Engine run-up de-icing

[Prober]

In the early days of the B757, severe weather run-up was 60%N1 for 60 secs every 60 mins. It was normal then to release brakes and start the T/O run (running, as it were). After some incident, Rolls examined the matter (RB211-535C) and revised the procedure to 60% N1 for 10 secs followed by a sharp reduction to idle before commencing T/O. The Management notice to this effect said that the rationale was that the ice shedding was accomplished by the twisting of the fan blades as they changed from forward thrust to idle. It was (not quite tacitly) admitted that all the previous one minute shuddering at 60 % N1 had been in vain.
During a recent sim session with a company using 535C engines, I found that their SOP's did NOT require the reduction to idle before starting T/O.
My question is: has the original lesson been over-ridden, or just lost in history? Also, would this apply to the E4 (and others) as well?
Any help, chaps?
Prober

[Mansfield]

My Boeing FCOM for the 757 cites the procedure you saw in the sim:

"Runup to a minimum of 60% N1 for approximately 10 seconds duration and confirm stable engine operation before the start of the takeoff roll."

Although the notion of blade twist is interesting, it troubles me. The fan blade angle of incidence needs to be pretty rigid in order to predict performance; I'd be surprised if there was much change in the angle with RPM. The blades will seat in their channels rather smartly upon acceleration. But other than a certain degree of creep (elongation), I believe the geometry needs to be stable. So that leaves us with centrifugal force. :confused:

[lomapaseo]

Most run-ups to 60% are to blow ice accumulations off vanes, etc. behind the fan that might have formed during taxi conditions where the inlet is depressed due to suckingt the air. Some run-ups also are designd to shed ice accumulated on the aft side of the fan blades again due to air being sucked through these blades at idle.

But on the subject of blade twisting with RPM. Yes they do twist-untwist and it is significant for the narrow chord blades that have those noisy shrouds or clappers between each pair of blades.

Yes the incidence angle is very important (at high RPM) and so is the ability to handle vibratory stresses and the ability to replace these blades on-wing. So the designer designs the blade to fit loosely in its disk slot and to accomodate a twisting action as it inceases in speed until all those shrouds. snubbers, clappers etc. between the blades twist into each other and form a soilid ring and freeze anymore twisting at the optimum incidence angle for high power.

It's always interesting to find out the root technical basis behind some of the recommended procedures published for the equipment one flies :)