Windmilling

[Xswqaz]

Hi, I'm in aviation enthusiast, I saw a video on YouTube of that plane turbine fan blades spinning the engine didn't seem to be running completely though, It was like the starter was engaged but the engine start Levers were not flipped. In the comments someone said they were windmilling the engines. What does that mean? And why do they do that? Oh and the video was of an A320.

[kharmael]

They may well have been motoring the engine to either cool it down or blow out any unburnt fuel.

They're essentially running the starter motor but not pumping in any fuel.

[Torque Tonight]

That is motoring the engine. Windmilling is simply when the engine is off and the wind causes the engine to rotate - like a windmill! A jet engine rotates freely unlike a piston engine and about 5-10kts of wind through the engine in either direction is enough to get it spinning quite fast.

[Armchairflyer]

Maybe a stupid question, but is there sufficient lubrication in a "non-powered" windmilling jet engine? Or would this wear be simply negligible compared to what the engine sustains in normal operations?

[grounded27]

Maybe a stupid question, but is there sufficient lubrication in a "non-powered" windmilling jet engine? Or would this wear be simply negligible compared to what the engine sustains in normal operations?

Yes it is not an issue, some bearing seals leak normally as a result. Other motors require to be strapped or pinned in place as they can sustain damage around the fan.

[DaveReidUK]

It's worth bearing in mind that the above comments relate to engines windmilling on the ground.

When engines are ferried externally as 3rd/5th pods then positive means are normally used to ensure that they can't windmill in the airflow.

I recall that the 3rd pod on the TriStar (well BA's, at least) used a wonderful Heath-Robinson arrangement of bits of wood and rope to stop the fan blades rotating when airborne.

[Peter47]

You often see a turbine rotating on the ground. Presumably engineers can apply a brake if they wish to inspect the fans - can anyone confirm this?

I know that wind milling props caused a lot of drag and I've read a couple of books where the flight engineer deliberately tried to freeze the engine by cutting off the oil supply - I suppose a seized engine is expensive - but not nearly as much as an aircraft that doesn't get down safely.

As a matter of interest does a wind milling turbofan create much drag?

[BOAC]

Hi-speed windmilling (eg in a strong wind) can cause lubrication problems and a trick wot I nose is to open the reversers to stop the draught.

[DTE]

Hi All,

With a breeze blowing, the front 'fan' can spin at quite a rate. I filmed this on a walk-around. The background noise is the APU running.

Windmilling Turbofan - YouTube (http://youtu.be/h-Qt17tUTGg)

Cheers

Owen
Author: '50 Tales of Flight'

[bcgallacher]

Peter 47
In answer to your question - there is no brake installed on the engine - some Turbo prop engines have a prop brake installed for safety reasons. Any writer that was under the impression that the oil supply to a turbine engine could be shut off had not done his research - a seizure in flight could do serious airframe damage.

[Peter47]

Thanks bcgallacher - as you can tell I'm certainly not an expert on the topic.

Presumably its far less of an issue where you can control the pitch of a prop and you can feather it (as long as the system is working).

[lomapaseo]

Windmilling jet engines are a typical everyday occurrence. The speed is a function of the wind direction and intensity but of little consequences to the engine itself. You can bet that the engine makers have to demonstrate this capacity even without normal lubrication. What's there as residual is good enough since the thrust loads are minimal. (just like your automobile wheel bearings)

The seizure bit is a red herring and not appropriate as an argument at these miniscule rotating energy levels.

[WASALOADIE]

In my Herc (C-130) days, we used to be able to windmill start an engine. In the event of the Gas Turbine Compressor (GTC) not working or there being a problem with bleed air for starting, we could back another herc up close to the one with the problem. Then power up the good aircraft and the resultant prop wash would spin the engine on the bad aircraft, once at a suitable speed the receiving crew would initiate the start and all being well get one engine going, this would then provide bleed air to start the other engines.

[Burnie5204]

Lomapaseo

Going back to my days as a R-R gap year student I know that we had to look at the capability of the engines to windmill and not shake themselves apart following a Fan Blade Off event and also to windmill safely in-flight in an engine off condition I.e. that windmilling in-flight iunder emergency conditions would not cause dangerous situations (as I was involved in the load calculations) but I am unsure whether we had to show on-ground windmilling capability.

Obviously the first 2 situations which we definately had to show were emergency situations in which you could quite happily wear your bearings out as long as it remained intact and didnt eject any materials.


That said any wet seal will retain some oil inside it which would provide limited lubrication and windmilling will allow the bearings to rotate preventing 'egging' where a stationary shaft would cause deformation of the seal due to its weight resting on the same point of the bearing for long periods of time.

[lomapaseo]

Burnie5204

Agree

Windmill unbalanced-in-flight is the limiting situation, although I suspected that the OP was talking about what he saw on-the-ground.

In flight it is regulated to remain within safe damage tolerance limits (as you said). While on-the-ground it's an economic thing if any thing begins to collect detectable wear (fan-blade bumpers. dampers, etc.).

I suspect that most who read these posts will think that higher air speeds mean more wear. Not true in most cases for the fan blades that hang loose in their slots when assembled or for bearings which depend on pressure balancing of their aerodynamic thrust load.

[P6 Driver]

Back in my Lyneham days, we in ATC used to witness two types of C-130 engine starts apart from the "normal" ones.

1. Slipstream.
2. Windmill.

What WASALOADIE described, ATC used to refer to as the Slipstream start, when one C-130 was backed up to the one needing the start and used propwash to turn the prop of the engine/s to be started.

For us, the Windmill start was when a C-130 would gather speed on the runway and at the appropriate time, unfeather the prop of the engine to be started, using the airflow to start it.

Happy days.

[lomapaseo]

For us, the Windmill start was when a C-130 would gather speed on the runway and at the appropriate time, unfeather the prop of the engine to be started, using the airflow to start it.


Would that all be accomplished before V1 was achieved?

[sycamore]

Lom, it was a high speed taxi to start an engine,which had a bleed air valve problem,or a starter failure...never an attempt to take-off before one had checked it.Not too bad if it was an inboard(on a C130),but an outboard could be interesting..! Usually happened at night,after the freight had all been loaded,and the loaders were busy elsewhere....Have seen the end 300ft of tarmac appearing out of the dust cloud on 12000ft of runway as we coasted to a stop.....