a safe altitude and fly AP. But if the AP uncouples for some reason, you may get a full nose down trim at the same time.
Frankly, would either of you like to fly a GA plane which has only halfways such suicidal tendencies? Not me.
I find that hard to believe, because it would not be human-flyable with electric trim switched off.
A tail wheel aircraft is also unstable in yaw. Humans take off and land them all the time though.
LeSving wrote:
A tail wheel aircraft is also unstable in yaw
I think you can get away with GC vs wheel position than with GC vs elevator/wing
Just remember if you want to fly with GC well back to not to flare when landing
If you fly with GC well in front just don’t bounce
Peter wrote:
* One could also argue the airlines are complicit in this, by demanding the same TR for everything. Boeing gave them what they wanted. Everybody is playing the same game… the CJ type rating covers the CJ4 as well.
And this ^ I think is the root cause here. The airlines want to save on crew training and certifying the MAX as a different airplane would have meant another TR.
