Freewheeling engines on the ramp in wind

[southern duel]

I am sure I have read it or heard it somewhere but when an aircraft is parked up the blades are allowed to freewheel. I am sure there is a reason why they cannnot be locked but cannot find it anywhere

anyone out there can help

cheers

[KBPsen]

It's called windmilling. There is no reason to prevent it in the short term, which is why it isn't done.

If an aircraft is parked for any extended period of time, engine covers are usually installed which in addition to preventing foreign objects, dust, moisture, birds etc. from entering the engine also prevents windmilling.

[grounded27]

It all depends on the motor, most require no short term care. Some will leak oil (PW4000 I think) at the C1, I remember a MM ref that stated it is a normal condition. The JT9D-Q could if spinning in reverse flow could lock up in the abradable shroud and do damage, we used PVC pipes to lock them down.

[Burnie5204]

Allowing them to windmill on the ground is actually better for the engines than locking them in the short term

If you lock them then the weight of the shafts will rest on the same point of the bearings and seals and cause warping and damage, particularly the heaviest LPC/LPT N1 shaft.

By rotating it ensures that oil is properly distributed around the engine bearings and any oil seals and keeps the bearings moving which keeps them circular.

Some engine manufacturers will often, whilst transporting engines covered up on low-loaders, attach a low speed electric motor to the engine to rotate the shafts slowly to help protect the bearings and seals.

[Gas Bags]

An engine should never be allowed to windmill. There is no oil supply via wind milling to the bearings and this will cause damage in the long term.


This is why the manufacturers supply fan straps with each engine.

[TURIN]

Suggest you take a look along the flightline next time its windy. No straps anywhere.


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[Burnie5204]

Gas bags

Yes, if gas turbines are left long term then yes, the lack of fresh oil supply will result in the bearings eventually running dry but you would have to leave them windmilling like this for weeks on end. Straps are provided to assist in safe working on the engines, boroscoping and other such activities. Some people will strap down when the wind picks up but actually the faster the shaft spins the better it will float and balance on the residual oil, but you wouldnt be able to safely stop it spinning to inspect the engine.


However, in a gas turbine that is used or at least run every so often then when it runs the oil in the bearings is refreshed and when it is then shutdown and left to windmill there is residual oil in the bearings and oil seals which prevents damage.

Like I said, by rotating it keeps the oil spread round and cushions the boundary between the shaft and the bearing and prevents 'egging' of the bearing from the weight of the shaft resting on the same spot on the bearing/seal by spreading the load and reducing pressure.


My source for this - I used to work for a major gas turbine manufacturer in their Whole Engine Modelling department, my specialty being in calculating rotor tip clearances, seal rub/wear profiles and movement/vibration patterns. And we would connect an N1 motor when shipping long distance to rotate the N1 shaft to ensure it arrived with the customer in best condition.

[Gas Bags]

Common sense mate....you aren't going to stop that fan spinning in high winds to strap it. Most times you can and strap them overnight.


If the straps are not for that purpose please explain what they are for?

[Tom Sawyer]

Gas Bags....Fan straps are to lock the fan to the fan OGV's when you need to do just a "core engine" run. I've done it a few times for under core cowl leak checks. As the fan is locked you get no by-pass airflow so can get your head under the cowls for inspections with the engine running.

As for windmilling - there is probably a time limit, wind speed or such like in the AMM which stipulates how long windmilling is allowed for. I'm pretty sure there is limits for in-flight shut down windmilling as well where inspections are required if windmilling was over a certain time.

[Gas Bags]

You cant be serious.....Where do you guys learn this stuff......Surely the industry has not been 'dumbed' down to this point....F*&^ me I hope not!

[Terry McCassey]

We used to deploy the thrust reversers, cut down windmilling significantly, especially in high winds.

[Krystal n chips]

As Turin correctly stated, have a look, and a listen, on any flight line and you will hear the sound of blades happily freewheeling....depending on the wind direction and strength at the time

These fabled straps ?. Why use straps when the engine manufacturers kindly supply....engine covers and blanks..... for obvious reasons and, as TM also correctly points out, there is the T/R option available as well.

On a, ahem, lighter note, a "well known airline" once tried a, lets call it an "alternative method" to prevent freewheeling. This alternative method, whilst fine in principle, would not actually meet with universal approval however.

The method was simplicity itself.....take one chock....place in intake and....noise stops !......alas, there was but one flaw in this plan. Sadly, when the engine was started for a ground run, the score draw that resulted was Chock 1 -Engine 1.

The above is known as dry humour before the more pious start on the "OMG! how could they !" response.

[Tom Sawyer]

Gas Bags, I'm not sure of the intent of your reply re dumbing down, but if you doubt that there is a procedure to strap the fan and do a core EGR, then go and find a P&W JT9 manual and it is detailed in there. Like I said, I've done it a couple of times.

The other time I have strapped the fan was for a 3 engine ferry, however this depends on type as I've done another 3 engine ferry on an A340 and it was not required because flight time was below X (?) hours and engine was allowed to windmill as there was no damage to the oil system. Was a bit weird raising a DD along the lines of Engine No1 inop - not to be used.

So to answer your question, this is what fan straps are for.

As for the original question - I've had a look in A330 Chp10 Parking and Mooring and I can find nothing specific as to securing the fan when parked, but only to fit intake / exhaust covers if parking for up to 15 days or more. Basically I would say (for the types I have access to), windmilling on the ground is ok. This is based on the pretty basic checks required after an IFSD dependent on failure reason, e.g. even after a IFSD allowing windmilling for over 30 mins you basically just need to check EMCD and oil level before running the engine and rechecking the EMCD. So if an engine can windmill in flight for a considerable amount of time with pretty basic checks (post fault rectification), then windmilling on the ground at lower rotational speeds should not cause a problem.

Other aircraft /engine types will more than likely have different criteria of course. Maybe B737 NG's have some tighter criteria as they are the only ones I can specifically remember seeing fans secured whilst parked on the ramp.

[crackling jet]

Sitting here and listening to the steady tick tick tick of the compressor blades rattling around as they windmill in the wind outside, surely there must be some ware occouring on the blades and central core of the fan assembly as there is movement when you hold one of the blades and move it with your hand there is a fair bit of lateral play. Over prolonged windmilling say months, does not anything ware ?

[grounded27]

You cant be serious.....Where do you guys learn this stuff......Surely the industry has not been 'dumbed' down to this point....F*&^ me I hope not!

You are the only one here of this opinion, the only engine I have ever strapped was for ferry flight after failure. You have had numerous professional responses, when you feel everyone around you is wrong it may be time for a bit of introspection.

[KBPsen]

Sitting here and listening to the steady tick tick tick...Whenever there is metal to metal contact (which is the sound you are hearing) there will be wear, but it is insignificant.

Where do you guys learn this stuffFrom the manufacturers. Where do you learn your stuff?

I have yet to see a notice in any form from either Rolls, Pratt or GE saying that "An engine should never be allowed to windmill.". In fact, after 25 years of turbine engine experience in line, base, overhaul and operational environments you are the only one who have ever said such a thing.

[woptb]

We would always blank (not gag) older marks of RB211 to prevent windmilling,the reason;It would cause low speed vibration - a known issue with BA's original 75's as DHL, (who bought them) found to their cost.

[slapdash8]

Turboprops on the other hand....

We tie them down on windy nights to prevent the prop windmilling in the wrong direction as it can occasionally lock the hydraulic pump.

It's also more hazardous on the ramp than a nice contained and shielded fan.

[Capt Quentin McHale]

Ladies and Gents,


I can see both sides of the argument here, but as I am on sick leave, I can't get near a Maintenance Manual for my own investigation. Can someone possibly produce a Maintenance Manual reference for all to see and read. I would assume different engine manufacturers/companies would have different procedures.

GasBags, with respect, is it possible that you are quoting a company procedure and not Maintenance Manual?


McHale.

[BARKINGMAD]

Another tech log entry?

Another item for the flight crew to miss on the walkround?

A FoD hazard unless properly installed/removed?

It's the ones which DON'T clatter when they rotate which used to bother me, indicating "blade cluster" and therefore potential for irritating inflight vibes.

[TURIN]

Blade cluster, or shingling as we used to call it.
When the fan mid span snubbers would get tangled/jammed together. A stout hide face mallet and a flick of the wrist would solve that problem.


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