Taguilo From Argentina, joined Aug 2005, 74 posts, RR: 0 Posted (9 years 8 months 1 week 2 days 1 hour ago) and read 2506 times:
For Boeing aircraft, on startup
1) If fuel is selected ON before N2(N3) reaches magenta bug (16-25% depending on engine), does the engine accelerate upon idle stabilization at normal speed with high EGT numbers (hot start chances increased) , does it accelerate at a reduced rate BUT still reaches idle speed, with high EGT, or it has chances NOT to reach idle speed (hung start)?
2) Extending perspective of first question, how near of N2/3 magenta bug rpm percent would be safe to insert fuel and obtain a normal startup, with maybe a high EGT but within tolerated parameters? Just thinking of, maybe 90-80%?
Fr8mech From United States of America, joined Sep 2005, 6138 posts, RR: 16
Reply 1, posted (9 years 8 months 1 week 2 days ago) and read 2482 times:
There are a few variables. The closer you are to the fuel on bug, the more 'normal' the start will appear.
The engine will attempt to accelerate to idle, but you do stand a real chance of a hung/hot start.
Really can't give you much more of an answer than that.
Fuel on prior to min N2 is not something I've ever done, though I have had air drop off shortly after light-up. At that point you make your decision based on the engine parameters. It's usually best to go to cut-off and re-establish air.
Jetlagged From United Kingdom, joined Jan 2005, 2605 posts, RR: 25
Reply 2, posted (9 years 8 months 1 week 1 day 18 hours ago) and read 2429 times:
The problem with applying fuel early is that there will be too much fuel for the airflow. So there is a real risk of a hot start or a wet start. If the engine lights up, it should run up to idle OK, if a little hot. If the engine type has a high EGT under normal circumstances, then it may well hit the red line.
As Fr8mech said, the real problem comes if the starter quits early for some reason. Acceleration will be reduced and EGT rises higher. A hung start is quite likely, possibly with EGT exceeding limits, depending how close you were to self sustaining N2 at the time. Usually best not to take a chance and so cut the fuel off. This is more of a problem on aircraft with manually controlled start valves, like the 727, 747-200, etc. More modern aircraft with latching start switches and auto starter cutout will only do this if there is a malfunction.
On aircraft with FADEC engines and autostart, timing of fuel on and any start aborts are automatically taken care of.
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