Attitude Indicator intermittent

Flyingfanatic

Flyingfanatic

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Flying Fanatic
The attitude indicator in our Archer indicated a turn during straight and level flight for about 5-10 minutes. It was fine for the 20 minutes before,it corrected itself and it was correct the rest of he flight.

The suction gauge indicated 5 psi and the HI worked properly the whole time.

The pitot static inspection was completed 11 months ago.

Is this an indication the AI is about to fail or is it an anomaly that we should just monitor?
 

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And I doubt the Pitot system was inspected at all. The periodic inspection for IFR and transponders only looks at the static system.

If you can believe the suction gauge, then it's either the AI or some connection to it (filter, tubing, etc...).
 
Time for an overhaul. I usually remove it myself and send to https://www.centuryinstruments.net/ or http://rudyaircraftinstruments.com/Home_Page.html. They do the overhauls at a flat rate that is significantly below other shops. If your vacuum hoses have never been replaced, I would do that while the instrument is being repaired. The hoses deteriorate on the inside and spew black particles that will wind up in your gyros. That will trigger another overhaul. You can replace the hoses yourself if you are licensed and own the plane. Be sure to sign off in your logbook:

"Replaced all Vacuum Hose inside cabin with Aeroquip AQ 306 3/8” I.D. braided low pressure hose. Hose conforms to MIL-H-5593."

You will need about three feet of hose. You will need an A&P to install the overhauled instrument and sign off.
 
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The vacuum gauge can read ok but an instrument might still be getting insufficient vacuum or airflow. It all depends on how the system is plumbed. Besides that, I've often found the central and relief filters totally clogged or otherwise shot. Annual inspections, indeed. More like drive-by inspections.

It would be a shame to overhaul or replace the instrument only to see the same problem still there due to a plugged filter or kinked hose.
 
Flyingfanatic said:
The attitude indicator in our Archer indicated a turn during straight and level flight for about 5-10 minutes. It was fine for the 20 minutes before,it corrected itself and it was correct the rest of he flight.

The suction gauge indicated 5 psi and the HI worked properly the whole time.

The pitot static inspection was completed 11 months ago.

Is this an indication the AI is about to fail or is it an anomaly that we should just monitor?
Click to expand...


Sorry all, I posted that before thinking. I know the AI isn’t part of the pitot static. Ugh.
 
MauleSkinner said:
Are you prepared to fly partial panel in whatever conditions you may be flying?
Click to expand...

There’s very little IFR flying in this airplane currently. I think I’m the only one and I won’t do it until this is fixed.
 
Do you want it fixed right, or fixed right away? For right away, I would overhaul the instrument and replace the hoses. For "right," I would put in two G5's in place of the AI and DG and remove the vacuum system altogether. It would cost an order of magnitude more, but you would never have a vacuum system failure again. I may be a little biased, since I had a vacuum pump failure followed by the new overhauled pump working fine during a ground run but dying on the first flight.

Given the intermittent nature of the problem, I don't know if I would trust just doing the hose replacement and then seeing good results as proof that the instrument is working. It could decide to take a break from glitches until you are flying over unlit terrain at night or in a cloud, then go wacky on you again. The gyro in the AI spins pretty fast. Spinning things wear out and have a MTBF. Sometimes you get caught on the early side, sometimes on the late side, otherwise it would just be TBF.
 
Look. Check the vacuum system. An AI that won't erect is sometimes suffering low airflow for whatever reason. Pressure and flow are two different things. If you don't have the flow, the erecting valves on the gyro case inside the instrument don't have enough thrust to get the thing upright.

The vacuum gauge is normally plumbed across the inlet and outlet of one of the gyros. It reads the pressure differential on THAT instrument. If the hose to the other instrument is kinked or is coming apart inside and clogging up, that second instrument ain't gonna work right no matter what the gauge says.

And some gauges were only plumbed to the vacuum side of the system, with its atmosphere port left open. That gives an even less useful reading.

Did this airplane have a vacuum pump failure a while back? When pumps fall apart inside, the vacuum in the instruments (there's lots of volume there, and it acts as a reservoir) will suck carbon debris back into the system and into the instruments. If it gets into the gyros you have big problems. Even if it doesn't, it's laying in those hoses and the new pump promptly sucks it into itself and shortens its life or trashes it right away.

I wish I knew why people don't buy the Rapco or Tempest pumps that have the vane wear inspection ports. If that inspection is done at annual, the pump gets replaced before it blows up and costs you money or introduces risks in flight. The inspection takes five minutes. It maximizes the life of the pump without running it to failure. It's a no-brainer, yet we constantly read of vacuum pump failures on this forum.
 
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