Thrust From United States of America, joined Sep 2003, 2691 posts, RR: 9 Posted (4 years 3 months 4 weeks 15 hours ago) and read 11318 times:
Hi there. Recently, I watched two videos on youtube of two different pushbacks (view from the outside) of American Airlines Boeing 777s at NRT. In each instance, I only heard what sounded like one engine start, but I've heard rumors that a 777's APU is strong enough to start both engines at once, at least for RR as far as I know. The sound was crystal clear. My question regarding that is that is it true? I know for a fact that a Boeing 777-300ER's APU can only start one GE-90 up at a time. My next question pertains to the length of time it takes to start an engine. I might start a new thread that asks a similar question, only more general than the one I'm about to. Is there a fixed amount of time of dry crank before ignition, or can it vary because dry cranking an engine is done manually (is it?). If my question is not clear enough, let me know so I can rephrase it. I've been pondering this for several months now.
Tristarsteve From Sweden, joined Nov 2005, 4150 posts, RR: 33
Reply 1, posted (4 years 3 months 4 weeks 14 hours ago) and read 11282 times:
Yes the B777 with Trent engines can start both together, and it is regularly done.
This engine starts quite fast, takes around 30 secs to idle, if the engine was cold.
If the engine is hot, the FADEC will not select fuel/ignition on until the EGT falls below 100degC.
Some engines have a dry cranking time built into the autostart. The V2500 has for one. They call this de-bow as it is meant to allow the shafts to straighten out before ign/fuel on.
Modern engines do start much faster than older engines. The surge margin is much greater which allows you to put more fuel in for quicker accelleration. Listen to an old RB211-22B start up. The accelleration is nearly zero. and the engine rumbles away for ages as it slowly accellerates.
I like watching the CF6-80E on the A330. With a good tailwind, the fan is still rotating backwards at light up, which the GE FADEC allows much earlier than RR.
AA737-823 From United States of America, joined Mar 2000, 6076 posts, RR: 12
Reply 2, posted (4 years 3 months 3 weeks 2 days 16 hours ago) and read 10855 times:
Quoting Thrust (Thread starter): Is there a fixed amount of time of dry crank before ignition, or can it vary because dry cranking an engine is done manually (is it?).
It varies because of other things, not manually vs. autostart. You're looking for a specific %RPM on the N2 spool before you throw in fuel. If you had a tailwind, it will take longer to reach that %RPM. If the weather is very cold, it will take longer. If the starter is aging, it will take longer. If you forget to turn off the AC Packs, it will take a VERY long time..... not that I've ever done that...... ahem.
Quoting Tristarsteve (Reply 1): I like watching the CF6-80E on the A330. With a good tailwind, the fan is still rotating backwards at light up, which the GE FADEC allows much earlier than RR.
I loved this too, when I did line maintenance on GE-powered 747-400s. That computer doesn't care which way the fan is spinning, so long as it can sense rotation, it knows the fan isn't stuck, and will throw in the fuel!