"Matt Young" wrote in message
ink.net...
[...]
The AI works on the principle that a spinning gyro will remain in the
plane in which it spins, right? If taking a long flight, ie
transatlantic, why does flying around the curvature of the Earth not
affect this.

As Paul said, the AI has a mechanism in it that pulls the AI straight up and
down relative to gravity.

This mechanism is always active, even in turns, and so you can theoretically
remain in a coordinated turn long enough to create a significant error in
the AI. In practice, you almost never remain in a coordinated turn long
enough for it to be a problem.

A picture would be better to explain how it works, but basically there are
air ports at the bottom of the indicator that are normally covered by a
little "door" that hangs from above the port. The same vacuum that runs the
gyro pulls air through the ports.

Whenever the indicator is not straight up and down with respect to gravity,
the doors on each side open to the extent that the indicator is tilted. The
ports are configured so that a port that opens pushes the indicator in the
correct direction to right the indicator back to straight up and down (this
includes that they are positioned to account for gyroscopic precession with
respect to the force they create).

This mechanism is the same thing that brings the AI to vertical after you
start the airplane. Look at the AI before you start the engine; you'll
almost always find that it's showing a bank and possibly also pitch. After
you start the engine and the vacuum pump is working, the righting mechanism
brings the AI to the correct position.

Pete