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Jet Engine Start Attempt On Running Engine  
User currently offlineTaguilo From Argentina, joined Aug 2005, 74 posts, RR: 0
Posted (8 years 1 week 3 days 15 hours ago) and read 4271 times:

Hi,

I would like to know what would happen on this hypothetical scenario:
(757/67 but I guess is the same for other a/c)

Eng(n) is running at idle or with power. Eng(n) starter switch rotated to GND.

Case 1:
ECU make switch cuts immediately to AUTO, Eng(n) bleed valves are not commanded to close, start valve NEVER opens and APU bleed valve is kept in the current position.

Case 2:
Start valve opens, Eng(n) bleed valves are commanded closed, APU bleed valve, if APU on, is commanded open. As Eng N2/N3 RPM are greater than 50 %, Start switch cuts to AUTO, Start valve closes, Eng and APU bleed valves perform the same as in a normal start.


Case 3:
A mix of Case 1 and Case 2...

Many Thanks

Tom

6 replies: All unread, jump to last
 
User currently offlineKaddyuk From Wallis and Futuna, joined Nov 2001, 4126 posts, RR: 26
Reply 1, posted (8 years 1 week 3 days 15 hours ago) and read 4270 times:

My Guess is that the ECU is smart enough to realise that the engine is already running and will do nothing but possibly flag a warning message on EICAS (as you refered to the '57).

Perhaps case 1 is the more likely scenario.

I'm also pretty sure that above a certain speed the start motor is physically disconnected from the gearbox and so even if the start valve did open, it would not affect the running of the engine.



Whoever said "laughter is the best medicine" never had Gonorrhea
User currently offlineDc10hound From United States of America, joined Dec 2000, 463 posts, RR: 5
Reply 2, posted (8 years 1 week 3 days 12 hours ago) and read 4168 times:

My first thought is why would you want to even do this.
It's sort of like putting your car in reverse a 60 MPH to see what happens...

 Wink

Case 1:
ECU make switch cuts immediately to AUTO, Eng(n) bleed valves are not commanded to close, start valve NEVER opens and APU bleed valve is kept in the current position.


You'll never get the start switch to latch in the GND position.

Once the engine is running at greater than 47% N1, the ground in the DGCU(Dedicated Generator Control Unit) is removed from the latching coil in the start switch. There are enough time delays in the pneumatic valve circuits to keep any of them from closing or opening.

Now if you hold the switch in the GND position, some of the events in Case 2 will occur.

You will also get a Level B STARTER CUTOUT message from EICAS, a VALVE light on the start panel and I would imagine a ruined starter...



"Eagles soar. But weasels never get sucked into jet intakes.."
User currently offlineTaguilo From Argentina, joined Aug 2005, 74 posts, RR: 0
Reply 3, posted (8 years 1 week 3 days 10 hours ago) and read 4126 times:

Quoting Dc10hound (Reply 2):
My first thought is why would you want to even do this.

Actually I won't, but probably some people that are going to fly my panel eventually will   That's the reason I need to know what I should code to resemble the real stuff as much as possible.

You know, I had a hunch it would be the way you describe, so I programmed a small latch time once switch is rotated to GND and released, after half a second it returns back to auto, firing the typical "clack" sound when reaching the AUTO position. Now I have to program the "hold in GND position" case 
Tough work!


Many thanks,

Tom

[Edited 2006-04-11 05:45:29]

User currently offline777WT From United States of America, joined Jan 2005, 874 posts, RR: 1
Reply 4, posted (8 years 1 week 3 days 10 hours ago) and read 4119 times:

Quoting Dc10hound (Reply 2):
My first thought is why would you want to even do this.
It's sort of like putting your car in reverse a 60 MPH to see what happens...

Been there, done that in a rental Chevy Colorando pickup truck, had it for a month.

I was excepting to hear gears or clutches grind or see a trail of transmission parts in my rearview mirror, but none of that happened. I shift into reverse and you feel a light braking action, the engine is still doing a high idle at highway speed (1,500 rpm) and adding throttle makes it slow down!

Sometimes I would go into a nearly empty parking lot, shift into reverse and gas it up til it stops then I jam it in park  Wink I like to call this "thrust reverser" Big grin

It was still running when I returned it  Wink


User currently offlineKaddyuk From Wallis and Futuna, joined Nov 2001, 4126 posts, RR: 26
Reply 5, posted (8 years 1 week 3 days 1 hour ago) and read 4014 times:

Quoting 777WT (Reply 4):
It was still running when I returned it

Abeit it had to be carried into the lot on a pick-up because it had become jammed into park...

Haha!



Whoever said "laughter is the best medicine" never had Gonorrhea
User currently offlineVC-10 From United Kingdom, joined Oct 1999, 3695 posts, RR: 35
Reply 6, posted (8 years 1 week 2 days 21 hours ago) and read 3932 times:
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B757 (RB211)

The Eng Start Sw is held in the GRND position by a holding relay. The relay releases the Sw back to Auto when the Dedicated Generator Unit senses N3 speed >43%.

So th senarios you cite cannot happen, the Start Sw will not hold in GRND.


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