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.