David Megginson wrote:
Dave Butler wrote:

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?



No and no.


Personal experience, credible sources, or just a guess?

1A. Will it keep the vacuum pump going? No, just a guess. You might have a
better shot at it with a controllable prop. That assumes, of course, that
whatever caused the engine to stop making noise didn't prevent it from spinning
'round.

1B. Will the instruments keep spinning enough? No, just a guess, reinforced by
my experience testing the Precise Flight backup vacuum system when it was
installed in the Archer I used to own. To calibrate the altitude limits on the
placard, you have to fly with varying levels of vacuum and note the instrument
performance. The vacuum doesn't have to be much below specified minimum (is it 4
inHg?) before the gauges get really squirrely. Noting how long it takes the
gyros to spin down while the plane is stationary on the ramp after you shut down
is not a useful measure.

2. It's easy to test. Try it for yourself if you really want to know.

3. I wouldn't rely on a usenet answer if it were 'yes'.

4. Even if I tested it and it worked, I wouldn't rely on it working when I need it.

Short answer: it was a guess.