taguilo
Posts: 73
Joined: Tue Aug 02, 2005 7:51 am

Fuel Insert On Startup

Mon Oct 31, 2005 10:58 pm

Hi,

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%?

Thank you
Tom
 
fr8mech
Posts: 6672
Joined: Mon Sep 26, 2005 9:00 am

RE: Fuel Insert On Startup

Tue Nov 01, 2005 12:08 am

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.
When seconds count...the police are minutes away.
 
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Jetlagged
Posts: 2562
Joined: Sun Jan 23, 2005 3:00 pm

RE: Fuel Insert On Startup

Tue Nov 01, 2005 6:08 am

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.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
taguilo
Posts: 73
Joined: Tue Aug 02, 2005 7:51 am

RE: Fuel Insert On Startup

Tue Nov 01, 2005 7:57 am

Ok thank you guys for the replies.

I wonder whether a 757 has some kind of protection at startup, or shares the same system as her "oldie" sisters 727 and 747.

Tom
 
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Jetlagged
Posts: 2562
Joined: Sun Jan 23, 2005 3:00 pm

RE: Fuel Insert On Startup

Wed Nov 02, 2005 2:03 am

Something in between. The start switches hold in automatically, but selection of fuel on is up to the crew to get right (no autostart).
The glass isn't half empty, or half full, it's twice as big as it needs to be.

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