Engine in-flight restart
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Engine in-flight restart
I've a doubt about general aviation jet engines in case of in-flight flame-out: is it true that having a small fan diameter, those engines are very hard to restart in flight by windmilling? and therefore needing bleed air (APU) or gen/starter to be restarted. The Citation X manual recommends the use of APU below 18000ft; is that for the windmill restart problem? Can someone enlighten me about this problem?
B_D
B_D
The diameter of the fan doesn’t have much to do with it. What is important is the volume of air passing through the core, which of course varies from one engine type to another. The flight manual for each type should contain the information you require, including an envelope (speed vs altitude) where a windmilling relight is possible.
The diameter of the fan doesn’t have much to do with it. What is important is the volume of air passing through the core, which of course varies from one engine type to another. The flight manual for each type should contain the information you require, including an envelope (speed vs altitude) where a windmilling relight is possible.
You could reliably windmill start at CF6-80C2 at 220 knots (we demonstrated down to 200 knots, but needed to allow some margin for the inevitable deterioration that occurs in service) - the GE90 needed 270 knots.
On the GEnx, we needed to depressurize the engine driven hydraulic pump during in-flight starting to get enough N2 to reliably start the engine at 270 knots.
3 spool Rolls engines are also a problem and require relatively high speeds for windmill starts since the incoming air has lost so much energy going through the first two spools that it doesn't spin N3 very fast.
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Thank you NZScion and tdracer for your comments, you have put me on the right track to research more on the issue. So the fan diameter is not the "culprit" in the windmill relight so much as the by-pass ratio is.
I've found confirmation on that looking at the in-flight restart envelop charts: MD82 (Low by-pass engines JT8D-217) 180kts or more, A320, B738 (Hi by-pass CFM56) respectively 260kts+ 300kts+ (talking about pure windmill relight).
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I've found confirmation on that looking at the in-flight restart envelop charts: MD82 (Low by-pass engines JT8D-217) 180kts or more, A320, B738 (Hi by-pass CFM56) respectively 260kts+ 300kts+ (talking about pure windmill relight).
B_D
But for windmill starting, none of the rest maters if windmill N2 (N3) is too low to open the fuel shutoff valve and - depending on the circumstances (like an all-engine out) - to power the FADEC.
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Having flown turbine powered helicopters most of my four decades of earning a crust, it’s been taken as read that a jet engine can’t be started by windmilling - because those fitted to helicopters aren’t generally exposed to forward airflow.
I’ve never worried about it. On the thankfully few occasions that I’ve had to carry out an inflight restart, it’s not been a problem.
I’ve never worried about it. On the thankfully few occasions that I’ve had to carry out an inflight restart, it’s not been a problem.
Speaking of hard starting, the old P&W JT3D (B707) had a problem at high altitude airports such as Mexico City, Jo-burg, Tehran etc to get a light off. The burner pressure, Pb, generated in the burner section during start was not high enough to open the FCU fuel valve so no start. P&W in a lovely piece of engineering cut the Pb line from hot section to FCU and teed off a pipe to the drain cluster. This pipe was fitted with a tyre valve core and a metal valve cap. To start, the ground engineer would remove the valve cap and connect a bike pump to the pipe. As the engine wound up on the starter you pumped like crazy and usually there was a thump and she lit off. Then disconnect, refit valve cap and move to the next engine.
I often wondered what the pax thought when they saw a guy pumping up the engine with a bike pump. FE used to do it at some ports when the engineer who was trained was away. I did a few coz I really didn't want any untrained people that close to a running jet engine.
I often wondered what the pax thought when they saw a guy pumping up the engine with a bike pump. FE used to do it at some ports when the engineer who was trained was away. I did a few coz I really didn't want any untrained people that close to a running jet engine.
Musta' , back in mid '70s ,[ time of casing ovalisations ] August in Tehran , Hot and High , P'nW JT9s on transitting B747s of BA 'n Qantas had hot start problems . Engineers always had catering van standing by full of catering sacks of ice . Hot start , cowlings open , pack the core 'Full' of ice sacks .
Wait 5mins restart , dispatch . Opened my young eyes , only used to mainline European Ops with 3 Speys .
rgds condor .
Wait 5mins restart , dispatch . Opened my young eyes , only used to mainline European Ops with 3 Speys .
rgds condor .