Aviation Stack Exchange is a question and answer site for aircraft pilots, mechanics, and enthusiasts. It only takes a minute to sign up.
Sign up to join this community
In the recent Netflix documentary, a claim is made that once MCAS activated, after several activations, the pilots physically could not trim the aircraft due to the speed and pitch.
Is this true? I didn't think the trim wheel required pounds of force like the yoke does.
The problem was they could not manually trim the a/c without performing the Boeing "offload the jack" procedure developed for the 737, which requires altitude to perform because you are forced to let the airplane dive to do the offloading (in the case of a nose down runaway).
The basic problem is, if you are holding a large displacement of the elevator surface from neutral, it induces a large additional torque force, or pitching moment, within the horizontal tail system itself, much higher than with the elevator at neutral.
This torque force is absorbed by the trim screw jack and is relatively light when the elevator is at neutral, aligned with the chord line of the stab surface. However, at very large elevator displacement, the load on the screw jack, induced by the twisting force built up within the tail assembly by the elevator displacement, is more than can be overcome with the leverage available to the manual trim wheel system.
It's like you are jacking a car with one of those little scissors jacks they come with, and normally you can turn the screw fairly easily with just the normal weight of the car, but then an elephant climbs onto your car (the up elevator input), and the extra weight loads up the jack so much you can't turn it by hand any more.
The only way to turn the screw is the make the elephant get off the car, but if the elephant climbs off, the car will tip over, so you are kind of stuck.
The Boeing crew were holding elevator prevent the plane from nosing over, but this prevented them from manually working the wheel, but to work the wheel, they would have to offload the jack (make the elephant climb off) by relieving the elevator back toward neutral, but doing so pointed them at the ground, and that was that.
Here is a Youtube video presentation of the situation, filmed in a simulator:
Mentour Pilot: Boeing 737 Unable to Trim!! Cockpit video (Full flight sim)
The core of this question and questions like it is based on a misunderstanding of a particular characteristic of the pitch control systems on the 737.
The general notion is that the yoke operates the flight control surfaces and the trim operates the flight adjustment surfaces. This would give the yoke pitch authority beyond that of the trim control, just like in most small aircraft.
On the 737 it's the other way around. The yoke still operates the elevator surfaces like in most other aircraft, but the trim controls adjusts the entire horizontal stabilizer, elevator surfaces and all. This gives the trim control system way more authority over pitch than the yoke, effectively turning it into the flight control system and degrading the yoke to the flight adjustment role.
The only way out of a runaway automated trim is by first disabling that automated trim control system and then manually readjust the trim control surfaces. Switching off the automated trim control system separately is for various reasons not possible. The only way to disable it is by switching off all powered pitch trim control. That includes the trim switch on the yoke. After that the readjustment of trim has to be done by turning the trim wheels ONLY.
At least one of the MAX crashes could have been prevented by simply keeping it that way. The fatal mistake was to repetitively reactivate the powered pitch control, thereby also reactivating the malfunctioning system. This not only makes the problem reoccur, but also makes it increase. That process repeats until the sheer authority of the trim control supersedes that of the elevator and the airplane goes into an unrecoverable dive.
The pressure problem arises as one pilot tries to maintain level flight by pulling the yoke, while the other pilot tries to adjust the trim wheels. These two forces work directly against each other. The harder one pilot pulls, the harder it is for the other to turn the wheel.
The solution is a roller coaster like routine that allows the spinning pilot to briefly adjust the trim wheel as the yoke pulling pilot briefly gives full slack. This decreases the problem with every turn of the wheel. This is by no means easy. Many experienced pilots still have a problem wrapping their head around it in unannounced simulations. It demands almost immediate response to something that occurs completely out of the blue and at first feels like ordinary turbulence.
The core of the story is that flight control input isn't always flight control authority and between automated pitch control and manual pitch control there is the switch on the yoke. It provides powered pitch control, which is neither automated nor manual.
A revealing part of the story is the fact that it shows how a 737,... correction, how any 737 can actually be trimmed straight into the ground. No pilot in his right mind would ever do that, but the intuitive notion that it's impossible is not correct. An intently obscured malfunctioning MCAS replacing a pilot who's not in his right mind proves that.
Meanwhile the impact of the MCAS related crashes continues to ripple through the aviation world. Reason for that is the one thing that everyone agrees on. These crashes should have been absolutely impossible, yet they happened. No NTSB investigation will ever determine the cause. No court case will ever convict those responsible. No FAA rule will keep it from ever happening again. Complacency is not reflective and mere ignorance does not provide closure. There's no excuse for these crashes, like there is no exception to what's impossible.
