The International Cessna 170 Association

So how does an A/P IA test old style vacuum artificial horizon without flying the airplane?
C170B has dual venturis.

These vacuum instruments have long been known to have a slightly extended time for erection.
Especially if cold.

Robert N Buck had advise about the time factor in his book WEATHER FLYING.

BUT the intent of this post is to learn how would you test the AI without flying the airplane?
Really how?

You have to apply a metered vacuum within the limits of the gauge to the gauge. This, and the knowledge how to use it, is what instrument shops have on hand and most others will not. And, in theory, the instruments shops also have the performance data for the gauge they are testing so they know whether the gauge is working correctly or not.

MoonlightVFR wrote:So how does an A/P IA test old style vacuum artificial horizon without flying the airplane?
C170B has dual venturis.

These vacuum instruments have long been known to have a slightly extended time for erection.
Especially if cold.

Robert N Buck had advise about the time factor in his book WEATHER FLYING.

BUT the intent of this post is to learn how would you test the AI without flying the airplane?
Really how?

Grady, you can always put a shop vaccum exit in front of the venturiis. (edit) or place the suction hose of a shop vacuum at the rear of a venturi)
As for erection time... I think we all have that problem when we're cold.
My AN Gyro is erect by the time I'm over the departure end of the runway or by 200' AGL every time.

Grady
Expanding on the shop vac idea, you could direct the suction side of the shop vac thru your aircraft vacuum regulator and check the gyro in-place in the plane. Don’t know what level of vacuum a shop vac will produce but it would be interesting to find out. I bet it puts out much more than a venturi. Start out with the regulator backed all the way off and slowly adjust it up to normal operating pressure. Then you can turn it off and let the gyros spin down then turn the shop vac back on for instant suction and see how long it takes to spin up under “normal conditions”. Bearings need lube and I bet it’s been decades since they had any.

How long has it been since your aircraft instrument air filter has been changed? Some of the old A-N gyros had a screen on the back but no real filter, might check that too. They can fill up and restrict flow but it would have to be real dirty to affect much, in my opinion.

I used the same general idea to leak test and calibrate a NOS oil pressure gauge for my C180 using a differential pressure compression tester. It worked like a charm.

There used to be a requirement to be able to individually check each vacuum instrument. Does your 172 have this valve George?

That is interesting, I have never seen that kit installed on ANY 172 I have ever flown. To me, that would create a vac tubing nightmare behind the panel. Thank goodness Service Kits are optional.

I've never seen this in ANY aircraft, is it possible this requirement was later canceled ? This seems a vacuum system gauge should be enough, not one for each instrument.

Ditto here. First I've heard of it. But I was also late learning that girls could be FUN.... I think I was almost seven or possibly even seven and a half...