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teopereira

Engine start: no APU bleed, no fuel, but N2 keeps speed?

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You have the starter valve on though (set to GND). It's expect to keep a bit of speed until you give it full fuel.

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Maybe windmilling! Starter switch (on) activate some elec. power for the display etc.

Regards

Jovabra

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Windmilling on ground? You probably shouldn't fly in tornado.

 

But well it's a bug.

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If you can reproduce it, send a ticket at support.precisionmanuals.com

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I just tried this and I could not reproduce it. With the APU bleed off and do duct pressure, attempting an engine start resulted in no N2 or N1 rotation.

 

Dave

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I just tried this and I could not reproduce it. With the APU bleed off and do duct pressure, attempting an engine start resulted in no N2 or N1 rotation.

 

Dave

Just do the normal procedure to engine start: APU bleed on, packs off etc... When N2 reaches 23% or whatever close the APU bleed... The N2 will remain where it is, it won't slow down and stop. 

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Teo;

 

Ok now that you've mentioned the exact steps, I can reproduce what you have described. So it does appear to be a bug and I would submit a support ticket. After confirming that the engine was not going to spin down, I did move the starter switch to off and the engine then did spin down. To me, this is a very minor issue since I don't know why you'd do as you describe. Normally you'd either just go ahead and start the engine or, if you decide to abort the start, just move the starter switch to off. Not sure why you'd ever turn off the APU bleed at that point. But good find.

 

Dave

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The engine is not started in the OP's picture. And that's his point, why is it still motoring with no fuel and no air source?

 

Dave

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J

 

Teo;

 

Not sure why you'd ever turn off the APU bleed at that point. But good find.

 

Dave

I didn't know whether it was a bug or something that for some reason worked that way in the real thing. So it seems to be a bug.

 

Thanks

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Er, if the engine is started isn't the APU unneeded at that point?

The engine isn't started, it's cranking over with no ignition and no air source. It could happen in real life if the APU failed during engine start before light off but in that case the engine N2 would run down to zero and the start valve would close.

 

The start valve needs air pressure to open. If air pressure is lost it should close.

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Windmilling on ground? You probably shouldn't fly in tornado.

 

But well it's a bug.

Absolute NO. I speak as a experienced ground engineer.

Regards

The engine isn't started, it's cranking over with no ignition and no air source. It could happen in real life if the APU failed during engine start before light off but in that case the engine N2 would run down to zero and the start valve would close.

 

The start valve needs air pressure to open. If air pressure is lost it should close.

The r/h engine is in the "GND" position and is not only for cranking. Thats the way you start the 737 engine. At 18-20% N2 you open the fuel cntrl valve. Read your engine strart procedure!

Regards Jovabras

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Absolute NO. I speak as a experienced ground engineer.

Regards

The r/h engine is in the "GND" position and is not only for cranking. Thats the way you start the 737 engine. At 18-20% N2 you open the fuel cntrl valve. Read your engine strart procedure!

Regards Jovabras

 

All due respect, I think you should take your own advice.  I'm not sure what "ground engineer" means, but over here, we use that term to fancy up the term "rampie."  That being said, a rampie is telling off a 73 mechanic (I think) and a sim guy (almost called you a sim tech again Kevin...haha), and not even getting the right procedure.

 

Boeing specifies 25% N2 or max motoring, if 25% isn't reached (defined as a lack of change in % N2 for at least a second, I believe).

 

Believe me - I've been in your position more than I'd like to admit.  While a lot of us appreciate enthusiasm, be careful where you use it, and how you use it.  I speak from very unfortunate experience.

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Absolute NO. I speak as a experienced ground engineer.

Regards

 

The r/h engine is in the "GND" position and is not only for cranking. Thats the way you start the 737 engine. At 18-20% N2 you open the fuel cntrl valve. Read your engine strart procedure!

Regards Jovabras

If the APU bleed air failed as the engine lit you'd have a very expensive engine repair bill unless you rapidly cut the fuel again. Whether you turn the fuel on at 18% or 23% the engine still needs assistance from the starter motor to avoid a hung start. As an experienced ground engineer you ought to know that.

 

In the situation the OP described fuel is off, therefore no ignition and no light off. Therefore the engine is cranking. Please read the post I originally replied to get the context in which I replied. My knowledge of start procedures is at least as good as yours.

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