How long should an AI last?

Chip Sylverne

Chip Sylverne

Final Approach
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Quit with the negative waves, man.
We installed a rebuilt AI in the Cherokee about 2 years ago. Another one went TU about 3 years before that. This one is already occasionally getting the "leans", about 5deg left and slightly nose down about 30 mins into flight.
Vacuum is good. Bird flies about 10 hrs a month.
Are we just unlucky with the instruments, or is there something else we can look for?
 
Chip Sylverne said:
We installed a rebuilt AI in the Cherokee about 2 years ago. Another one went TU about 3 years before that. This one is already occasionally getting the "leans", about 5deg left and slightly nose down about 30 mins into flight.
Vacuum is good. Bird flies about 10 hrs a month.
Are we just unlucky with the instruments, or is there something else we can look for?
Click to expand...

Who made it? I've had much better life out of SigmaTek than R C Allen.
 
I think it's a Sigma Tek, but I'll have to check.
 
One of the things that destroys gyros is old rubber hoses going to the instrument. After several years these hoses deteriorate and a black "dust" begins going into the instrument and causing premature failure. If your instrument hoses are the rubber type and more than 5 years old they need to be changed. Chances are when the instrument is opened it will be contaminated, along with the DG.
 
Agree with R&W, you have deteriorating crap in your vacuum system downstream of the intake filter. Get that addressed.
 
My experience is that good AI's last 1000-2500 hours. If yours rolled over after only two years, it's probably not the AI's fault, and the problem probably lies in the vacuum system. Even if it turns out the AI is dying, I'd suggest digging into the vacuum system to see if that's what actually killed both this one and the last one.
 
Our DG is an old barrel type, been going strong since the 40's.
Going to go poke holes in the air tomorrow, so I'll check the logs to see if the vac system has been addressed in recent history.

Thanks
 
Ron Levy said:
My experience is that good AI's last 1000-2500 hours. If yours rolled over after only two years, it's probably not the AI's fault, and the problem probably lies in the vacuum system. Even if it turns out the AI is dying, I'd suggest digging into the vacuum system to see if that's what actually killed both this one and the last one.
Click to expand...

We replace vacuum hoses every 5 years or so. And even then some of them show signs of deterioration. In a hot climate, parked outdoors, both hoses and gyros will suffer. In a cold climate, the fine oil in the gyro bearings will gel and get squeezed out, and the bearings run dry and wear out prematurely. We find that a heated hangar saves more than engines--it saves radios and instruments too.

The attitude indicator has a gyro wheel inside a case, and the case is gimballed so it can rotate in two axes within the instrument's outer case. Vacuum is applies to the outer case, and air is let into the aft gimbal bearing, where it travels through a hollow gimball frame into the inner case and spins the gyro wheel. It then leaves the wheel case through four tiny holes at the bottom, facing outward horizontally: one forward, one aft, one each left and right. Two pendulum valves hang from pins in the wheel case and are set up so that air is restricted or allowed free flow from each of those four hose. The purpose is to erect the gyro when it starts and to keep it erect; closing one hole will create a reaction from its opposite, and precession causes the wheel case to erect in the desired direction.

Now, if bits of rubber or rubber dust or the solids in tobacco smoke are allowed into the case, they will accumulate around and in those tiny jets and clog them. Clog one, and the gyro will go off level as the OP's has.

Dan
 
Dan Thomas said:
We replace vacuum hoses every 5 years or so. And even then some of them show signs of deterioration. In a hot climate, parked outdoors, both hoses and gyros will suffer. In a cold climate, the fine oil in the gyro bearings will gel and get squeezed out, and the bearings run dry and wear out prematurely. We find that a heated hangar saves more than engines--it saves radios and instruments too.

The attitude indicator has a gyro wheel inside a case, and the case is gimballed so it can rotate in two axes within the instrument's outer case. Vacuum is applies to the outer case, and air is let into the aft gimbal bearing, where it travels through a hollow gimball frame into the inner case and spins the gyro wheel. It then leaves the wheel case through four tiny holes at the bottom, facing outward horizontally: one forward, one aft, one each left and right. Two pendulum valves hang from pins in the wheel case and are set up so that air is restricted or allowed free flow from each of those four hose. The purpose is to erect the gyro when it starts and to keep it erect; closing one hole will create a reaction from its opposite, and precession causes the wheel case to erect in the desired direction.

Now, if bits of rubber or rubber dust or the solids in tobacco smoke are allowed into the case, they will accumulate around and in those tiny jets and clog them. Clog one, and the gyro will go off level as the OP's has.

Dan
Click to expand...

Excellent info Dan, thanks. We're based FDK in a hangar, but no heat.

There must be some sort of inlet filter on the outer case of the instrument. Is that an owner-serviceable item, or does it require a shop?
 
If replacing the inlet filter isn't listed as one of the preventative maintenance items, then the owner can't sign it off. And I couldn't it in App A of part 43.

fwiw - I'm just an owner, not an A&P.
 
Chip,

I also found out (the hard way) that there is a BIG difference in the quality of intrument overhauls from one shop to another. Shortly after I bought my 182, I did a panel upgrade and pulled every instrument (including MP gage & Tach) and sent them out for overhaul. This was in April, 2004. We also replaced all the lines and the filter.

Every instrument in my plane has been subsequently overhauled and/or replaced again. My ASI failed in less than a year, was replaced under warranty and then again failed in less than a year. Two other instruments lasted for less than two years. Two more failed in less than three years. The AI lasted the longest. I replaced it last summer and replaced all the hoses again. The filter gets changed every other year at annual.

Like I said...I found out the hard way that Aircraft Quality Instruments was NOT the place to go. I've since had much better luck with Mid-Continent. Both shops are in Wichita.

YMMV
 
Chip Sylverne said:
Excellent info Dan, thanks. We're based FDK in a hangar, but no heat.

There must be some sort of inlet filter on the outer case of the instrument. Is that an owner-serviceable item, or does it require a shop?
Click to expand...

There should be a filter mounted on the inside of the firewall or someplace handy to the instruments. But the deteriorating hoses are between the filter and gyro, so replacing the filter won't help if those hoses are shot. Perhaps the filter is rotting and sending chunks of paper into the instrument.

Some really old (big) gyros had a screen or maybe a felt filter on their inlets. Don't see those much anymore.

vol3-44.jpg


To understand the gyro erection mechanism: http://www.faatest.com/books/IFRH/4-4.htm


Dan
 
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They now make and install in-line filters for the inlet of the instruments - this filter prevents the device from getting contaminated when another gyro or the vaccum pump disintegrates. They also protect against hose rot.

I've had good luck with The Gyro House for rebuilds and exchanges.
 
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