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A Turbofan Spinning Backwards?

Sun Jan 06, 2002 7:24 pm

I was at kualar lumpar airport last year watching the 777 that was going to take me to Perth WA, and as i looked at the fan blades they were spinning backwards. I had to look twice to make sure i was right and yep, i noted the blade orientation and they were going in reverse at about 5-6 rpm.
I had two thoughts why this were happening:
1: Wind was blowing through the back of the engine windmilling the blades backwards.
2: The pilot was running some sort of starter motor test by running it in reverse (if that is possible).
Im sure it will be the first one but i would just like to clarify it with someone who knows better.
Thankyou in advance.  Confused

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RE: A Turbofan Spinning Backwards?

Sun Jan 06, 2002 7:33 pm

Option 1 is the correct one. AFAIK, the fan (and it's turbine and the shaft that connect the two) aren't connectected to anything so they are free to turn in the wind.
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RE: A Turbofan Spinning Backwards?

Wed Jan 16, 2002 5:06 pm

Would a case like this, no matter how common, prevent proper engine start-up since the main fan connected to the turbines are spinning in the opposite direction thereby sucking in uncommpressed air into the combustion chamber?

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RE: A Turbofan Spinning Backwards?

Wed Jan 16, 2002 6:58 pm

If proper starting procedures are followed the fan windmilling in the wrong direction will not cause any problems. You may have to have the starter motor running a bit longer or in extreme cases point the aircraft away from the offending wind.

Just out of interest a CF6 can actually have the core engine up and running with the fan stationary for a maximum of 30 seconds.

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RE: A Turbofan Spinning Backwards?

Thu Jan 17, 2002 8:28 am

My flight instructor told me that engines made in England spin in opposite directions. Of course the fan and compressor blades are angled different. That could have also been the case, that the wind could have been blowing in from the front.

Ejay MD-11
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RE: A Turbofan Spinning Backwards?

Mon Jan 21, 2002 5:11 pm

I don't know about all the engines made in England, but I do know the Rolls-Royce RB-211's all turn backwards from P&W. Hence the "RB" designation. (Runs Backward)
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RE: A Turbofan Spinning Backwards?

Wed Jan 23, 2002 2:20 am

The "Runs Backward (RB)" designator isnt to do with direction at all. I think it has something to do where it was made. The RB are the first letters of the Two worded town in which it was made. Then you have got Rivers (Trent, Avon) I dont know about the Olympus engine though
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RE: A Turbofan Spinning Backwards?

Wed Jan 23, 2002 12:25 pm

Reverse airflow from the rear of the engine is what caused the 'backwards' rotation. Neither the GE or PW engines have a limitation on the maximum amount of tailwind for an engine start. I have operated both GE 90B and PW4090/98s and have never had a problem. The autostart system will take care of all of the hot/hung start problems that can occur with this type of condition should it arise.

That said however; airmanship should tell most pilots that if they suspect too much tailwind for a normal start, an alternate pushback arrangement should be made.
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RE: A Turbofan Spinning Backwards?

Wed Jan 23, 2002 2:32 pm


Agreed, in the case of "split spool”,” free turbine", and fanjet or turboprop engines.

In the case of older technology turbojet engines or direct drive turboprop engines airflow up the tail pipe is a genuine concern.

As some of us know, in the free turbine engine the fan, or Prop, as the case may be, the rotating group is split from the gas producer section of the engine from the power section except by "air coupling".

In any case, no pilot or technician would start a turbine engine with a hurricane blowing up its butt.

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