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Engine limitations during tail/cross winds

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Engine limitations during tail/cross winds

Old 18th Nov 2021, 14:34
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Engine limitations during tail/cross winds

Why jet engines have limitation for ground operation in tailwinds and crosswinds? For example, the 777 engines (General Electric Model) should be limited to 75% N1 between 30 knots and 45 knots tail/cross winds. And the 787's RR Trent 1000J should not be run for winds greater than 65 knots.
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Old 18th Nov 2021, 14:54
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My bet would be that high crosswinds negatively disrupt the airflow through the engine, and could potentially cause a stall or surge.

High tailwinds will definitely increase the EGT due to the increased backpressure and I guess with very strong direct tailwinds, the EGT could very well go over limit during engine start.
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Old 18th Nov 2021, 15:04
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Also during start strong tailwinds put a huge strain on the starter motor, hence lots of engines have a tailwind start limit.
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Old 18th Nov 2021, 15:06
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Old 18th Nov 2021, 15:51
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Engines also "windmill" when not running therefore with a tailwind they are initially running backwards so the first task of the starter is to overcome this and the stronger the tailwind the greater this becomes
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Old 18th Nov 2021, 17:27
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Although the N1 (fan section) of the engine may run in the opposite direction to normal in a strong tailwind with the engine shut down I do not believe that the N2 or N3 section , as appropriate, also run backwards since these drive ancillary gearboxes and therefore have significant mechanical drag to overcome. It is the slowing of the N1 and then rotation of the N1 in the correct sense that gives the impression of the starter being worked harder. It is simply running for longer!

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Old 18th Nov 2021, 17:30
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Locked door

I don't think there is much more strain on the started motor, as in all large turbine engines I know of, the HP or N2/N3 shaft is turned by the starter motor.
The only extra strain is in the time it takes for the Fan or N1 to stop rotating backwards and start rotating forwards due to the induced airflow before the fuel can be turned on.
On the CF6 the starter had a 5 minute limit and I don't recall ever going past about 1 minute even with the max 40 kts tailwind.
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Old 18th Nov 2021, 19:32
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The big concern for tailwind/crosswind is inlet separation. Inlets are pretty much optimized for cruise operation - which means a relatively thin inlet lip. As a result they don't work all that great statically - the air from the side and behind the inlet is making a big, sudden turn over that inlet lip - the higher the power setting, the faster the air has to turn around the inlet lip. A tail wind or cross wind just makes that worse. If the airflow over that inlet lip separates, the resultant distortion into the front of the engine can result in an engine stall/surge. Some engines/inlets are more susceptible to this than others.
Rolls engines have another issue - both the RB211-524 series and the Trent series have issues with fan flutter when operating at low airspeed. In fact both the RB211-524G/H and Trent 800/1000 have incorporated 'keep out' zones that will prevent steady state operation in certain N1 speed ranges when the airspeed is less than 30 knots.

Starting is something else. Most modern (i.e. FADEC) engines don't have much of an issue, but some older engines (JT9D being a prime example) do. The problem is that when starting with a tail wind, it increases the back pressure on the core exhaust. Engines like the JT9D don't have much compressor stall margin during start to begin with - and engines typically lose some compressor stall margin as they deteriorate. The small amount of back pressure on the core due to a tail wind during start can push a marginal engine over the edge resulting in a 'start stall'.
Many years ago, one of the engine companies was using the very first 747 RA001 as a flying test bed for a new engine type. I was on board for engine ground runs prior to the first flight with the new engine type - the new engine worked just fine, but there was a tail wind and the old, tired JT9Ds simply refused to start until we got the aircraft turned around and pointed into the wind
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Old 18th Nov 2021, 23:26
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td, is ingestion of exhaust gases an issue at all with the likes of your Boeings, certainly was the problem in helos if attempting a start with a tail wind.
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Old 18th Nov 2021, 23:44
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Not that I'm aware of. Even at idle there is significant exit velocity from the core with a jet (over 100 mph exit velocity at idle -much higher than that for a pure power turbine), so I suspect the exhaust gases during start are going far enough aft that what gets back to the inlet is sufficiently diluted that it doesn't cause a significant problem.
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Old 19th Nov 2021, 21:30
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Thanks for all the responses
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