Hi guys,
The inflight re-start envelope is about ensuring sufficient compressor rotation speed ('windmilling'), and therefore stabilised, steady and smooth airflow through the combustion chamber.
On the 744 Rolls-Royce fleet, our re-start envelope (with a windmilling engine) is as follows:
GND-FL200 240kt IAS - Vmo
FL200-FL300 280kt IAS - Vmo/Mmo (lower of the two)
Boeing and Rolls-Royce essentially guarantee a light-off within 30 sec if the aeroplane is within this envelope. Starts may be attempted outside the envelope (eg. FL360 at 300kt), but they may or may not be successful, and would likely take quite a long time. The envelope for GE engines is essentially very similar, with only minor airspeed differences. I'd imagine the Pratts would also be similar (Mike?).
We also have the option of using the air Starter Motor to assist inflight starts at lower airspeeds. The envelope for the starter motor-assisted relight (RR aircraft) is:
SFC-FL140 0-240kts IAS
FL140-FL180 160kts - 240kts IAS
FL180-FL200 200kts - 240kts IAS
FL200-FL280 200kts - 280kts IAS.
Above these speeds, a purely windmill start should be attempted.
Andrew and Rhys, I thought your comment about oxygen availability was interesting, so I looked up the Rolls-Royce jet engine handbook, and it too says that the envelope is solely predicated on ensuring compressor speed (ie. combustor airflow) for start.
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