Matt Young wrote:
Ok, a curious question just popped in my mind. Say that one was flying
IFR in a piston single, maybe a 172 or 182. While enroute, either in
actual or above a cloud layer, the engine fails. Will the windmilling
prop keep the vacuum pump going enough to make the AI and DG usuable
during descent through the clouds, or will the gyros keep spinning fast
enough long enough to make the vacuum pump irrelevant?

Whilst doing a engine runup at about 2000 rpm my vacuum indicates
'normal', and even during taxying at around 1000 rpm the AH is stable.

So., if the engine fails *but the prop still keeps turning* at a
reasonable rate (eg above 1000rpm) then I'd expect that there
should still be sufficient 'vacuum' to operate the gyro instruments.
With *no* vacuum, (eg after engine shutdown) my gyros starts
drifting after about a minute.