The trouble with the 737 is, that up from a certain airspeed you can no longer use manual trim to rectify the situation
Isn’t that true on any big jet?
Peter wrote:
Isn’t that true on any big jet?
Not that I am aware of. That is why it got such huge attention
Reverting to electric trim isn’t the only way to correct a mistrim situation on the 737 at high speed, and as a matter of fact it is not the procedure that is taught.
Mooney_Driver wrote:
Personally I think the trim system of the 737 is dangeorusly flawed as it can NOT be unloaded when you need to do this most. I would not be suprised if more accidents could be atributed to this. And I fear in the future more will be.
It surprises me that Boeing hasn’t done anything about this. Redesigning the stabilizer jack system can’t be that difficult, can it?
Any stabiliser jackscrew that is manually actuated will be hard to move when under high aerodynamic loads (mistrim). This is not a Boeing only or 737 only problem.
The “problem” of the high aerodynamic loads on variable incidence tailplanes for transport category aircraft is known (and in theory trained for) since the early 60es.
Well…yes and no.
A 737 can be manually trimmed at any airspeed within the approved envelope.
The problem is manually trimming on a stabilizer with a heavy moment load. This will typically happen in presence of a strong elevator command, as in a heavy out-of-trim condition concurrent with a strong and opposite yoke command. You need to unload the elevator force or else you will not be able to manually trim.This is also a problem with most (if not all) aircraft with pitch trim via movable stabilizer and manual trim backup. Some aircraft, however, do have electric backup for the stab trim so a failure or runaway of the primary is easier to tame
On the manual version, whichever the type, be it 737 or a320 or whatever, once you are in a seriously out-of-trim condition you are quickly running out of options. However, though I don’t know the exact time,it will easily take half a minute from a medium stab trim to a seriously out-of-trim condition so it is not like the situation needs instant action.
The manoeuvre I saw suggested in a couple of places with regards to the Max issue, once in a heavy nose-down out of trim condition, is a sequential loading/unloading of elevator to try and maintain altitude while trimming during the unloaded portions of the sequence.
The key is detecting the trim m runaway and disconnecting primary trim before it gets out of hand. Most if not all modern large aircraft have different types of aural and visual warnings whenever stab trim is running continuously for more than a few seconds, precisely to avert of this situation. Things like “stabilizer motion” warnings or the likes, although I cant confirm the case for a 737.
Antonio wrote:
Most if not all modern large aircraft have different types of aural and visual warnings whenever stab trim is running continuously for more than a few seconds
Even our STEC 1500 autopilot on the PA46 does that. A lady will complain and say “trim in motion”. That is very a good feature. For example if you level off with the autopilot and forget to add power.
Sebastian_G wrote:
STEC 1500 autopilot on the PA46 does that
I did not know that. That is great!
Sebastian_G wrote:
For example if you level off with the autopilot and forget to add power.
…you mean as you level off after descent.
Hmm. that requires another level of thinking: let me see…connecting “trim in motion” warning with “forgot to add power”…
This kind of mental connection you are implying is exactly the one whose lack the thread title warns about…
Antonio wrote:
Hmm. that requires another level of thinking: let me see…connecting “trim in motion” warning with “forgot to add power”…This kind of mental connection you are implying is exactly the one whose lack the thread title warns about…
To my simple brain the words in the warning (and any connection) are less important than the fact that there is a warning telling you that something’s wrong, stop dozing, wake up and fly the bloody aeroplane by hand for a bit until you figure it out.
I’ve done very little flying behind automatics, but before I did that little bit a good instructor told me:
“The instant it does anything you’re not expecting, turn it off and hand fly. Diagnosing a problem is much easier straight and level than whatever funky attitude the automatics might be trying to make you assume.”
