Boeing 737 Unable to Trim!! Cockpit video (Full flight sim)

> ... it can become impossible for a pilot to physically turn the manual trim wheels to get back into trim I feel like this isn't talked about enough. I had no idea how bad it can actually get: ▶ this video

The pre-MAX 737 has a cut-off switch that switches of the automatic trim system, without also switching of the electric trim (there's another one that switches off the electric trim). The 737 MAX also has two switches, but they both switch off the automatic trim (including MCAS) and the electric trim. So you can't switch off MCAS without also switching off electric trim, leaving only manual trim. But in situations like the Ethiopian flight, you need inhuman strength to control the manual trim (as Mentour Pilot demonstrates in this video: ▶ this video ). You need electric trim to have any chance to fight MCAS, but you can't enable electric trim without also enabling MCAS. Must be horrific to be in such a situation.

> it's not fully known yet why that procedure didn't prevent the accidents The only way in the 737 Max to disable MCAS, is to disable electric trim altogether. That means only manual trim is available. When MCAS has trimmed all the way to one of the extremes (as happened in the Ethiopean flight), it requires many revolutions of the trim wheel and superhuman strength to return it to a more appropriate setting (see e.g. ▶ this video ). Previous generations of the 737 had two buttons related to the electric trim: one disabled the automatic trim system, the other one disabled electric trim altogether. The 737 Max still has two buttons, but they both have the same effect: disable the electric trim altogether. The old system could easily have prevented the crash by disabling MCAS but leaving the electrically assisted trim enabled. Boeing's procedure to make manual trim adjustments possible is to put the airplane in a dive, because that lowers the aerodynamic forces on the trim surface. When you're only a few hundred feet above the ground, that's not really an option. The pilots re-enabled the electric trim in the hope they could use it in their fight against MCAS. It's the only thing they could think of in the stress of the moment, faced with near-certain death and an airplane actively trying to crash. In hindsight, after reading accident reports and thinking about it in the comfort of chair that's not crashing, there were things they could have done. Lowering the throttle would have le

They accurately simulate the forces on the controls, see this youtube of 737 pilots unable to manually trim in the simulator [0], but I don't think they could anticipate and simulate every single possible fault. ▶ this video

>> Source? I'd assume, in fact, that in most planes that is not the case - superhuman strength is not required to trim. ▶ this video Showing the strength required to trim a 737 in certain circumstances.

At least some simulators are capable of modelling those forces (I don't have any knowledge of the domain, but I would assume it's standard in "professional" simulators): ▶ this video

> Having it repeatedly come on and run is runaway trim And they recognized it as such and completed the runaway trim checklist, but they couldn't use manual trim (as advised) to recover from the situation, because aerodynamic forces acting on the stabilizer overpowered them. Two Swedish pilots demonstrated this scenario in a 737NG simulator: ▶ this video Ethiopian pilots were in even more stressful situation, because they also had broken AOA sensor with unreliable airspeed indication, active left stick shaker, occasional terrain proximity warnings and overspeed clicker. No procedures or training existed for the situation they were in. Pilots of early 737s were taught the so-called rollercoaster maneuver to temporarily relieve the load to allow manual trimming, but that was scrapped decades ago.

Even more than that, you pick up speed due to the nose down configuration. This means the forces on the control surfaces are so great that you can no longer manually trim via the trim wheels. This was, apparently, a well known handling characteristic of the 737, with original versions describing a "roller coaster" maneuver where you dive briefly to relieve the forces on the control surfaces and incrementally trim. Unfortunately, because of how MCAS was set up, it was inactive while flaps were set, but kicked in immediately upon flaps up, meaning that they essentially had no altitude to work with. See: ▶ this video for a flight sim recreation of this situation on an NG. Everything here seems to be that exceptional pilots can (and did) save the situation, but that it's not straightforward even with training.

Very good points. And in the case of MCAS, physically unable to exert the effort to overcome the automated trim settings. (Posted above): ▶ this video

> Boeing 737 Unable to Trim! Cockpit video (full flight sim) - ▶ this video Start at 13:20 to skip straight to the relevant bit.

Depends on the aircraft. Some modern jetliners are using fly-by-wire, while others have hydraulically-boosted controls. Some still rely on cable-and-pulley. Either way, as I understand, it wouldn't matter with the MCAS-related MAX aircraft crashes. The elevator trim is able to move the entire elevator (literally, the entire elevator moves in response to the jackscrew controlled by the trim system) to such an extent that, depending on CG, airspeed, and other factors, it cannot be overpowered by moving the control surface to its full deflection. So whether or not the MAX had boosted controls or fly-by-wire probably wouldn't have mattered in the case the aircraft were trimmed fully nose-down. The pilots would have been struggling to maintain control either way. That's why there's stabilizer trim cut-out switches and manual trim. I'd highly recommend Mentour Pilot's videos on the subject, links below: Runaway Stabilizer: How to stop MCAS - link Boeing 737 Unable to Trim! Cockpit video (full flight sim) - ▶ this video Five questions about the Boeing 737 MAX Answered - link

Mentour Pilot has a video where he does that exact simulation for a 737NG flight simulator in order to show how difficult it is to trim manually [1]. [1] ▶ this video

This video by a 737-NG captain shows the kind of forces you get: ▶ this video It is not MAX but the systems are identical. That's why it is so essential to recognize and stop the runaway trim (for whatever reason) ASAP or you could get into a situation where it would be physically impossible for the crew to do so.

Well, the "Stabilizer wheel - grasp and hold" item of the procedure doesn't necessarily mean the crew will be able to override it. That's the last ditch option on the checklist, when everything else has failed, even turning the motors off (there could be e.g. a short circuit powering the motor). There is no clutch on the 737, AFAIK, because the forces are such that it wouldn't work. Here is a good video by an actual 737 captain explaining how the runaway stab trim procedure works and what you have to do. link And this simulator video shows both how the 737 horizontal stabilizer is controlled and what kind of forces are on the trim wheel and control column when severely out of trim: ▶ this video Should give you an idea what were the Ethiopian pilots facing when unable to use the electric trim, plane trimmed nose heavy by MCAS and at high speed ...