DG and AI gyros not spinning up w/main switch on

M

mikehoover

Filing Flight Plan
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Jan 11, 2008
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Mike Hoover
I was hoping someone might be able to enlighten me as to why, after having a battery replaced (Cessna 172H), my DG and AI gyros do not spin up when I turn on the main switch. The gyros seem to operate okay once the engine is running, then after I shut down, the gyros spin down, even with the main switch still turned on. Apparently they no longer respond to battery power, but only generator/altenator power. Any ideas?

Thanks!
Mike
 
Your DG and AI aren't electrically powered. They're driven by a vacuum pump that's driven by the engine.
-harry
 
mikehoover said:
Any ideas?
Click to expand...
Nothing conclusive, but look for a wire that was connected to the battery terminal (most likely the positive terminal) that was not reconnected when the battery was reinstalled.

A guess, but the first instinct is to check over what was recently completed.

-Skip
 
Mike,
They are working fine. The DG and horizon are vacuum powered by the engine driven vacuum pump. The only thing that should "run" with the master on is the turn coordinator, which is electric. If you're used to hearing something run or hum with the master on, and now you don't hear that your turn coordinator may have quit.
CJ
 
PT13Pilot said:
Mike,
They are working fine. The DG and horizon are vacuum powered by the engine driven vacuum pump. The only thing that should "run" with the master on is the turn coordinator, which is electric. If you're used to hearing something run or hum with the master on, and now you don't hear that your turn coordinator may have quit.
CJ
Click to expand...


CJ is right. AI and DG should not turn on with the master switch as they run only when the vacuum pump is on which means the engine has to be running. What you normally hear is the electric turn coordinator.

Is your TC running ok? Does the flag go to black to indicate that it is running up to speed?
 
Groucho: "Doctor! Doctor! After surgery will I be able to play the violin?"
Doctor: "I would think so."
Groucho: "Good, because I could never play it before!"

So, did they spin up when the master was turned on before the battery was replaced?

Y'see, it is possible, though unlikely, that Mike's plane has electric AI/HI. If so, further investigation is merited. If not, then Mike has learned about the vacuum pump that powers his AI/HI.
 
Thanks for all of the replies. I believe it must be the Turn Coordinator that is not spinning up when I put the master switch on. The AI and DG are indeed vacuum driven (probably should have known this already :) ) I'll have to get the TC checked to see if it is broke.

Regards,

Mike
 
mikehoover said:
Thanks for all of the replies. I believe it must be the Turn Coordinator that is not spinning up when I put the master switch on. The AI and DG are indeed vacuum driven (probably should have known this already :) ) I'll have to get the TC checked to see if it is broke.
Click to expand...

I believe the other possibility is an avionics cooling fan... might want to just tell the shop that you used to hear something spin up and now you don't.
 
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