GE CF56 EGT 'Bloom'
 

Been having a discussion with various members of the 'two winged master race' who are convinced of this phenomenon on GE powered 747s. However none of us techies can find any info (reliable or otherwise) about whether it actually exists or not.... any ideas?

Yes thats the one, affects us for take off, N1 can go way past the original thrust lever setting if not hauled back....:eek:

One question - what's a GE CF56 ?

It's a typo GE CF6 , not CFM 56

ASFKAP thanks for your input, it was a typo, but any information is useful:8

This is a well know phenomenon of the CF6. It is known as "Bloom and droop". It affects both the EGT and N1. Also one of the reason there is a two-stage EGT limit for TO. Usually EGT peaks at around 500' AGL then starts to decrease again. This of course affect the tip clearence and efficiency of the turbine, and this will again affect the N1. So it is not unusual to see a 2% variation of the N1 due to this characteristic.

Also, there is no need to adjust the throttles unless limits are exceeded. Since there is a good N1 margin, it is, as usual, the EGT that tends to be the reason for a bit of throttle pulling.

There used to be Operations Engineering Bulletins describing this phenomenon and procedural handling of it. Trying to trim N1 at 120kt on a rough runway isn't a exact science. :ouch:

Bloom is generally worse on the first TO of the day when the hardware isn't well warmed up. A part-power runup for a minute before TO will be helpful in this case.