Engine in-flight restart

[Black_Dawn]

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

[NZScion]

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.

[tdracer]

Originally Posted by NZScion

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.

To elaborate a bit, it's the bypass ratio more than the fan diameter. The earlier generation of high bypass engines had pretty good windmill starting characteristics, but starting with the higher bypass on engines like the PW4000/112" and GE90, the combination of less air going through the core and the core being more highly loaded (higher pressure ratios to generate the work to spin those big fans), windmill N2s dropped relatively to airspeed, meaning you had to go faster to get adequate N2s for a windmill start. You need an N2 of about 10% before there is adequate pressure from the engine driven fuel pump to operate the fuel control and open the high pressure shutoff valve (varies slightly between engines, but you need ~300 psi to open the shutoff valve in the fuel control).
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.

[Black_Dawn]

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).
B_D

[galaxy flyer]

tdracer,

Would it more accurate to say it’s the compression ratio that matters?

[tdracer]

Originally Posted by galaxy flyer

tdracer,

Would it more accurate to say it’s the compression ratio that matters?

It matters. but that's only part of it. Of course, the whole in-flight start thing is fairly complex (the PW4000/94", RB211-524, and CF6-80C2 had similar overall compression ratios, but the CF6 windmill start was significantly better, while the 3 spool RB211-524 was way worse). Subtle things - like the profile of the splitter (that separates the core and fan airflow) can make a big difference. Other things, like the way the fuel is injected into the combustion chamber, and the increasing complex burner/combustion chamber designs also play into it. The GEnx was a SOB to start - even on the ground. GE had to play around with the fuel scheduling a lot to get it to the point where we could get reliable light-off characteristics - and even they we had an unpleasant discovery late in the flight test program when they took a 747-8 to Alaska to do very cold temp ECS and other system testing (-30 to -40 degrees C) - only to discover that they couldn't get the engines to light off when it came time to come home. We ended up doing a panic trip to Iqaluit, Canada (it was late March and it was the only place we could find where they were expecting sub -30 degree C temps before winter ended). GE had to try a few different starting fuel schedules before we found one that would reliably light-off so we could get the engines started when it was super cold...
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.

[ShyTorque]

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.

[mustafagander]

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.

[condor17]

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 .