B737 Vibration Indicator

[Sink Rate]

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?

[Al Weaver]

Neither answer above sounds right. I suspect that like other engines it is due to a critical shaft speed as the engine rotors spool up and the oil pressure comes up.

You probably don't get it on the way down since the bearings have lots of oil under pressure from running.

[rr892igw]

Not that I'm sure,it seems to be related to the EGT.The higher vibration always occur at the peak EGT and drops as the EGT comes down.
Any expert can add to this? :confused:

[Redpanda]

Unstable combustion...... during light-off, the flame front in the combustion chamber starts from the ignitor plug and tries to spread out. At low rpm's during the start cycle, the air flow and fuel pressure are low enough that the flame front sort of bounces around within the combustor and cant establish itself. Thats the low frequency rumble heard during the start. As rpm increases, higher air flow and better spray patterns from the burners allow the flame front to settle, and the start cycle smooths out to stabilised idle.