# Where is the inclinometer “ball” in a knife edged turn?

Discussion of steep turns has lead to a question about where the inclinometer ball would be if the plane rolled to 90 degrees. Also, would there be any difference if the plane simply rolled to 90 degrees, rolled to 90 degrees and maintained altitude in a straight path with rudder, or rolled to 90 degrees and executed a "knife edged turn?

Does the ball become irrelevant at these attitudes?

As it was pointed out in the other thread, the ball just rolls around inside a curved glass tube. It doesn't know what the angle of bank is, or anything other than what direction gravity, and/or centrifugal force is moving it. Therefore:

At 90 deg AOB in coordinated flight at positive G, the ball would be centered. The aircraft in this case would be in a descending spiral.

At 90 deg AOB in a "knife edge" straight pass where altitude is maintained with rudder, gravity will pull the ball towards the earth. Same thing if you were able to make a knife edge turn holding altitude with rudder.

• It isn't possible to be in coordinated flight at 90 degrees angle of bank, is it? – Fred Larson Jan 14 at 18:00
• Why not? You can be in coordinated flight inverted. I didn't say you could do it while maintaining altitude. – Michael Hall Jan 14 at 18:01
• Have you seen the video of Bob Hoover pouring iced tea while doing a barrel roll? – Michael Hall Jan 14 at 18:07
• Yeah, I was thinking too narrowly. – Fred Larson Jan 14 at 19:47
• I think if you are at 90 deg and still pulling, there is zero vertical lift vector so you are kind of in a ballistic spiral. – John K Jan 14 at 21:31