The engine power-off sound is played when disengaging the starter. Maybe I'm doing it wrong, but I don't think the starter is required for more than a few seconds and oddly enough the sound only plays when starting the engine the first time.

Steps:
1. Start the game
2. Power on to batteries
3. Starter on
(ticking sound plays)
4. throttle to idle
5. Starter off
-> engine power-down sound plays, yet the engine is running idle.

if you repeat these same steps after landing, the power down sound no longer plays.