Jet Engines

[Craig Lindsay]

I was watching an episode of ACI and it was about and air china flight.No.4 engine flamed out,they tried to start it but they were flying too high.Question.how do the engines operate with so little oxygen,and why cant they start any engine that flames out at cruising altitude

[Andrew McLaughlin]

I was watching an episode of ACI and it was about and air china flight.No.4 engine flamed out,they tried to start it but they were flying too high.Question.how do the engines operate with so little oxygen,and why cant they start any engine that flames out at cruising altitude

In short, there's enough oxygen at altitude to keep the fire burning, but not enough to start it!

I'm sure there's a more scientific way of explaining it, but I'll be buggered if I know it!

Will? AJ? Bueller? Anyone?

[Rhys Xanthis]

I really have no idea, but heres to guessing.

What Andrew said sounds correct.

And im going to go on and take a random stab...

Burning hydrocarbons needs the hydrocarbon itself and O2, oxygen. When the supply of oxygen is limited, such as at high altitudes, the extent of the reaction would be limited too.

So a lack of oxygen would lead to a lack of the overall reaction which would lead to a flameout. I will take another step further and say temperature and pressure also have a pretty important role as well. Temperature and Pressure are proportional to kinetic energy. A specific amount of energy for a reaction to start is needed (activation energy), so a certain amount of molecules must have equal to or greater than the Activation Energy for the reaction to proceed. So if Temp and Pressure are decreased, so will the amount of energy.

So lower temperature and pressure at the increased altitude, as well as decreased oxygen could all play a part.

This is all guessing by the way, so im probably wrong:p

[Will T]

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.

[Mike Scott]

Sorry about the delayed response (been on the road with no access) yes Will, Pratts are almost identical..our 400's upper limit is FL280 with similar speed numbers to what you have posted. Dont have the AC manual anywhere within range otherwise I would list them off.

Regards

MS