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anchorriver
25th Mar 2009, 11:17
My fellow PPRuNe users,

I would be grateful if you can enlighten me with your technical knowledge regarding this procedure. QRH states;

When N2 below 20%
HP BLEED VALVE (affected engine)…..OFF
ENG START selector…..CONT RELIGHT
SECOND START ATTEMPT…..PERFORM

According to FCOM1-17-60, p3/4, the pack valves are controlled closed if the INGNITION selector is on START A, START B or CRANK position. So, with ENG START selector in CONT RELIGHT position, in this case, the pack valves are controlled OPEN, I assume.

So, now you have HP BLEED VALVE (affected ENG) OFF, as well as the PACK VALVES open. Would you explain to me why this would improve the ENG start performance after experiencing hung start?

Thanks in advance. Safe flight, guys.

Anchor river

hetfield
25th Mar 2009, 11:32
Hi,

if the selector position is CONT RELIGHT, the pack valves are closed by the latching of one of the two START pb-switches. In this case, each ARM light is illuminated when the relevant N2 is lower than 45% (excepted if the relevant start valve is controlled open).


IGN CONT RELIGHT 'cause this gives you both igniters.

Kind regards

tubby linton
25th Mar 2009, 17:20
The closure of the HP valve helps to regulate the airflow demand within the engine.HP air is drawn from the last stage of the compressor(14th).

anchorriver
27th Mar 2009, 01:23
Dear Hetfield and Tubby Linton,

Thank you both for your quick response. I really appreciated. I did not know that the pack valves would be closed with ENG start selector in CONT RELIGHT position.

During normal ENG start, HP Bleed valves are in AUTO position, and controlled OPEN (FCOM1-16-20, p5/6). Clearly, as you stated, Tubby Linton, by closing HP Bleed valve on the affected side it must “help to regulate the airflow demand within the engine.” I don’t mean to be a nit picky, folks, but I want to know how the closed HP valve helps regulate the airflow. If so, and it must be so, how come we don’t do the same to start ENG all the time?

During Hung Start, N2 is usually hanging below idle. I wonder if by closing HP valve, you force IP valve OPEN so that N2 can spin up easier…

I wish I have time and money to join the engineering school.

Anchor River

tubby linton
27th Mar 2009, 09:34
Anchor river I would suggest that you ask this question on the engineer's forum and you may get a reply from a powerplant expert.However here is my basic understanding of it as a pilot.By closing the hp you are no longer drawing air off the engine during start.As I mentioned my fcom states that it draws it from the last stage of the compressor.This is obviously the point before the air is then mixed with fuel and ignited.I do not know how much air is drawn through that valve but by closing it I can only assume that you are adding more compressed air into the combustion area and changing the fuel air mix..
Closing the hp valve means air is bled from the Ip stage through the Ip bleed air check valves.The fcom also notes that normally IP air is used as a bleed source to minimise engine pressure losses.I would therefore infer that the HP is closed in order to get the maximum amount of compressed air into the combustion chamber.
I would imagine that through testing it was discovered that for the majority of starts this procedure is not necessary.

rider of the storm
28th Mar 2009, 15:24
Hi Anchor river,
Hetfield has already stated that the reason for using the CONT RELIGHT position is the energize both igniters in case the hung start is due to weak igniters.

Now for the HP VALVE closure I would say the reason might be an air bleed leak through the associated HP VALVE that might have caused the HUNG START and we want to isolate the pneumatic lines to prevent this.

Then again, this is only a pilots point of view so I stand to be corrected.


Happy landings

ROTS

anchorriver
3rd Apr 2009, 05:40
Dear Tubby Linton and ROTS,

Sorry I couldn't say thanks to you earlier. Just got home this morning. Anyhow, I will visit the engineering office and ask them.

AR


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