Discussion
A jet transport is trimmed in level cruise. The pilot pulls the nose up slightly, returns the column to its trim position and holds it there, then watches the instruments for two minutes.
Which behaviour is being demonstrated?
Answer: C
Returning the column to trim and holding it is the classic phugoid excitation: the aeroplane leaves trim with a small speed error at essentially trimmed incidence, and then wanders in speed and height over a period of a minute or so. A short period would need a sharp doublet on the column and would be finished long before the two minutes are up. Spiral divergence is lateral, requiring a bank disturbance and showing itself as a slowly tightening turn. Neutral static stability would show no tendency to return at all, whereas the oscillation itself proves the aeroplane is statically stable.
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