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I'm assuming this is being noticed because of the recent ANA plane aerobatics : http://www.youtube.com/watch?v=aunUFwyxsF0 (spoiler : plane landed safely).

that was centrifugal, not gravitational force

Centripetal. Centrifugal force is imaginary.

Centrifugal is imaginary in a non-inertial reference frame (as the case is here), but is a "real" force otherwise.

There was a discussion on this exact subject recently on Reddit (/r/askscience) which might be interesting:

and the entry on Wikipedia: