Mentour Pilot
7 HN recommenders · recommended in 5 months, 2019–2022
22:58 · 421K views · Mar 22, 2019
A Boeing 737 pilot explains the runaway stabilizer checklist, the procedure for stopping unwanted horizontal stabilizer movement such as an MCAS activation on the 737 MAX. He walks through the steps, the trim cutout switches and the jackscrew mechanism, and shows a simulator run including the manual trim wheel effort.
During the 737 MAX crash coverage, HN commenters linked it as a clear, pilot-led demonstration of what an MCAS runaway looks like and how hard manual trim is to crank.
13 comments from Hacker News
> You probably have nothing to worry about with air travel. Well, the conclusion is already made in the statistics, and air travel is the safest, yes, but it's worrying that it's safe despite the enormous fallacies, despite the insane pressure of market forces. A very eye-opening thing is to watch what the pilots should have done: this video Runaway stabilizer scenario has associated memory items (meaning the pilot must have the checklist committed to memory). There are big wheels that turn, with noise and a big visibility marking. This was Boeing's initial defense too, that the pilots fucked up, because disabling any electronics works the same way. (There's a cutoff switch.) So if something is doing something, shut off the power, rotate the wheels manually. And in this regard Boeing is right. Pilots decisionmaking was suboptimal. However, it is Boeing's fault that they opted for a no-training-required strategy. A similar WTF is that pilots don't keep up with the aviation industry news. How - the fuck - can you pilot a plane that you know nothing about (except that your license is valid for it)!?
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 - this video Boeing 737 Unable to Trim! Cockpit video (full flight sim) - [LINK] Five questions about the Boeing 737 MAX Answered - [LINK]
I'd highly recommend anyone who wants to see what an MCAS runaway looks like and the steps needed to correct it watch the following video by Mentour Pilot.[1] He's a 737 pilot for a European budget carrier, and I believe he is qualified to certify other pilots. In the video he goes over what an MCAS failure/runaway stabilizer would feel like to the pilots, the troubleshooting steps, and at the end of the video he shows a simulated runaway MCAS failure in a sim. He also has a few other videos on the MCAS accidents. I'd highly recommend watching all of them; he does a very good job at putting everything into perspective in a way that the news does not. [1] this video
I highly recommend everyone suggestion engineering solutions to watch this video from an actual pilot performing a runaway stabilizer procedure in a 737 simulator. this video What happened is a series of failures; this video frames it perfectly.
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. this video 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: [LINK] 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 ...