Anders Cornelius Olesen
3 HN mentions · mentioned in 3 months, 2018–2019
1:40 · 247K views · Jun 11, 2014
A short clip shows the horizontal stabilizer jackscrew of a Boeing 737NG cycling through its motion. It illustrates how pitch trim is physically adjusted.
HN used it during 737 Max crash discussions to show how the stabilizer trim works and why manual trimming takes time.
3 comments from Hacker News
Here's a video of the wheels in motion: [LINK] Disregard the text on the video description as it's blatantly wrong (describing that the trim tabs still move without the wheels moving; wrong on two accounts: the trim doesn't move without the wheels moving and the 737 uses a jackscrew for horizontal stabilizer trim rather than trim tabs [which is why the pilot can't simply override the aerodynamic force as they could with a trim tab]) Jackscrew operation: this video The 7-3 has balance tabs to make the control forces lighter in the event of a hydraulics failure, but these are not trim tabs in any sense of the word.
There's no trim tab on a 737 - the whole horizontal stabilizer is moved to trim the aircraft in pitch. Normally this is done electrically, but the stabilizer jackscrew can also be moved using manual trimwheels that move the jackscrew using physical cables. Whether this is possible at high airspeed while the stabilizer is trimmed nose down, and the elevator is being pulled up as hard as you can to avoid diving - that's the question that is being raised. Edit: this shows the jackscrew in operation: this video
> In theory then, solving the stabilizer problem that caused the crash is as simple as flipping both switches to cutoff, then using the wheel to set the stabilizer back to a sane value. this video The issue for the Lion Air pilots was time. At 5,000 ft you don't have time to adjust the trim before you hit the ground. While I think that the stabilizer trim can be adjusted more quickly than the video shows you'd still be looking at somewhere around the order of 30 seconds end to end.