Laser9 Posted June 14, 2022 Posted June 14, 2022 A noticed that during a windmill start the manual advises you to turn off the bleed air switch and enable the cross feed. What exactly is the purpose of doing these two steps? If someone could explain it to me that would be awesome. Thanks!
Solution jaylw314 Posted June 14, 2022 Solution Posted June 14, 2022 (edited) You turn bleed air off because it feeds off the engine compressor, causing drag. That can actually slow down the engine acceleration during a windmill start. Also why you put the throttle at max and use IGN, because the automatic start sequence when you put the throttle to idle includes opening up the bleed air valve (to the air turbine starter). Bleed air off = faster engine acceleration You turn crossfeed on so that both fuel pumps (each powered by one engine) can feed fuel to both. Nice if the other engine is still running. If both are out, and you're having to do a manual reversion dive to windmill the engines, it will allow either engine the best chance of lighting ASAP, in case one isn't working (you're also having a very bad day). Otherwise the left engine would have the advantage since it has the DC fuel pump for backup (why you start the left engine first on the ground). Edited June 14, 2022 by jaylw314 1
Laser9 Posted June 14, 2022 Author Posted June 14, 2022 (edited) Thank you for responding. I'm not sure if I understand you correctly about the bleed air. I'm not sure what you meant when you said leaving the bleed air valves open will cause drag. Do you mean that by leaving the bleed air valves open you are basically letting the bleed air system steal air that could be used for starting? If you could elaborate on that I would really appreciate it. Edited June 14, 2022 by Laser9
jaylw314 Posted June 14, 2022 Posted June 14, 2022 (edited) 31 minutes ago, Laser9 said: Thank you for responding. I'm not sure if I understand you correctly about the bleed air. I'm not sure what you meant when you said leaving the bleed air valves open will cause drag. Do you mean that by leaving the bleed air valves open you are basically letting the bleed air system steal air that could be used for starting? If you could elaborate on that I would really appreciate it. Sort of. The spinning fans are like an air compressor. If you let its output bleed off, you lose energy, and that energy comes from the RPM's. With one engine running, you might be able to spin the other engine using bleed air from the good engine and the air turbine starter. However, by definition, if you're attempting a windmill restart, that has already failed to work. So now you're stuck with trying to use airspeed to spin up the dead motor. You've already determined you can't start the engine using the air turbine starter powered by bleed air (either from the APU or other engine). Now you just want to get the dead engine spun up as fast as you can. It's not that you want to save bleed air to start the engine. You don't want it being used at all. Since there's only one bleed air switch for both engines, you turn it off, and you make sure the dead engine is not in IDLE (which will override the switch and open the bleed air valve to that engine). That's at least my best understanding from the info out there Edited June 14, 2022 by jaylw314 1
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