I’m sitting in Manchester waiting for my flight and there’s a 747 at the gate getting ready to load. It’s a good 30 minutes before the flight and the one engine I can see into (port side outboard) has been turning consistently for at least 20 minutes. Not fast but maybe 10 RPM? So my question is this - why is it turning? Is it wind? Is it running at a slow rate to provide power? Can jet engines even run that slow? Just curious.
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It is windmilling. Jet engines can’t run that slow, or even close.
Power is either provided by a cable from the aerobridge, the APU, or possibly a ground power cart.
I love watching windmilling engines like that. It seems to convey the engineering precision they are made with. You know it is a giant powerhouse capable of generating ridiculous amounts of energy, yet there it is, so well balanced, and assembled with such precision, that it just sits there gently turning in the breeze.
(I’m sure that the inner shaft (or shafts in a Trent) are not turning, but the fan, its turbine, and shaft are where the power hits the road as it were.)
I am going to make a SWAG here. They are probably spinning the engine to keep the shaft from deforming.
In the days of steam ships. Jacking gear would turn the steam turbine over about 2 RPM to keep the turbine shaft from sagging.
No, it is definitely windmilling.
Probably more for Richard or LSL Guy, if he wanders along, but is there a mechanism in the engines to prevent them from windmilling when shut down, or is it just the luck of the prevailing breeze?
I’m with you on getting a kick out of getting that sense of insane precision in huge things. Years ago, San Francisco’s MUNI replaced the cable car turntable at Powell and Market as the old one was however many years and years old and required enough muscle to turn a car that they were being troubled by back injuries. The new turntable sits on a bearing so smooth that one of the workers was able to sit on the edge of the new ~40 foot diameter platform and push off with his hand. He spun around at probably less than one RPM and went around at least once before getting bored. Presumably, it was a smooth ride.
Not in the engines but when the plane gets “put to bed” for the night there are straps to stop the fan from windmilling endlessly. You do need a reasonable wind to get them going and from the right direction. Sometimes all of them will be spinning but often it’s just one or two.
Edit: LSLGuy is taking a sabbatical from the SDMB.
Sorry to hear that. I enjoy reading his posts.
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I agree that it’s windmilling, but gotta give Snnipe 70E props for the cool factor—
I was always fascinated by the jacking gear in the engine room: it’s a (relatively) tiny motor that is connected via three or four worm-wheel reductions to the main shaft, so a small motor spinning at hundreds or thousands of RPM is turning a three-foot-diameter shaft (and its screw) at an amazingly slow pace, to keep the steam turbine shafts from warping while steam is present, but the engine is not under load, and to spread oil about. And if you want to lock a shaft, engage the jacking gear without turning on the motor–it’s like leaving your car in 1st gear when parked.
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I have a video somewhere i’ll try and find and post of a similar situation, but using an inching drive on a large bucket wheel. Sounds pretty drab, but when you start the main drive @ 1500 rpm and have forgotten to mechanically disengage the inching drive (electric AC motor), which then is forced to spin at some ungodly speed backwards, becoming a powerful little generator for about 200 milliseconds before it explodes violently. very amusing 
I hope you can find that video. It sounds like those sea stories we used to hear about sailors forgetting to disengage the jacking gear and then opening the main throttles. According to one story I heard, applying 70,000 horsepower to the opposite end of all of those reductions has an amazing effect, launching the jacking gear motor in orbit (or at least through the overhead)