First point - don't operate the engine like an Allison 250!
Or an LTS for that matter.
I have seen so many operators doing just that.
Prolonged idling - progression from the 250 and or 101 and doing 2 minute run downs. Habit.
Not good due to low oil flow around the rear bearing and heat build up. This progresses to carbon buildup in the scavenge and vent. If the scavenge blocks it is possible for oil to migrate to the rear of the GG 2nd wheel. After that there is a good possibility of losing a turbine blade. 30 seconds is the required run down and then turn it off as any more is detrimental. There is a graph around somewhere that shows the temperature initially reduces in the area of the rear bearing then after 30 seconds it climbs again and does all sorts of strange things. If you have been idling run it up to flight again for a while then back to idle then shut down.
Any maintenance in the rear bearing area requires plenty of attention to details and procedures.
The bearing is after the 2nd wheel on the extreme right.
ref the AD here is a picture of the buildup that comes off unevenly which causes the vibration and rear bearing failure.
