Originally Posted by
L39 Guy
A stab trim runaway affecting the fly ability of the aircraft is no time to “figure out “ anything. The figuring out happens back on terra firma. Whether the source of the runaway is a short circuit in a switch or motor, MCAS or anything other source, when the aircraft experiences uncommanded flight control movements in pitch it’s a stab trim runaway for which there is a clear and simple memory drill. The instinctive thing would to pull or push to restore the desired path then trim for it with the electric trim switches, even if it means long burst of trim. Whatever it takes. If the trim continues to be uncontrollable, turn it off and manually trim with the wheel, however one has to be flying at something less than Vmo to do that which means controlling the thrust or, in other words, flying the airplane.
In the isolation of a simulator session where the sim instructor selects a runaway trim malfunction, fully agree with you.
Press play below before reading on...
This was not a classic runaway trim. No trim issues were encountered until the flaps were retracted. Retract flaps, now some forward trim starts, trim back with the electric trim and everything is good for 5 seconds, just enough time to start thinking about else, there goes the trim again, fix it with electric trim, all good for 5 seconds, repeat until end of sequence. You are also dealing with an airspeed unreliable. Not easy with that racket going on the background.
To the pilots in all three event flights, this was a
trim fight, not a continuous runaway in the QRH condition statement for a Runaway Stabilizer. Judge for yourself from a current in-service document:
Condition: Uncommanded stabilizer trim occurs continuously
The trim worked as they expected, sort of. If English was not your first language, it would be even more difficult.
The Lion Air crews were in the worst position, they had no prior knowledge of the subtle system changes and faults that had led them to
trim fight, not a runaway.