Hi all,
The accident to the FlyDubai B738 at Rostov-On-Don recently could have been due to "mistrimming". The vertical profile of this accident is almost identical to that of the Air Tatarstan B735 that crashed at Kazan, Russia in 2013.
In both cases, during a missed approach, the aircraft reached a height of several thousand feet above the aerodrome then rapidly pitched down, diving into the ground beside the runway. Both crashes were fatal. Neither appears to have stalled.
Is the pitch trim system of today's airliners poorly understood to the point that aircraft are crashing?
Please refer to this article on the subject:
Roger-Wilco | Do you really understand how your trim works?
Here is an extract from it:
I have watched, in the simulator, a 737 go-around from a Cat lll fail passive approach (as described above) with its marked pitch up; HP kept his arms locked forward to contain the attitude whilst simultaneously running the trim forward with the thumb switch. I am sure he was expecting the trim to reduce push needed and he either didn’t know, or had forgotten, that it wouldn’t. We duly pitched straight back quickly into the ground as the tailplane incidence ‘bit’.
This account sounds a lot like what could have happened in the two accidents I referred to above.
So, what does actually happen when, in a type like the B737, you hold a particular attitude - against the feel forces - then run the trim. Do the forces reduce to zero, or do they stay the same?
Incredibly, the linked article doesn't actually say! Here is the first paragraph:
Picture yourself in a conventional airliner, say a 737 of any generation. You have to do a low level go-around, perhaps because your fail passive Cat lll has just failed, er, passively. You apply GA thrust, and the aircraft pitches up. If you are low enough, you may already have some extra helpful nose up trim applied thanks to the ‘design feature’ that ensures that in the event of AP failure at low level, the aircraft pitches up not down, and so a few units of nose up trim are applied late in the approach. Your speed is low, about Vapp and the thing is pitching firmly upward. You need ample forward stick/elevator to restrain it. You don’t want to carry this load for long so you retrim. Question: if you run the trim forward while maintaining forward pressure on the wheel, what happens? Hands up all those who think the load reduces to zero. I see a lot of hands. My unscientific polling to date suggests that just about everyone is convinced that this is what happens, but it doesn’t.
Frustratingly, the author doesn't clearly spell out what does actually happen, but reading between the lines, he seems to be saying that the forces DO NOT reduce to zero.
What happens on the B737? And, is it any different to all the other airliners that have trimmable horizontal stabilisers?