4 ms·

It probably is a memory item, but it's not something they got to immediately, and I'm guessing here because runaway trim symptoms are slightly different to what

by _s 8y ago

It probably is a memory item, but it's not something they got to immediately, and I'm guessing here because runaway trim symptoms are slightly different to what MCAS was doing. This happened to me a while ago involving trim, and I'll try and go through my thought process and why it takes a little while in the cockpit to figure things out: - Autopilot is engaged, holding a heading of X, altitude of Y and I've got a power setting for speed Z - My hands are off the stick, and I'm planning the next 20 mins of the flight. - The nose has slowly started creeping downwards, I don't notice until I look up a few seconds later - I glance at the altimeter and see we're now Y-300ft, and descending at 500ft/min. - I check the altitude hold on the autopilot flight director; see yes, it is still set to Y, and am rather confused, as the stick is fairly pulled back to pull the airplane up (autopilot), so the pitch hasn't changed that much. - Airspeed is slow, and I'm even more confused - how are we both descending and slowing down. - Glance over to the passenger, she's fast asleep, a bit restless but not touching the yoke or rudder. - ATC comes over the radio asking if we're descending and what our intentions are. - It's only been about 30 seconds but we're now Y-900feet, at a decent rate of of -1400ft, the AoA indicator is starting to beep as we're now in the warning arc, speed is Z-20kts. - Unsure of what's actually happening, I push in full mix/throttle to keep our airspeed up. - I grab the stick (already fairly back) and attempt to get us level. - Tell ATC over the radio I'm having difficulties with the autopilot as I disengage it. - Feel an immediate / tremendous force on the stick pulling it forwards, have to use both hands to keep it back. - I'm now wondering if the elevator control is broken. - Try to start trimming it back (electronic trim), but a warning flashes on the instrument panel. - Electronic trim usage also immediately disables the autopilot completely, so the wings are now rocking too, giving a weird sensation as I feel like I'm fighting the controls. - Reach over to the trim wheel, realise the passengers knee is propped on it. - Move her leg out of the way, and start wheeling it back and feel pressure easing on the stick. - Keep trying to use the electronic trim and it kicks in, and I can see the trim position on the panel (set pretty far forward). - Aircraft is now finally level, airspeed is increasing. Elevators / pitch control feels normal. - Set airspeed for best rate of climb, and tell ATC I'll level off at my previous altitude. Put us into a climb. - Re-engage the autopilot and all seems well That was about a minute and a half. I went from 8500ft to around 6800ft in what felt like a few seconds, and it's only after I was in the climb that I realised her shuffling was knocking the trim wheel around. She had been probably doing it for the better part of an hour, and the autopilot had been opposing her inputs. I didn't pick up on the airspeed changes as we're a slow plane flying into a strong headwind so I expected to not be flying the usual speeds. I pored over the Pilot Operating Handbook that day, and also the Garmin G3X/GFC500 to figure out what was happening and to understand the systems. The trim wheel itself doesn't disengage the autopilot, only the electric trim control does. When the servo motors can't engage (too much force), the system just shows a red X on the instrument display where the control display would be. Etc etc. The 737MAX pilots don't have that luxury, as they're told it's just a 737 type. Go through the above scenario, but have no knowledge of the fact that theres something moving the trim. You do not have the luxury of altitude, nor airspeed. The sensors are providing weird inputs. Plus, everything is happening around 10x faster, not a slow build up. You reset the trim, all is well for a few seconds and bam, nose down again. Reset again, it happens again. If I don't know about the MCAS, the 5 sec delay etc, my immediate thought is a faulty computer and I'll try and restart it. And so forth.

js2 8y ago
Thank you for a pilot's perspective. Here's my bit of confusion though. Once you realized the stab trim was the issue, you were able to reset it and get back to normal flight. Had the problem recurred that flight, you'd probably have recognized it immediately. In the Lion flight, the pilots realize the stab trim is off because they correct it. Once they realize that, regardless of what keeps setting the stab trim to pitch down, and even if they've forgotten there's cut-out switches, why don't they keep resetting the stab back to 0? And how do they not eventually notice the stab wheels turning? It's almost as if the pilots weren't even aware of the stab wheels and stab indicator, and how those related to the electronic trim switch and the stabilizer itself. But I can't beleive that. The thing that would be most helpful is knowing what their mental model of the plane was, but I'm not sure the investigation will be able to tell us that.
- _s 8y ago
  I think context is what helps - - I’ve just taken off, and am going through the after take off checklist: Set the airspeed, power, climb rate, heading, talking to ATC, planning the next stage of the flight - Perhaps I’ve just raised the flaps and the plane starts behaving erratically - My first / immediate thought is that my previous action led to an undesired aircraft state. - I work from that point on - I touch the electronic trim as habit, which resets the system briefly, and all seems well for a few seconds. I don’t know that the system reset itself. - It goes all weird again all on its own - Now I’m even more confused Etc Edit: I'm a low time private pilot flying a LSA, I genuinely don't know the procedures or processes in place for a 737, but I can empathise with their situation as I know how quickly things can go start going amiss in a plane.
  - js2 8y ago
    I think that, not being a pilot, I can't really understand, but thank you again for the perspective. Edit: one last thing though - when the plane isn’t behaving as expected, why isn’t the first thing to check that all the control surfaces and the throttle are where you expect them to be?

by _s 8y ago

js2 8y ago

Thank you for a pilot's perspective. Here's my bit of confusion though. Once you realized the stab trim was the issue, you were able to reset it and get back to normal flight. Had the problem recurred that flight, you'd probably have recognized it immediately. In the Lion flight, the pilots realize the stab trim is off because they correct it. Once they realize that, regardless of what keeps setting the stab trim to pitch down, and even if they've forgotten there's cut-out switches, why don't they keep resetting the stab back to 0? And how do they not eventually notice the stab wheels turning? It's almost as if the pilots weren't even aware of the stab wheels and stab indicator, and how those related to the electronic trim switch and the stabilizer itself. But I can't beleive that. The thing that would be most helpful is knowing what their mental model of the plane was, but I'm not sure the investigation will be able to tell us that.

_s 8y ago
I think context is what helps - - I’ve just taken off, and am going through the after take off checklist: Set the airspeed, power, climb rate, heading, talking to ATC, planning the next stage of the flight - Perhaps I’ve just raised the flaps and the plane starts behaving erratically - My first / immediate thought is that my previous action led to an undesired aircraft state. - I work from that point on - I touch the electronic trim as habit, which resets the system briefly, and all seems well for a few seconds. I don’t know that the system reset itself. - It goes all weird again all on its own - Now I’m even more confused Etc Edit: I'm a low time private pilot flying a LSA, I genuinely don't know the procedures or processes in place for a 737, but I can empathise with their situation as I know how quickly things can go start going amiss in a plane.
- js2 8y ago
  I think that, not being a pilot, I can't really understand, but thank you again for the perspective. Edit: one last thing though - when the plane isn’t behaving as expected, why isn’t the first thing to check that all the control surfaces and the throttle are where you expect them to be?