Why do jet turbines turn backwards?

As I waited idly at the airport yesterday I watched the fans on the front of an aircraft’s engines (an A340). They were turning around slowly - I always assumed that they were sort-of on “tickover” when this was happening (I can see that’s stupid because they’re not turning fast enough to compress anything).

But then it occured to me that they were turning backwards, too. So are they just windmilling in the breeze when they’re doing this? Or is there some active energy input to make them turn? And if so why?

Airline pilot here …

Just windmilling. If the jet’s pointed into the wind they turn one way and if its parked tail-to-wind they turn the other way.

The typical parking arrangement at terminals makes it rare to have much headwind; headwinds are largely blocked by the building. So the global effect over a bunch of airplanes parked at a bunch of terminals on a bunch of days with the wind blowing in various directions is that you tend to see some rotating backwards and most others rotating not at all, and very very few rotating forwards.


It can be a serious problem for the engines, too. The rotors are turning but the lube pumps that keep the bearings lubricated aren’t. Windmilling time and speed, with the engine not even running, can actually be sizing conditions when designing the bearings. The clanking of fan blade platforms against each other can cause more wear on those surfaces than actual engine operation (when centrifugal loading tightens up the joints), plus, the noise can be annoying.

Many engine parts are actually at their hottest temperature after shutdown, too.

I noticed that one of the four engines wasn’t turning - I suppose now that it was just sheltered from the breeze. But why not apply some kind of brake to avoid the wear problem? A simple stick through the spokes ought to do it … could I patent this and make a million?

You could if it weren’t already done. A parked C-5 will normally have a 2x4 stick jammed into each fan for precisely that reason. Ugly as hell, but effective.

Most engines just live with the problems - they’re easily controllable, but a brake system would add cost/weight/reliability issues that might be even worse.