As olympus engine shaft was longer than other jet engines, the cooling down was not homogeneous along this shaft.
This caused a slight bowed deformation of the shaft.
This is the reason for the presence of the 4 debow switches on the starter panel.
They must be used as soon as a start occurs between 10 minutes and 5 hours after last engines shutdown.
Their use limits N2 to 32%, thus avoiding damage to the bearings and compressor blades during the uniformization of the shaft temperature.
This speed can be exceeded for 5 seconds during the debow start.
Debow duration is 3 minutes outside icing conditions and 1 minute if the outside temperature is below 3°C.
Cheers
Pierre