From here
Antonio wrote:
Needless to say in neither of the accident aircraft the trim was disconnected before things got too far out of hand.
They did but found that they don’t have the force to move the trim wheels by hand, so they had to re-engage the electric trim. This is something else which makes me really uneasy about ALL 737’s and is why the Russian MAK at some stage actually considered withdrawing the airworthiness for them in Russia. The tragedy of these accidents really was that the information how the system works was not with the pilots.
The first Max which crashed briefly recovered after re-selecting flaps 1, as flaps deployed stops MCAS from “working”. Unfortunately, they did not simply reduce power, keep flaps and reland, which would have been entirely possible, but retracted the flaps again, causing MCAS to trim full nose down and crash.
The whole thing was a criminal neglect or even intent to tell pilots what they were dealing with. This combined with a trim system which IMHO should never ever have been certified in the first place, starting with the 737-100.
BTW, the same thing nearly happened with the SF50 jet as well, only there the uncommanded trim down could be overpowered by the pilot. Nevertheless scared the living daylights out of them.
And of course, the A330 incidents in Australia had a similar background too.
That’s not applicable to GA autopilots, however.
The servos can be overpowered by the pilot.
Certified autopilots are required to annunciate TRIM IN MOTION after x seconds, etc.
That’s not applicable to GA autopilots, however.
The servos can be overpowered by the pilot.
It wasn’t the autopilot servos they had to overpower. It was the aerodynamic loads on the stabiliser trim jackscrew using the manual handle on the trim wheels.
From the speculation I’ve seen on this, early 737 pilot operation handbooks contained a procedure for setting trim manually while in a severe out of trim condition, which basically involved letting go of the control pressure on the yoke while winding the trim back. I gather it was removed from later PoHs, presumably because convincing someone who’s plane was trying to fly itself into the ground to let go of the control force proved more difficult in practice than while sitting on an office chair.
The main takeaways for light GA are about knowing exactly how the systems with control authority on your aircraft work and how to disable them in a hurry e.g CWS button on the yoke, circuit breakers etc.
Peter wrote:
That’s not applicable to GA autopilots, however.
True in the sense that no such design like the 737’s trim has ever made it beyond certification bodies in a GA airplane.
Peter wrote:
The servos can be overpowered by the pilot.
The servos were not the problem, but the fact that in the 737, ALL MODELS, you can not rectify an out of trim situation using the hand wheels while there is aerodynamic load on the stabilizer and the trim jackscrew. You have to let go of the controls, then trim as fast as you can, stop the airplane from hitting the ground or pointing up in the sky and do it again and again until you reach a trim where the airplane will fly straight. The power you can exercise on the hand wheels is not sufficient.
What they were faced with is a system, which due to the single faulty AOA sensor insisted on trimming the airplane’s stabilizer to the forward stop every time power was applied to it at ferocious speed, combined with lack of elevator authority to counteract this and the inability of the manual trim system to rectify the situation near the ground.
The only way they could have saved the situation is to put the flaps lever to Flaps 1 position, which disables MCAS and then use the electric trim to straighten out the airplane. The first crew actually did that initially but then retracted the flaps again as they did not know that connection. The 2nd crew (Ethiopian) never did, they however disabled the trim to find they could not trim by hand and could also not let go of the yoke, as they would immediately hit the ground if they did, so they reactivated the electric trim and tried to trim up, but were counteracted every time by the MCAS until they hit the ground.
Apart from the Max Accidents, there is ample evidence that two more 737’s fell foul of the lack off manual trim possibility, both in Russia. One was a 500 series (which goes back to the 300/400 systems and are close to the classic 100/200 variants) and the other an NG. Both came to grief due to upsets, the -500 during a go around, the other due to bad weather. That is why the MAK originally wanted to ban the 737 in Russsia, but were overruled by the authorities, as the implications were too severe.
GA examples of similar problems included the Cirrus Jet, which had some incidents in the early exemplars, where also an uncontrolled trim down was encountered due to faulty AOA sensors, but which had sufficient control authority to counteract the trim.
In any event, the relevance for GA planes, or actually ANY plane equipped with electric trim, is that it is vital that you know how to really fast disable electric trim and auto trim if you have to. Depending on the situation, you can very quickly run out of height to correct a situation like a full nose down trim or stall in a full up trim before you have the chance to correct it.
in the 737, ALL MODELS, you can not rectify an out of trim situation using the hand wheels while there is aerodynamic load on the stabilizer and the trim jackscrew
That is true for all airliners. They have a large speed range and thus have a large trim range.
is that it is vital that you know how to really fast disable electric trim and auto trim if you have to
True. That is why one checks the trim position indicator, and autopilot=off, before takeoff.
This is nothing new.
There was a Cessna crash where it was speculated that as the PF moved the throttle lever fully forward, his knuckle engaged the AP, which wound the trim fully forward and crashed the plane.
Have you met people who can’t disconnect the mechanical steering lock in their cars? You know how it goes: the steering is left in a position which is pushing against the lock then you cannot turn the ignition key to disengage the lock until you turn the wheel to unload the force on the lock then you can disengage it…
That is the issue with manual trim on the 737. That is not unique but, pertinent to GA…just try and turn the pitch trim on a Mooney while strongly pulling or pushing on the yoke in flight and you’ll notice how much harder it is. Try and turn the rudder trim on a Cessna while steering on the ground in the opposite direction…It is a situation that just almost never happens.
You first have to unload forces on the jackscrew then it will turn easily.
By the time the MAX accident crews disconnected the trim things were too far out of hand to unload the stabilizer and manually trim. That is what I wrote and meant. They also tried engaging autopilot which as we all know won’t work on a severely out-of-trim aircraft.
MCAS was a bad design, yes.
However, and this is coming from someone with my aerospace engineering and flight-test background, a pilot is not required to know exactly how or why the electric trim is running away…(be it faulty switch, shorted wiring, stuck relay, pilot overpowering pitch servo, MCAS or, as in my case, misbehaving autopilot).
A pilot IS REQUIRED to know how to control his aircraft and, pertinent to this thread, quickly disconnect the electric trim regardless of the reason that is causing it. Then, and ONLY THEN, once you have control, it may or may not be useful to understand what happened.
A deep understanding is a plus, but not a requirement. MCAS was one disgraceful case, but there are hundreds more cases where pilots do not understand or are taught the logic behind control or warning systems and are flying safely. This on Boeing but, much more prominently, also on Airbus aircraft. The stall warning deactivation logic at low IAS on AF447 springs to mind as one example…
Peter wrote:
That is true for all airliners. They have a large speed range and thus have a large trim range.
That may be true for all airliners, but it is not a law of nature that stabilators with a large trim range are impossible to trim by hand if the stabilator is under load. It is simply bad design.
Quite a heroic thread drift from an Arrer to transonic auto stab trim systems :)
https://www.aerosociety.com/news/downfall-the-case-against-boeing-reviewed/
It is not a law of nature but that does not mean it is not widespread, widely accepted and almost never caused an accident before MAX.
My point is the focusing on MAXs is plainly wrong.
The focus should be on keeping the pilot in control by training and design.
The aerodynamic forces on a transonic airliner are such, that if there is an auto stab failure, or it is disconnected by the crew, they need to decelerate or accelerate to trim speed to apply manual trim. This assumes they remembered the memory item to shut off the trim if there is a runaway in the first place.
As @Peter points out, in light GA the pilot should be able to overcome the pitch servos, even on a one Weetabix breakfast :)
