Windmilling Prop & Vacuum Pump

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.

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

Depends on your pump and what RPM's you can expect the prop to windmill
at. My pump will produce 4" at idle and if you just shut off the fuel
to the engine, the prop will windmill in the glide at well above idle
(actually, you can't really tell by the RPM's that the engine isn't
running).

Of course, if your engine siezes, you're not going to get any windmilling.

during descent through the clouds, or will the gyros keep spinning fast
enough long enough to make the vacuum pump irrelevant?

It's amazing how FAST the gyros spin down (and how long it takes them
to spin up). I had a pump crump on takeoff roll and before I was at
pattern altitude the AI had started to lean over (fortunately in VMC).

In article et,
Matt Young wrote:
during descent through the clouds, or will the gyros keep spinning fast
enough long enough to make the vacuum pump irrelevant?

I question the wisdom of relying on an instrument that you already know to
have failed. You would need to know just how long you can stare at it
in a very stressful situation before you can no longer believe what it
tells you. I would think that immediately covering the instrument would
be your best course of action in this scenario.

No Such User wrote:
In article et,
Matt Young wrote:

during descent through the clouds, or will the gyros keep spinning fast
enough long enough to make the vacuum pump irrelevant?


I question the wisdom of relying on an instrument that you already know to
have failed.

What makes you think it has failed? If the vacuum guage still shows a vacuum,
then the gyro's got to spin (unless you are unlucky enough to have a second
failure of the vacuum system at the same time that the engine crumped).

What I've never understood is why there isn't more obvious indication that
there is no power (vacuum or electric as required) going to the instrument.
Jeez, the VOR which isn't as essential to instrument flight has a better
indication that it's not on.

On Wed, 13 Oct 2004 04:39:47 GMT, 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?

Your 're going to get answers all over the spectrum, but my take would
be it might happen, but don't count on it to save your bacon. IOW,
I'd not count on them working, or staying accurate which is
_far_worse_ than not working when you are in the clouds.

There are many variables such as the actual engine, prop, and vacuum
pump combination as well as best glide speed.

Its been my experience (which may not be typical) that a wet pump will
do better with the low RPM than the dry pump. This is assuming you
still have RPM which with a catastrophic failure you probably won't.

In my particular airplane which has a wet pump and a constant speed
prop, once the instruments are spun up even vacuum at idle (which is
well below the minimum) will keep the AI and DG working. The AI stays
accurate. The DG will start precessing but slowly although it does
hold well with a prolonged idle on the ground. Actually it's close to
being in tolerance. With full vacuum I don't have to reset it during
a 3 to 4 hour flight.

So, I'd expect to see the vacuum instruments "on mine" hold for some
time, but even knowing them as well as I do, I'd still depend on
partial panel and consider any use out of the vacuum instruments a
bonus.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com