[quux]

Private pilot here. Mechanical attitude indicators don't need to be reset on level ground, when they first start spinning they're almost always tilted but once they spin up they level out on their own. Because they're driven by vacuum from the engine, sometimes they don't fully spin up and level out until the engine's being run up or takeoff.

I've always wondered how the attitude indicator self levels, and the OP's explanation sounds plausible. Next time I go flying I'll have to spend a few minutes in a constant rate turn to see if I can make the attitude indicator drift like he says it should.

Edit to add, here's a link to a youtube video showing the inner working of an AI, including the pendulous vanes, neat:

http://www.youtube.com/watch?v=KUSklh3MKtA

[brendn]

Yeah, I kind of glossed over the vacuum aspect of the attitude indicator because I don't understand all its intricacies. The video pretty much confirms my understanding of how the gyro is kept spinning. The original author's description of air puffs sounds a lot like how the filtered air is ducted in to the housing and vented off in quadrants. I suspect that has more to do with maintaining the gyroscopes precession (http://en.wikipedia.org/wiki/Precession) than leveling the gyro in mid-flight.

I've heard that during prolonged banks or pitches, the AI can eventually find a different "level" than the true horizon. I'm trying to find more info into that phenomenon though.

In the mean time, I've found this article that seems to be talking about the same accuracy and drift issues as the original post, but explains the issues with more clarity and also includes sensor graphs and code samples: http://myahrs.wordpress.com/2012/04/24/turning-errors-contin...