Boeing 737NG horizontal stabilizer jackscrew cycle
Why Hacker News recommends it
HN used it during 737 Max crash discussions to show how the stabilizer trim works and why manual trimming takes time.
What it is
A short clip shows the horizontal stabilizer jackscrew of a Boeing 737NG cycling through its motion. It illustrates how pitch trim is physically adjusted.
Recommended by 0 HN commenters across 0 different months, 2018 to 2019.
What Hacker News said
Cites inferiorhuman · 2018-11 · on “Third Pilots' Union Raises Concern About Boeing 737 Max Jet”
> 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.
Cites sokoloff · 2019-02 · on “Behind the Lion Air Crash, a Trail of Decisions Kept Pilots in the Dark”
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.
Cites mhandley · 2019-04 · on “Ethiopian Airlines Boeing 737 pilots 'could not stop nosedive'”
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]