PW120 Series Engines
 

Can some explaine why boost pumps on these engines need to remain on until engine speed drops below 10 percent?

Engine driven fuel pumps are typically lubricated with fuel. The boost pumps need to be on to do that. Don't know the 120 but I know Pt6s are only allowed to operate with the boost pumps off for a max of 10 hours. More than that and pump comes off. Boost pumps need to be on even during engine maintenance such as compressor washes. My two bits worth.

Although I don't have specific data, I would think that centerline engines such as DC-9/727/BAC111 and like bizjets have a similar requirement.

The ATR is fitted with PW120 series engines, and I have heard this comment many times.

This is either pure invention, or some procedure carried from another PW engine that is totally unrelated.

There is (was) no such thing in the ATR manuals.

Follow the manufacturer's manuals. If it was causing any damage, they would have included this procedure. It may well be on the PT6, but not on the PW120 series, at least not the ones mounted on ATR aircraft.

Do you mean the aircraft boost pump as the engine does not have a switchable "boost pump"? If yes then it's likely to prevent engine driven pump cavitation.

Correct. There is no mention in the ATR manual that I saw about leaving boost pumps on after shutdown and it wasn't in our checklist. Don't remember it on the PT-6 either but it has been a while.

Physically impossible on the Dash too - aux (boost) pumps are AC driven and AC goes offline when you go to start/feather.

In addition to what Kiwiconehead has rightfully stated, the aux pumps were procedurally switched on only for takeoff and approach on the -300 (just like they are now on the -400). Between climb check and approach check they were only required to run for some abnormal procedures (typically involving high oil or fuel temperatures).

And the engines ran just fine under this kind of treatment.

Maybe captjns could specify which type he is referring to?

Based on the terms used in captjns post ie "boost pumps" and "engine speed" (Ng) I think he refers to the PT6 series. Never heard those terms used on the PW120 series, but hey I coud be wrong.