Normally this run up is done if the Vibration Ind 'increase showing Ice accretion on the blades(Inbalance).After the time (30 secs)the engine is reduced to 'idle' to see if the Vib's are still'High"..
Thence one can assume the imbalance wont be there for TO.
Once knew of a L1011 doing it 3 times,finally canx flt,in certain conditions at Edmonton....
