View Full Version : Engine Anti-Ice Runup Question
Globally
19th Dec 2003, 02:17
Our flight manual requires running up our GE CF6-802C2B5F engines on our B744s at 60% N1 for 30 seconds in the takeoff position prior to takeoff, during icing conditions. My question is after we have completed the 30-second runup should we then reduce the thrust to idle prior to beginning the subsequent takeoff procedure, or can we begin the takeoff roll from the existing 60% N1 and then set takeoff thrust directly from there? Our book doesn't discuss what to do after the runup in the takeoff position. Appreciate any thoughts/comments.
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....:rolleyes:
lomapaseo
19th Dec 2003, 05:34
The reason for the 60% runups before takeoff on the big fan engines is to generate enough air blast (not hot air) to blow small amounts of ice off the innards behind the fan as well as to shed any ice that might have accumulated on the blades or spinner.
Once having been completed, you should be ready for takeoff assuming conditions permit.
One of the problems is that if you delay this runup for too long a time it may actually result in large amounts of ice being shed which could damage your engine. Or if you only use a 50% runup instead of the recommendation assuming that by holding it longer it will melt the ice (definitely this won't work).
The biggest problem with all this is the problem that occurs on taxi in during icing cnditions and nobody notices it behind the fan and then attempts takeoff without applying the 60% runup, poof all the built up ice goes down the engine.
Assuming that you have followed the manual, there should be no problem in continuing the takeoff from your last spoolup.
Interesting topic. There was a school of thought that said it was the reduction of RPM that shed the ice however we have just been looking into this and both IAE and Rolls do not require the RPM to be reduced before take-off. Both say that it is the RPM (and therefore as mentioned before the high airflow) around the IGVs and centrifugal force of that RPM that will shed the fan ice. I would have thought that if there was still a high level of vibration after the runup, which would indicate fan ice and not IGV icing, then it would be a good idea to go back and have it checked.
Had this happen to us in Calgary in freezing fog. during the preflight noticed (very slight) ice accreation on the fan blades, but didn't consider it to be too serious. However, after deice, on engine start up there was a lot of vibs. Spooled up engines again, vibs dropped and off we went.
Prior to t/o spooled up the engines again and vibs were off the scale. Throttled back and tried again, couldn't even get to the 60% target. After 3 unsuccessful attempts, taxied back to stand where I supervised a de-ice of the fan blades with hot air blowers, making sure the blades and inlet area were completely dry afterwards.
Another de-ice (airframe this time) and we were off. When we got to the runway, spooled up the engines again and vibs at the 60% target were in limits, so without spooling down we set our derate target and took off.
As we were going down the runway the vibs did creep up again and the right engine was just at the limit but not enough to abort the t/o.
When we got to our destination (OAT -35 degs C) the post flight inspection before close up showed not a trace of ice on any of the blades.
P.S. Blade de-icing with hot air blowers is an acceptable MM proceedure for severe blade ice accreation removal.
Globally
19th Dec 2003, 20:09
Many Thanks for your replies. Cheers!
Flight Detent
21st Dec 2003, 18:33
Hi all,
My thoughts regarding this question are this,
Continue the takeoff when the run is complete, don't reduce power then subsequently increase it! - further, your manual will also point out that if you begin to slide over the icy runway surface during the run, just release the brakes and go!!
Cheers
;)
I just finished continuing qual training yesterday. They tell us for the GE engines (CF6-80C2B6F) the reason for the 30 second run-up is to shed ice accumulation off of the stators. They tell us to make this run-up every 30 minutes during the taxi. The run-up before takeoff is also done to 60%N1, but only long enough to see that the engine instruments are normal. Then we may release brakes and go. Seems logical enough.