737NG Pack Flow

[QAVION]

It's written in the Boeing 737NG Maintenance Manual that, with the packs in AUTO and both engine Bleed Switches OFF, the packs will go to High Flow. The Pack Wiring Schematics are pretty complex, but the logic appears to be valid with or without the APU running or with the aircraft in the air or on the ground.

Is there any operational reason for running the packs in High Flow in this configuration? If you only have the APU (or External Air) to feed the packs, rather than two engines, why would you want to make a single bleed air supplier work so hard? Why would you switch off the Engine Bleed switches anyway?

If you wanted High Flow, why not just select High Flow with the Pack Switches?

Thanks.
Cheers.
Q.

[B73567AMT]

Power to the Pack Low Flow Solenoids of the flow control valves is removed when the Eng Bleed Switches are turned off.
If you had both engine bleed switches OFF, then you would want the packs to supply a larger volume of air since the APU cannot supply as much as the engine bleed air can.

You could switch the Pack switches to high mode, but that is why they are kept in Automatic....so it can be done for you when necessary.

Why would you switch off the Engine Bleed switches anyway?
Dual Bleed problems (not Dual bleed light)?

[QAVION]

"If you had both engine bleed switches OFF, then you would want the packs to supply a larger volume of air since the APU cannot supply as much as the engine bleed air can."

Thanks, B73567AMT.
I guess that's one way of looking at it. I think I am more used to aircraft where the bleed air is used more extensively, such as for ADP's, Reversers, NAI, etc. In the air, it looks like the APU on a 737 is under less stress than other aircraft (if there is no air coming from the engines): It looks like the major loads for the 737 are limited to WAI and Aircon (or is there any way APU bleed can be used for Nacelle Anti-Ice?). I suppose engine start is another use.

"Why would you switch off the Engine Bleed switches anyway?
Dual Bleed problems (not Dual bleed light)?"

I was wondering that myself. Is there a procedure for smoke in the cockpit which involves shutting off both sources of engine bleed air?

Re GR's post (which mysteriously disappeared)....
"The only person normally capable of answering this type of question is you, and as you dont the answer I very much doubt anyone else does !"

Unfortunately, GR, I am very much the amateur when in comes to 737's... I think anyone can sound intelligent though, when they have the BMM, the FIM, the Schematics and the Wiring Diagrams in front of them... ;) ... not forgetting fellow engineers and unlimited internet resources!

Cheers and thanks!
Q.

[CaptainSandL]

Q,

Are you sure that your extract from the AMM is correct? My Vol 2 Systems manuals for the 3/4/500 and 700 all state that “AUTO - When operating one pack from APU (both engine bleed air switches OFF), regulates to high flow.” I can’t believe that you would want to drive the APU so hard when it is your only source of air. I may just try it on the ground tomorrow under engineering supervison!

Re both engine bleeds OFF scenarios. The various smoke/fumes drills do not call for any change in engine bleed position until you come to smoke/fumes removal, which then asks you to verify that both engine bleeds are ON. The best way to deal with a dual bleed problem is to switch the APU bleed OFF, not either of the engine bleeds. Even wing-body overheat drills only ask for the bleed on the affected side to be switched OFF, not both.

The only time I ever switch them both off is not for a non-normal, but for a bleeds off take-off or landing. Bleeds off take-offs are relatively common (maybe 1 in every 100 flights depending on your destinations and loads), but I have only ever had to do one bleeds off landing in 10 years on type. That was on an 18.5K engined, –500 on a flap 40, Cat IIIa, autoland in icing conditions. If you go through the perf books for that config you will find that you are so weight limited for the go-around climb gradient that you can barely shoot the approach with enough fuel for a diversion even if you have only a modest pax load – a very uncomfortable situation! :eek:

Regards

S & L (http://www.b737.org.uk)

[QAVION]

Are you sure that your extract from the AMM is correct? My Vol 2 Systems manuals for the 3/4/500 and 700 all state that “AUTO - When operating one pack from APU (both engine bleed air switches OFF), regulates to high flow.”

I could only prove it correct by doing what you're going to do, S&L ;) ... try it next time I'm on the aircraft.

Both the text and the Wiring Schematics seem to reflect the same thing. Also, the one pack scenario activates the same relay as the two engine bleed off scenario. Each provide an (different) earth/ground to energise the Norm(Low)/High relay.

Thanks for the tips on procedures, gents.... and I look forward to the test results.

Cheers.
Q.

[CaptainSandL]

OK, I have just got back in from experimenting with a 737-700. The aircraft was on the ground with the engines stopped and the APU running. Both packs were in AUTO, isolation valve OPEN and APU bleed ON. Note for non-NG pilots: You are supposed to run both packs from the APU when on the ground on an NG, unlike the classics where you are only supposed to run a single pack from the APU.

With both engine bleeds ON (normal config on a turnaround) the packs were running at the normal flow, APU EGT was 460C. I switched one engine bleed off – no change. Only when both engine bleeds were switched OFF did the packs go to high flow, APU EGT now 490C. Now, switching either engine bleed back ON made the packs return to normal flow.

I suspect that the Vol 2 (Page 2.10.4) is either wrong or badly worded when it says “When operating one pack from APU (both engine BLEED air switches OFF), regulates to high flow.” Because from the experiment it is also the case if operating both packs from the APU.

I think the EGT rise is also significant, because it is only a 30C rise for both packs to go from normal to high flow. I suspect that this is why the increase to high flow is scheduled (allowed) to happen because it is well within the capability of the APU. Remember that the Allied Signal APU on the NG’s is much more powerful than the classics and has no EGT limits.

Finally, bear in mind that the scheduling of flow from high to normal is dependant upon many other factors ie ground/air sensors; pack switch position; APU bleed valve; engine bleed switches; flap position and the position of the other pack switch. These factors can give many combinations, not all of which give predictable results in the flow rate. As a pilot I am happy to have the luxury of any AUTO switches in my flightdeck which make the necessary adjustments to bleed demands or whatever as required to save me from having to remember to do them, especially at times of high workload eg bleeds off take-off, EFATO etc.

S & L (http://www.b737.org.uk)

[QAVION]

Many thanks, S&L!

Something 737NG pilots may not be aware of, is that, with the APU running, there are two forms of high flow on the ground... The one you described in AUTO mode and the manually selected HIGH. The latter forces even more air into the ducts.

Different solenoids in the Pack Flow Valve are used to force the pack into these modes.

The normal AUTO "high" air flow (with both engine bleeds switched OFF) is 80 pounds per minute. Normal AUTO Low flow is 55 pounds per minute by the way. The manually selected HIGH (on the ground with APU running) is 100 pounds per minute. Selecting "HIGH" with only the engines providing air or with the aircraft in the air only produces the first mentioned type of high flow (80ppm).

Regards.
Q.

[B73567AMT]

My apologies if this has already been said, but the FCV APU High Flow mode is only available on the ground.