High altitude engine starting...

[tom775257]

Hullo all,
A quick question about something I have been thinking about after reading in ‘New Scientist’ about the 747 that flew for two hours powered by one engine only – the new G.E. engine for the 777 (GE90-115??)…anyhoo..it got me thinking re: restarting engines. After you have had a normal jet engine off at high altitude is it possible to start it again after cold soak, and in the low-pressure environment (OK, the APU can do it, I believe at lower than cruise altitude????). I assume in the aforementioned 747 airframe/777 engine GE engine test the aircraft was flown at lower altitude, however is it possible to start a cold engine at cruise altitude?
Many thanks for any info for a bored, mildly inebriated Cherokee pilot.
Tom.
:)

[ProfessorMCDU]

The simple answer is, it depends! Most of the aircraft you are describing can be restarted at high altitudes. The are several factors to consider however. The temperature of the engine oil is one of them.
Also, the maximum altitude for the restart would be a factor. And, the APU limitations could come into the picture.

Personally, the most important question I would need to have answered is, "Why was the engine off in the first place?"

If it was shut down to conserve it for future need (Possible go-around at destination) and it was previous to the shutdown suffering some sort of issue enroute (Decrease in oil quanity or pressure, or an unusual vibration) then the restart MIGHT be an option.

Hope this helps....from a bored, completely inebriated Airbus pilot

[expedite_climb]

On a two engined airliner you will not be able to maintain your cruise altitude on one engine for a start. On the 757 you will 'drift down' around 10000' When at this drift down cruise alt then you should be able to restart it. The eicas will show you the restart altitude and speed envelope.

[lomapaseo]

The cold isn't nearly the problem that lack of O2 is.

Basically you've got to descend from cruise at a high enough forward speed to windmill start an engine. I know that cross-bleeds from other running engines (if you have any) may work along with a working APu, but you can't always count on these.

[tom775257]

Thanks for the replies...
As a ball park figure, assuming you have descended to a altitude where on the specific aircraft you are in, the engine can be restarted. OK, so you have the engine spinning nicely via wind-milling or driven by the starter from X-bleed or APU bleed air…what sort of pressure and temperature is there going to be in the combustion chamber prior to fuel burning while in-flight….when I think of jet fuel I think of something around about the consistency of diesel fuel, which is very difficult to ignite at low temperatures. Is getting the fuel to ignite a big problem at (cold) altitude, or with the compression of air is there a real good temperature rise? As a side note am a correct in assuming for the same N2 RPM on the ground at ISA pressure while starting prior to ignition (asssuming hitting the starter max speed it can drive the N2 spool..), and say a temp of -5 Celsius there will be a higher overall temperature rise in the air flowing through the compression stages of the engine than the same engine with the same N2 spool speed at 10,000' inflight with ISA pressure and temp, or is the temp rise the same?? Hope this makes some sense
Cheers,
Tom.
p.s. the reason the 747 story got me thinking was purely because after the two hours of flying on one engine, I assume there is going to be some in-flight engine starting…

[lomapaseo]

> p.s. the reason the 747 story got me thinking was purely because after the two hours of flying on one engine, I assume there is going to be some in-flight engine starting?
<

Not on the normal engines, they would simply keep these at idle.

[tom775257]

hehehehe.....
As I have been told by my tutors many times.....never trust non-vetted articles...didn't think they were idling. Question still applies though.
tom.

[Jet II]

I seem to remember that the RAF Nimrods used to shut down 2 engines whilst on patrol to extend the patrol times - are there any ex - Nimrod crews out there with the details?

[mono]

Can say I have been present when an IFSD has been restarted, but I have started an engine at altitude and at -35 degsC (Ulan Batar late Jan elevation approx 3000ft)

There were 2 problems here. 1, the fuel was cold (I seem to remember A1 freezes at about -45 C) and the oil was also cold ( the manual for the RB211 states that starting with oil temp -40 or below is a no no)

The start proceedure however is normal with the exeption that the fuel lever is set to rich to start the donk and back to run when oil temp rises to above 0 degs. The only observation was that there was a lot more 'mist' out of the back of the engine prior to light up (I suspect a combination of rich fuel and the cold temperature)

I imagine that the friction of a windmilling engine would keep the oil temp above the -40 limit, so that should not be a problem, neither should cold fuel.

On the temp rise question, any temp would be less at altitude. (it is one of the reasons that jets are more efficient at altitude - better EGT margin)

[Stan Evil]

Restarting turbojet engines shouldn't be a big deal. The airtest schedule on the Canberra required a shutdown and restart a 45000 ft and that was a 1940s technology engine!!! I guess on twin or triple spool engines there is probably an issue regarding getting the inner bits to go round and round.