Attitude indicator drifts off, and my new one is doing the same

I recently bought a new attitude indicator since I was having trouble with my previous one "drifting off". Now this new indicator is doing the same after only about 25 hours.

What happens:
- In both cases the issue first arose on a long flight, just doing straight and level. When I do frequent turns/climbs/descents it seems to encounter this problem less - though it still does drift.
- The symptoms were identical in both cases. First it drifted up and then eventually it starts to tilt sideways, etc. On subsequent flights it loses calibration faster and drifts up/sideways quicker.
- Occasionally it seems to bounce around and back into position and appears to "work" again, but it is obviously not an instrument I'd want to trust with any kind of reliability.
- This "new" indicator is actually used, but worked OK for the previous owner. It worked fine for me for the first 15-20 hours before this started occurring.
- Other vacuum instrument (DG) works great with no issue. The vacuum pressure gauge also shows a good reading the whole time.

I've included pictures to show what it looks like when it starts going crazy. Has anyone else encountered this issue, or have any suggestions? Since it's happened with 2 AI's I'm thinking about saying screw it and just buying a G5 replacement unit, but would like to avoid that if I can.

(I've also messaged my A&P but if I can help point him in a likely direction then maybe I can save a few bucks of labor time)

 

Is the vac gauge accurate? Checked the line for leaks or being pinched off or blockage?

 

Appreciate the suggestion. I haven't checked that (line leak or blockage), I'm not sure I'd know what to look for, but I can ask the A&P about that. I've done routine maintenance on the vacuum system replacing the filter, but that's about it.
Do you think that would mess up the DG too? Since that instrument seems to work without any issue.

 

If the line to the AI is the only thing messed up, it shouldn't mess up the DG. I would second the suggestion to check the vacuum system, as the pictures look just like the pictures I've seen of inadequate vacuum in the AI, whether that was localized or the whole system broke down.

 

This attitude indicator looks really old, the orange bank pointer and the blue sky color are faded (though the bank pointer could have been white originally). When was it overhauled? Does it have an off flag for low pressure?

 

If it's a leak in the line or the vacuum ain't sucking good, I'd think it would affect both. But if it's blockage somewhere down stream after they split off from each other it should only affect the one downstream from the blockage.

 

While theres always a possibility that 2nd used AI failed on its own, there could be a system issue that caused both to fail regardless if the DG didnt. At a minimum I would have your AP check the entire system to include the filter.

 

It is a little bit dated. On the installation docs I don't think it included an OH date, just a serial number and the tail it used to belong to. I'm sure I can get that information from him though. Should have asked for that before!

Sounds like the consensus is the vacuum system/lines, I'll have them work on a diagnosis from that. I just find it kind of unlikely that both the AI's would fail on their own with the same symptoms independently. Certainly possible, but probably a latent variable that I'm not directly seeing that's causing this.

Appreciate everyone's suggestions!

 

It does not have an off flag for low pressure. My previous one did not have that feature either.

 

You bought an old used AI with no idea how many hours on it. Sounds more likely you rolled the dice and lost. Gyros and vacuum pumps are the least reliable equipment in an airplane, you're lucky to get 1,000 hours out of a brand new one. If you're unlucky then it can be 300 or so.

 

Depending on how the vacuum gauge is plumbed in, you might have a vac leak or a blockage. If the gauge is connected, with both of its ports, across the AI, and it shows good vacuum, the AI is likely shot. I have seen vac gauges connected with only the vacuum port, and a plugged filter or collapsed air line will show good vacuum even though there's no airflow.

Vacuum pumps are quite reliable if one buys the Tempest or Rapco pumps with the vane wear indicators, and does the five-minute inspection at annual once it has reached 500 hours or so. Vanes wearing past limits is what breaks them 90% of the time. The other 10% includes the shattered carbon junk from the previous pump that was run to failure, sucked into the system by the stored vacuum in the instrument cases, and then sucked into the new pump. Every new pump comes with instructions warning mechanics about this, yet most will just bolt it on and shove it out the door. A few pumps fail when their plastic drive coupling is way too old and fails. 6 years is the limit. It's plastic so it fails if the pump seizes, to save the engine's gearing. Plastic subjected to hours of heat gets brittle and weak. And some pumps fail because the mechanic wasn't careful to keep cleaning solvent out of its open drive when washing the engine. That solvent gums up the carbon vanes and rotors, loading the rotor until it breaks. Tempest has that licked, too.

Owners wouldn't dream of running the airplane out of fuel, yet they're willing to run vacuum pumps, alternators and magnetos to failure. Go figure. All of those items are listed in airframe manufacturer's maintenance stuff as recommended inspections around 500 hours.

Gyros last a long time if the airplane is kept in a heated hangar so that the gyros aren't starting up cold. That wrecks them. The lube in those tiny, precise bearings is stiff in the cold and won't lube properly. If the vac filters aren't maintained, or if people smoke in the airplane, gyro failure will be normal.

 

Long shot here, in the Bo last year we failed the static test and it took awhile to find a set of pressure switches for the auto extend for the gear that were the cause. If you have a retractable with any sort of vacuum sensing look at those systems. BTW there was a pair of switches luckily we choose wisely and got the correct replacement for the one that had failed.


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I am betting that you have never replaced your vacuum hoses and they have interior deterioration which are contaminating your new instruments. If they are over 20 years old, replace them. Took 4 feet of hose to replace mine and you are permitted to change them out yourself.

 

Honestly, if you're on the fence about replacing it with a G5, I would say defiantly go for it! I have 2 G5s and would never go back to a vacuum system. Occasionally I fly a friends 172 with a vacuum system. It always reminds me how much I love the G5s. If you fly IFR its so much better!

 

One has to watch that the new hoses are installed carefully so that they don't interfere with any flight controls. I have found too many vacuum hoses limiting travels or snagging on the controls, something that could turn out very badly. I've also found wiring doing the same things. It would seem that a lot of folks replace or repair stuff under the panel without making sure that they haven't created more serious problems than they fixed.

 

This is exactly my situation - I've been watching my VP decline in performance, and worked with my AP yesterday to change it prior to failure.

I have 20+ hours of flying predicted for May, and don't want a pump failure to be included. It'd happen at the worst time, most assuredly.

 

Yes. I did one hose at a time and cut each piece to the exact length of the old one. Hard to screw up if you do that.

 

I would think that in the process of replacing the indicator the mechanic would have blown out the plumbing and installed a new filter. No?

 

My money is on the "new" attitude indicator being junk. It is faded as hell and is decades old. It does not appear it has been overhauled in decades either. It came out of an airplane because someone didn't want it anymore. It was needing to be overhauled or thrown away when it came out of whatever airplane it was in.

Anyhow - moving forward - either buy a new one or a freshly overhauled one from a reputable shop with a warranty. This is a complicated sensitive instrument with moving parts. I'm not surprised that it crapped out after 25 hours. I'm more surprised you even got that many hours out of it.

When I was running a flight school, I exclusively used these folks for gyro overhauls: http://www.flyaqi.com/. I never had a problem (though I've heard others have). They'll overhaul most attitude indicators for $400-ish, and it'll look brand new and not faded when you get it back.

 

AQI will evaluate/test your instrument for free. Talk to Hai.