737NG Air cart/normal vs Crossbleed

[Kev738]

Hello all,


Reviewing FCOM 1 I found:
Start no.1 first(Obvious reason that ground equipment will obstruct no.2)
Use normal start procedure... which means Isolation valve switch OPEN.

Then; for a crossbleed start it says the isolation valve must be positioned AUTO. Since the pack switches will be OFF, the actual valve should still be open.
Why is the isolation valve to be set to open during a normal and ground start and to AUTO for a crossbleed start?

Just wondering what would make the big difference, it seems to be pointless to move the switch at this stage.

Thanks in advance!!

[Jwscud]

Many companies will leave the Isolation valve in AUTO throughout engine start. I believe a certain large lo-co likes to keep it OPEN for start, which may be a hangover from the -200.

[Skyjob]

Correct, the isolation switch should not need to be moved from AUTO

In fact and even alone for the following reasoning: if the isolations switch found to be unserviceable, this is a NO GO item! Check your MEL/MMEL/DDPG!

There is NO requirements to OPEN the isolation valve switch and this is indeed may be a requirement on pre-Classic variants like Jdscud said, but not required on the Classic and NG.

Think about HOW will you determine if the isolation valve switch functions properly in AUTO if you do not let it perform the function designed to be completed when selected in AUTO?

The ONLY time the switch should be moved from the AUTO into OPEN is when APU Cabin Air is used for air conditioning as the fuel flow for single pack operation on the APU is higher then for both packs by ~10%.

Of course, there is a NTO from Boeing to allow always opening the isolation switch valve to start... Is there a provision attached? Who knows... Someone must test the function of the isolation switch at some time prior to flight if it constitutes a No Go...

[ant1]

Oh yes, there is a need for having the Isolation valve Open on the ground on the NG and it comes from the fact that you have to be able to supply APU bleed air to 2 packs.

On the other hand on the Classics you have only 1 pack on the ground and that is why there is no need to move the Isolation Valve to Open and hence leave it on Auto.

[Skyjob]

Ant1 you just repeated the ONLY exception as already mentioned in paragraph 5 of previous post for the NG, lol.

[ant1]

AFAIK, no exeption here: Open is the position in which you will find the isolation valve on the NG, on the ground, it's part of the shutdown and preflight procedure. Can't believe I'm writing this.

[Fullblast]

In my opinion the truth lies in the standardization. Type ratings in easa world includes all the models from -300 upwards, -200 also for FAA, so, K.I.S.S.
Anyway, here in the tech section the OP is looking for a technical answer, if there is any.

[Skyjob]

Open is the position in which you will find the isolation valve on the NG, on the ground, it's part of the shutdown and preflight procedure.

Ant1: That's a company procedure, not standard Boeing...

[ant1]

Really? It's in the FCOM: preflight and shutdown. I haven't found so far any company so far that contradicts Boeing FCOM, but there might be a few daring ones.

If your FCOM says differently, I'd love to have a copy, I'll frame it and hang it over the mantelpiece.

[BOAC]

Only required if both packs are needed for conditioning. 'Normal' conditioning is left pack, ISOL 'Auto', and check right duct pressure rises when left pack off for start (as per post by Skyjob) or you could all be flying with a u/s ISOL valve!

[Jwscud]

Ant1 - the FCOM is a company document, issued by Mr. Boeing containing both Boeing and company procedures. If you saw another company's FCOM1/2 you may be surprised.

[Kev738]

Thanks guys,


Was thinking/hoping there was a more in depth technical reason I couldn't find anywhere, but there isn't really!


Cheers!

[ant1]

Jwscud,

As you say, I might. Untill now I've never seen any document that says otherwise, so in my experience they seem to be a "rara avis" if they exist. Anybody feel free to post an NG FCOM where preflight and shutdown procedure saya ISOLATION VALVE Auto. I have a "penchant" for collector's items.

[latetonite]

Amazing, in this high tech era, with all kinds of gizmo's in the cockpit, that a 6 button electro-mechanical design from the fifties causes such a confusion.

[Skyjob]

The Boeing Template for 737NG FCOM in all procedures published (except those referencing specifically different) makes the following assumption: APU is used for air-conditioning.

The above assumption is usually NOT used by airlines in more recent times due fuel saving practices and noise restrictions at airports, therefore the placements of switches to facilitate this should be addressed when these procedures are implemented.

Therefore the template document makes the following collection of statements about NG Series aircraft in the pre-flight Air conditioning panel.............. Set:

AIR TEMPERATURE source selector - As needed
[737-800/900] TRIM AIR switch - ON
[737-600/700] Verify the DUCT OVERHEAT lights are extinguished
[737-800/900] Verify the ZONE TEMP lights are extinguished
Temperature selectors - As needed
Verify that the RAM DOOR FULL OPEN lights are illuminated
[737-600/700] RECIRCULATION FAN switch - AUTO
[737-800/900] RECIRCULATION FAN switches - AUTO
Air conditioning PACK switches - AUTO or HIGH
ISOLATION VALVE switch - OPEN
Engine BLEED air switches - ON
APU BLEED air switch - ON
Verify that the DUAL BLEED light is illuminated
[737-600/700] Verify that the PACK TRIP OFF lights are extinguished
[737-800/900] Verify that the PACK lights are extinguished
Verify that the WING-BODY OVERHEAT lights are extinguished
Verify that the BLEED TRIP OFF lights are extinguished


All steps below the statement APU BLEED air switch - ON require you to have an air source available to complete the annunciation checks of the described lights.
Therefore, the steps immediately prior to this facilitate what you are trying to achieve and by opening the isolation valve both sides of the duct are pressurised and thus the checklist can be continued to checked for accurate indications.

IF your company or airport thus does not (allow/prefer) to use the APU during this stage of flight or if the APU is INOP, the statements following describing the indications of lights cannot be checked and these checks should be introduced/moved at that point in the checklist flow where such checks can be performed. Two possibilities of which the second requires NO modification of the FCOM:

Immediately prior to the Engine Start Procedure when an External Air Cart or APU (therefore duct pressure) is available [thus having to physically move the check items in the checklist];
Inclusion of the check items in the Before Taxi Procedure.


* Setting up the panel for a pressurised takeoff at step Recall.......Check states: Verify that all system annunciator panel lights illuminate and then extinguish.

Note: standard Boeing also have the F/O or RHS start engines at all times, and assign designated areas of responsibility C and F/O or LHS/RHS for the Before Taxi Procedure, not PF/PNF, again this may be different in a variation of your company's FCOM...

[Aluminium shuffler]

It's interesting that every company claims to be "Boeing standard", but each operates completely differently.

Anyway, there is no technical reason to manually select the isolation valve open unless it is malfunctioning in the "auto" position. I'm not sure I'd want to take an aircraft with such a fault, because there's no way to tell if will lead to a complete lack of control over the valve with it being unresponsive to manual signals as well as auto signals. As long as engineers could verify that it's a sensing or signalling issue, not a valve or dc controller issue, then no problem.

[ant1]

I guess all the companies I've worked for/known have been using A/C. Even when at airport where there was a restriction on APU usage of x minutes b4 start we've been using the A/C during those x minutes for PAX comfort. So as far as I'm concerned, there's always been a good reason: the FCOM (Boeing) said so.

Whatever the template says (I've never seen it), I wonder what most companies do, in my own experience OPEN is the industry standard for the NG.