Venturi driven AI +DG working poorly

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Velocity83

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Loopadoop
Hello everyone, first post here so please be nice.... :)

Im a student pilot and have been lucky enough to be loaned an aircraft from a good friend to learn and save $$$

Recently i have been noticing the DG and today the AI totally not working correctly. These instruments are ran by the double venturi vacuum system as I'm sure you all know.
I looked at the suction indicator and noticed that I'm getting around 3 to 3.5, should this be enough to correctly run those instruments? If not is there anything we could do to improve this?

Any info and knowledge would be wonderful.

Thanks in advance.
 
What does the POH say on the Vac? I am looking for something north of 5 on my Archer. 3 seems a bit low to me.
 
So there is no actual vacuum pump, just a venturi mounted to the air frame? In that case they never work right on the ground...and they sort of work in flight.

What year airplane and model is this? What problem precisely are you having with the DG and attitude indicator? Be as descriptive as possible.
 
They should work fine in flight.

Most old GA planes flying today with or without vacuum pumps also have old hoses so hard they leak at the fittings and over the years the regulator was adjusted to compensate until one day there just isn't enough vacuum anymore. When the hoses get replaced and the regulator has to be tuned the other direction.
 
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Ahh,,.....so it's one of the old Cessna styles mounted on the right right just behind the cowl? No actual vac pump, correct?
 
3 to 3.5 inches of suction isn't enough to run most gyros. Typically 5 inches +/- 0.4 is required. I would suggest having a mechanic check the condition of the venturis and check the systems for leaks. Note that with a double venturi system, the two circuits (AI and HI) should be independent, so unless there is a system selector for the vacuum gauge, what you see there may be the suction in only one of the two systems.
 
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The double venturis on my Stinson worked fine.

Just set the runway heading once you're lined up and go.

To your question
Leaks, dirty filter, dirty Venturi, and/or gyros need service.
 
Ron Levy said:
3 to 3.5 inches of suction isn't enough to run most gyros. Typically 5 psi +/- 0.4 is required.
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True, but it is important to know that most gyros will not work with a Venturi system.

I don't recall the exact minimum at the moment, but the old AN gyros do not require as much vacuum to run properly.
 
Ron Levy said:
3 to 3.5 inches of suction isn't enough to run most gyros. Typically 5 psi +/- 0.4 is required. I would suggest having a mechanic check the condition of the venturis and check the systems for leaks. Note that with a double venturi system, the two circuits (AI and HI) should be independent, so unless there is a system selector for the vacuum gauge, what you see there may be the suction in only one of the two systems.
Click to expand...

It is NOT PSI.... It is inches... ".
 
3-3.5 inches should work. Note that it takes upwards of 15 minutes for things to get stabilized and that runs from the time you're airborne in venturis unlike the vacuum pump which pretty much sucks as soon as you crank the engine.
 
flyingron said:
3-3.5 inches should work. Note that it takes upwards of 15 minutes for things to get stabilized and that runs from the time you're airborne in venturis unlike the vacuum pump which pretty much sucks as soon as you crank the engine.
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Good point...:yes:
 
Check the inlet filter (opposite end of the system from the Venturi, usually mounted on the firewall on the pilots side) which can block flow without affecting the vacuum reading. Since the instrument operates on volume of flow more so than differential in pressure, this is a typical indicator of a clogged filter.
 
Thanks for all the great replies here. Lots to look into.

Im sorry i didn't state the model, its a 1956 172.

Again thanks for all the great info.
 
Henning said:
Check the inlet filter (opposite end of the system from the Venturi, usually mounted on the firewall on the pilots side) which can block flow without affecting the vacuum reading. Since the instrument operates on volume of flow more so than differential in pressure, this is a typical indicator of a clogged filter.
Click to expand...

May not be a filter, but great place to start.
 
Velocity83 said:
Thanks for all the great replies here. Lots to look into.

Im sorry i didn't state the model, its a 1956 172.

Again thanks for all the great info.
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Follow the advice above, eliminate any problems with the vacuum system first, if the vacuum system is performing fine then it's time to overhaul those gyros. It could quite possibly be a combination of all of the above problems on an air frame that old.

FWIW every venturi system I've used "works" but not to the point to where I'd be comfortable taking off into a quarter mile visibilty and 200 ft overcast. It takes time for those gyros to start working right so low IMC departures are something I wouldn't touch. There's a reason they don't use them anymore.

My Flybaby has a Venturi and a vacuum turn and bank installed and it does work once you're up to speed in flight. But if I do accidentally stumble into IMC Foreflight and my Stratus will be what saves my ass. Worst comes to worse I can do step descents to a runway on an approach plate and get down just as good as about any other airplane. The Stratus AHRS works great if you permanently mount the stratus in a fixed position like I've done in the Flybaby. Of course not going to help you right now in your training...
 
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jesse said:
FWIW every venturi system I've used "works" but not to the point to where I'd be comfortable taking off into a quarter mile visibilty and 200 ft overcast. It takes time for those gyros to start working right so low IMC departures are something I wouldn't touch. There's a reason they don't use them anymore.
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Two reasons. They also ice up.

Dan
 
Velocity83 said:
Im sorry i didn't state the model, its a 1956 172.
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Your friend is really doing you a great favor. The pre-60s 172 is one of the greatest aircraft to learn in that was ever built.

If you can get your friend to OK it, the fastest way to learn in a 4-place is to team up with another student at about the same level you are. You take a lesson with him/her watching, then (s)he takes a lesson with you watching. The learning curve gets VERY steep VERY quick when you see all the mistakes without having to correct for them yourself.

Jim
 
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