Mystery AI Failure

M

Mtns2Skies

Final Approach
Joined
Jul 12, 2008
Messages
5,622
Display Name
Display name:
Mtns2Skies
When I bought the plane my vacuum AI would periodically die off to the side in flight to return in a minute or so. It was a relatively rare occurrence, but I thought since I was doing my IR training shortly I should get a new one put in. So I did and it has worked fantastically for about 10-15 hours. Yesterday however, it tumbled while taxiing, and in flight exhibited the same symptoms as before. My suction gauge reads around 6" (of whatever units they use) in flight consistently. I'm not sure what could be going on?

When I sent my old AI out in exchange the shop called me to tell me it was full of sand somehow. So when I got the new one installed I got all the lines cleaned thoroughly and a new suction filter put in. It's also worth noting that my DG does not precess at all, it's pretty much solidly dead on.

I have the auto-vac 2 STC (suction is taken from the engine intake manifold) and a wet pump - both of which should be pretty reliable. What could be going on to cause my AI to be acting this way?
 
Last edited:
Crack in a line or fitting, cracked water catch bubble (I think your Cessna has one, mine does, it’s a POS), missing or ripped filter (down on the floor in mine, stupid place for it) or housing, pinched line, all sorts of things.

We’ve had more than one of those above on ours. Vacuum gauge doesn’t read what the Instrument is getting if other crap past it is broken or leaking.

If the last one got sand in it, I’d be looking the filter assembly and lines running to it near the floor. Sucked it right up when someone took your airplane to a beach, I bet.

Or is that Beech? :)
 
I'd replace all the vacuum/pressure lines, not try to "clean" them. Never heard of that. Also, gyro instruments have a life of their own. It could just need an overhaul, even if relatively new. Gyro instruments HATE being tossed around in the mail.
 
kgruber said:
I'd replace all the vacuum/pressure lines, not try to "clean" them. Never heard of that.
Click to expand...
It was an A&P from a shop on field with a very bad reputation. They lost my hangar neighbor's Cirrus. Anyway that's a different story. He ran water through all the lines looking for sand and then dried the lines back out.

I'm going to a very good A&P in a couple weeks to get my new (thanks OSH) engine monitor installed. I think I'll have him do that... just replace it all. The lines/fittings are probably relatively cheap.
 
Oh! I see you own a Cessna 180! Nice................I delivered several 180/185s from Wichita. One of the my own.

Also, be thinking Garmin G5. Although, I'm sure you already are.
 
Mtns2Skies said:
I'm going to a very good A&P in a couple weeks to get my new (thanks OSH) engine monitor installed. I think I'll have him do that... just replace it all. The lines/fittings are probably relatively cheap.
Click to expand...

Cheap as far as airplane junk goes, for sure. I almost thought the bill for the parts and labor on our static system repair was missing a zero, being used to the other airplane bills we get. Ha.
 
6 sounds a bit on the high side. I seem to remember a range of 4-5 was nominal.
 
My vacuum gauge was reading high last year. It was saying 5 when it was really only 4 which is not enough to get a good spin according to the guys in Rudy, AR. We upped my suction and my AI has been spinning better. No sand in mine though.
 
Got the plane back from Mx. My A&P replaced all of the vacuum lines and fittings and now everything is good to go. My suction gauge now reads rock steady at 5 rather than bouncing around between 6 and 7. My AI is solid and definitely doing better. Easy and relatively cheap fix!

My A&P said he inspected the old stuff and didn't see any obvious leaks or issues, but oh well, it's fixed now!
 
Mtns2Skies said:
When I bought the plane my vacuum AI would periodically die off to the side in flight to return in a minute or so. It was a relatively rare occurrence, but I thought since I was doing my IR training shortly I should get a new one put in. So I did and it has worked fantastically for about 10-15 hours. Yesterday however, it tumbled while taxiing, and in flight exhibited the same symptoms as before. My suction gauge reads around 6" (of whatever units they use) in flight consistently. I'm not sure what could be going on?

When I sent my old AI out in exchange the shop called me to tell me it was full of sand somehow. So when I got the new one installed I got all the lines cleaned thoroughly and a new suction filter put in. It's also worth noting that my DG does not precess at all, it's pretty much solidly dead on.

I have the auto-vac 2 STC (suction is taken from the engine intake manifold) and a wet pump - both of which should be pretty reliable. What could be going on to cause my AI to be acting this way?
Click to expand...

Mine did the exact same thing. We went through 2 rebuilt AIs from AQI. We replaced all the lines, etc. Turns out the vacuum pressure was way too high (AQI told us this and sure enough they were right) at like 6". We adjusted it down to 5" and have had no problems since.
 
I wonder if one of the vacuum line hoses was collapsing? If a line collapses, you'll get an excellent vacuum reading because no air is getting in beyond the collapsed hose, but the instruments will be starved of air. A long time ago, I discovered someone before I bought my plane had done a "do-it-yourself" repair of a vacuum line with a hose material that was not vacuum-rated. That line could easily collapse under vacuum, and it was a wonder anything worked.
 
You must log in or register to reply here.
Back
Top