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Wet versus dry vacuum pump

The wet pumps have been around for many years but one doesn’t come across them much nowadays – presumably because the dry ones are so much cheaper, around $2500 versus $400 for a top quality (Tempest) dry pump.

Here’s a good video

The wet pumps last much longer.

It would be interesting to see what the approval paperwork is. Is it really a Major Alteration, on an N-reg? It depends on the reading of the words “basic change”…

Shoreham EGKA, United Kingdom

Why not invest your money in not having a vacuum system at all?

If you have a certain range of King autopilot, the legal ways to get rid of the vacuum driven KI256 are

  • EFD1000+EA100
  • G500+GAD43
  • G600

None of these are what I want to do. I already have very nice kit.

The other method is the very nice Castleberry electric KI256-lookalike which I already have on the RHS but this cannot be legally connected to the autopilot – and mine isn’t.

Admittedly one can tell when a dry pump is on its way out gradually (i.e. via wear) – by knowing what the vacuum indication should be (especially at idle when the wear has the most visible effect) and keeping an eye on it over time. What you can’t pick up is the sudden failure mode.

I wonder where the dry pump gets its oil supply. It certainly can’t come via the standard dry vac pump attachment point.

Last Edited by Peter at 19 Jun 21:23
Shoreham EGKA, United Kingdom

I wonder where the dry pump gets its oil supply.

What should a dry pump use oil for?

LSZK, Switzerland

I meant the wet pump

Shoreham EGKA, United Kingdom

It certainly can’t come via the standard dry vac pump attachment point

the accessory drive attachment point must have this oil line, which I guess is blocked when using a dry pump.
On the Airwolf internet site FAQ they say:

A wet vacuum pump, commonly called a wet pump, is a vacuum pump that is internally lubricated by engine oil. All engines since the age of time, already have the accessory cases drilled for a wet pump so no additional modifications are needed from the installing mechanic.

EDxx, Germany

It is certainly true that the “hydraulic pump drive” on my IO540-C4D5D does have an oil feed (see the backup alternator thread) but I have not seen such a feed on my vacuum pump drive which is otherwise identical (or would be if Lyco had bothered to put in the right drive shaft in there…).

Maybe the hole is there but is normally covered by the dry vac pump gasket. If so, that is quite a potential for an oil leak because the oil feed hole is only about 3mm away from the gasket edge so if that little section of the gasket breaks, you will get oil squirting out of there at the full gallery pressure of about 70psi.

Shoreham EGKA, United Kingdom

Just had a pump failure and the mech is recommending a dry pump as a replacement.

I’ve read up on the basic differences between wet vs dry pumps but I’d like to find out about the practical realities of operating a dry pump rather than the stock wet pumps on aircraft engines.

Does anyone have any practical experience and wisdom they’d be willing to share about this?

Today’s dry pumps have a window to inspect the vanes. That makes it possible to determine their condition and also not have a fixed replacement interval under EASA reg. I don’t think anybody really installs wet vacuum pumps anymore these days.

However the vanes also sometimes fail catastrophically for no obvious reason.

There is a bit of wet vac pump installation going on in the USA.

Shoreham EGKA, United Kingdom
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