Qantas744er
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747-400 Engines Starts

Wed Oct 15, 2008 12:04 am

Recently reading on a forum i came across the statement of a ex 744 mechanic saying that during 744 engine starts for 1-2 the right ISLN valve will be turned off while during starts for 3-4 the left ISLN valve will go off. But if pack 2 and 3 are off during start how can bleed air from the APU get to engines 3 and 4 if the left ISLN valve is off with packs 2 and 3 off?

I have never seen a 744 checklist mentioning ISLN valve procedures.

How doe sit really work on the 744?

Leo
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jetmech
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RE: 747-400 Engines Starts

Wed Oct 15, 2008 3:24 am



Quoting Qantas744ER (Thread starter):
But if pack 2 and 3 are off during start how can bleed air from the APU get to engines 3 and 4 if the left ISLN valve is off with packs 2 and 3 off?

According to this diagram, the pack isolation valves and starter valves are arranged in parallel after the wing isolation valves. Thus, air does not need to pass through the packs to reach the engines for starting.



http://meriweather.com/747/over/bleed.gif

Regards, JetMech
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Qantas744er
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RE: 747-400 Engines Starts

Wed Oct 15, 2008 3:47 am



Quoting JetMech (Reply 1):
According to this diagram, the pack isolation valves and starter valves are arranged in parallel after the wing isolation valves. Thus, air does not need to pass through the packs to reach the engines for starting.

Ok now that makes some sense, so thanks for that! But i am still wondering about the actual use of these valves during engine start.

Not even on the few videos online were 744 engine starts are visible i see the use of these vavles.

Leo
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AA737-823
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RE: 747-400 Engines Starts

Wed Oct 15, 2008 6:53 am

I realize that this isn't an entirely technical answer, but I AND OLD: Guangzhou - Baiyun (CAN / ZGGG) (closed), China">CAN tell you that we can start engines on a 744 AND run a pack at the same time.

We got into a heated discussion on this board a few weeks ago, wherein someone swore up and down that a 744 could run all three packs and start all four engines at the same time..... somehow, I still doubt that....
 
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Francoflier
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RE: 747-400 Engines Starts

Wed Oct 15, 2008 10:03 am



Quoting Qantas744ER (Thread starter):
But if pack 2 and 3 are off during start how can bleed air from the APU get to engines 3 and 4 if the left ISLN valve is off with packs 2 and 3 off?

I'm afraid I don't really understand your question.

Isolation valves are left alone (open) during engine start.

If the left ISOL valve was closed, it wouldn't prevent #3 and 4 (right engines) to start since APU air would still get to them.

Pack 1 and 3 are usually shut for engine start, but since the isolation valves remain open, any of the three packs could keep running during start.

AFAIK the only time an Isolation valve will have to be closed during a start is when a start valve is MEL'd and a mechanic has to manually open and close it.
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PhilSquares
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RE: 747-400 Engines Starts

Wed Oct 15, 2008 10:30 am



Quoting AA737-823 (Reply 3):
We got into a heated discussion on this board a few weeks ago, wherein someone swore up and down that a 744 could run all three packs and start all four engines at the same time..... somehow, I still doubt that....

Well, here it goes again. Yes the 400 can be started, if fitted with auto start with packs on and just the APU. However, in normal line ops it isn't done. But, it is capable of doing that.

The L/R Isolation Valves are normally left in the open position for all normal engine starts. However, if you have a weak APU or two weak huffers, it's not a bad idea to close the opposite Iso Vlv. So, if you're trying to start 4 (aren't going to start 2 engines with a weak airflow) you would close the L Iso Vlv and isolate the L side and have the all the bleed going to the right side.

But as I wrote earlier, in normal line ops, the ISO VLV is left open on both sides.
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fr8mech
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RE: 747-400 Engines Starts

Wed Oct 15, 2008 7:17 pm



Quoting Qantas744ER (Thread starter):
744 mechanic

Very often the maintenance checklist for run and taxi will be different that the flight crew checklist for departure. Not really sure as to the reason, but sometimes it's true. maybe that's the case here. I do know that our B744 maintenance checklist has us ensure both isolation valves are open and packs off.
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