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Death of a vacuum attitude indicator


RobertGary1

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Over decades I've had both gyro and solid state attitude indicators fail. The understanding has always been you can't really predict when they're going to fail. But maybe you can??

About a year ago I noticed my vacuum attitude started a high frequency vibration. It still otherwise worked well. Since its really a third attitude indicator for me and not required for ifr flight I didn't do much about it. Then the other day it tumbled and spun.

Doing some research I think I know why. A build up dirt and a loss of lubrication slowly adds friction to a spinning gyro. As a result the gyro spins slower for the same amount of applied vacuum. The auto erection feature of the gyro works by throwing off some of the vacuum which works to put the gyro in straight/level flight. This force is very strong when the gyro is slow and spinning up resulting in the quick erection we see. In cruise this force is resisted by rigidity in space so its much less, but still helps to right the gyro (if you fly in a sustained turn long enough your mechanical gyro will slowly return to level). 

Well apparently this wiggle I noticed was the force of the erection feature slightly fighting against the rigidity of the my gyro because the RPM of my gyro had degraded.

Since we have no direct way to measure gyro RPM we don't know its failing but the force of the erection feature starting to gain over the force the the spinning gyro's rigidity essentially is telling you that.

Thoughts?

-Robert

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1 hour ago, PT20J said:

Sounds reasonable. I’ve always thought slow erection was the symptom of the bearings going out but I seems logical that any unusual motion in a mechanical is a sign of something amiss. Probably @Jake@BevanAviation knows.

Heck, I've always thought it a symptom of old age:D

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The usual expectation for gyros failing…

Their bearings wear…. Slowing them down…

They could also be getting dirty…


How they fail… probably doesn’t get covered very much… outside of IFR 101….

 

So… if you see any funny behavior… check the gyro’s age/hours….

It may be time for gyro OH…

PP thoughts only, awaiting for our gyro gurus to arrive….

Best regards,

-a-

 

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1 hour ago, carusoam said:

The usual expectation for gyros failing…

Their bearings wear…. Slowing them down…

They could also be getting dirty…


How they fail… probably doesn’t get covered very much… outside of IFR 101….

 

So… if you see any funny behavior… check the gyro’s age/hours….

It may be time for gyro OH…

PP thoughts only, awaiting for our gyro gurus to arrive….

Best regards,

-a-

 

Exchange gyro already ordered. Will be here tomorrow. But since it’s not required for ifr won’t affect my trip next week if I don’t get it jn. 

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Several attitude indicators I’ve had fail did give very subtle signs a month or more in advance.  Each time was either slow erection or not exactly level in the first couple minutes. One had warned me for a while, then suddenly in flight turned and parked itself showing a 90* bank. Two began giving erroneous indications in flight; beginning a descent caused a shallow right turn, leveling caused a shallow left turn. That was fun, getting vectors for a nighttime ILS until I figured out what was happening. Another time a fresh overhaul had one of the auto erection pendulum vanes stick. It would work properly then show a left turn for a while, then level out for a while…. Shop said it was a speck of dust.  Like so many things, they tell us in advance if we pay attention.

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I have a couple of old gyro capsules on the shelf that I play with occasionally. I doubt they slow down. The turbine wheel spins to match the airflow through it. Bearing drag tends to make them precess. If you are holding the spinning capsule in your hand you can see and feel it. The erecting mechanism will counter the precession until it can’t. This is why they seem to fail all at once. They have been getting worse over time, but when the finally overcome the ability of the erecting mechanism to right them they fail.

I took apart an electric gyro once, it actually had a little turbine wheel that created air flow for the erecting mechanism.

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As air is sucked in thru the cockpit, back when more people smoked they used to fail more often, and taking one apart from a smokers airplane was nasty.

So I believe filter condition has a lot to do with it, I’ve seen a few of the foam sock ones on vacuum adjustment assemblies crumbling because they hadn’t been changed in years

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Slow erection or excessive noise from the indicator are always signs that the bearings are worn and eventually the rotor speed will be to slow to maintain rigidity.  Try to make note of the gyro orientation as the gyro is spooling up, a fresh overhaul always has a nice shake out of the indicator as the rotor speed is increasing.  When the bearings get old and stiff you will see a reduced shake out or none.  If you witness the AI no longer shaking out during the initial start of the aircraft, that is a sign that the indicator is getting worn.  Always replace the filter when replacing the AI.

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