B737 Vibration Indicator
Why on the B737 -3/4/5 series during engine start does the vibration indicator suddenly rise to about 3/4 units and hang there, then suddenly drop shortly after engine stable?
Occasionally it drops in one unit stages down to it's normal level which almost looks like a self test function.
I've been told that it could be the N1 blades shaking themselves into an equilibrium point as they spin up, or a self test function.
Neither seem too likely to me. Is there a definative answer out there?