Weird problem with attitude indicator today

Rykymus

Line Up and Wait
Joined
Dec 24, 2014
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647
Location
Allen, TX
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Display name:
Rykymus
Looking for advice before I turn the problem over to my A&P.

Flying at 12000 MSL today, my AI started gradually indicating a dive, eventually reaching maximum downward deflection of the AI. Of course, I was flying straight and level, with no change in altitude, airspeed, or vertical speed. Also, my DG (which is vacuum driven) continued to function properly. (My Turn Indicator is electric.) Vacuum remained normal, and even increased when I turned on the backup electric pump. (Although doing so did not correct the problem with my AI.)

It was a CAVU day, so I finished my flight (using the AHRS on FF/Status) as a backup. (Not legal, but not even required considering it was VFR.)

Funny thing is, as I descended, the AI eventually righted itself and was working normally by the time I got down to about 8,000-9,000 ft. I conducted my business at my destination, then flew back home. Same thing. Everything was fine up to about 9,000 MSL, and then it started indicating a dive, the angle of which increased as I climbed. (I had to climb, to get over mountains.)

Is this a leak in the vacuum hose feeding the AI? When I got the plane back from annual a couple months ago, there was something rubbing when I pushed the yoke in during my check flight. They found a vacuum hose that was not secured properly that was hitting the yoke. Could that be related?

Any advice would be appreciated.

FYI: The AI was replaced about 2 years ago with a new rebuilt unit.
 
Loose vacuum hose makes sense. The air got thin enough at 9000 that it started affecting the AI. The DG probably doesn't need as much air being sucked through it as the AI to operate normally. That's my guess
 
Definitely something with the vacuum. Remember the instrument wants to see a differential between the IN which is just vented to the cockpit via (hopefully) a filter and the OUT which is whatever your pump is producing. Could be a wonky pump, a colapsing/kinked line, a hole in the system somehwere, etc....
 
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