Mentour Pilot
3 HN recommenders · recommended in 8 months, 2019–2022
24:08 · 369K views · Apr 19, 2019
Mentour Pilot, a 737 captain, demonstrates in a full flight simulator why a Boeing 737 can become impossible to trim manually at high speed. He explains the jackscrew forces and the roller-coaster manoeuvre.
HN pointed to it during the 737 MAX discussions to show how hard manual trim is when severely out of trim.
14 comments from Hacker News
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]
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 ...
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.
> ... 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
> 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