B737 Air Conditioning

[Snot Box]

I am in the midst of the -200 groundschool and I don't know why but I just seem to have a hard time understanding what the isolation valve switch (-200 Type) really do.

Perhaps some of you more experienced drivers can enlighten me on this topic. I thought I would have the most trouble with electrics but I guessed wrong!

[BOAC]

You've looked at this (http://www.b737.org.uk/pneumatics.htm) I take it?

In essence 'OPEN' or 'CLOSED' JOINS or ISOLATES (by selection) the left pneumatic duct to/from the right; 'AUTO' normally joins the two ducts but has 'automatic' ISOLATION functions in some pneumatic 'failure' conditions.

[Snot Box]

No I didn't know about this website before. Thank you for your help!

[PifPaf]

This is really a good site, but make sure that the text (and figures) are for -500 and -400. 737-200 packs doesn't have the AUTO position (only ON or OFF). Since I've never flown any 737 model but the -200, I don't know if the systems operate at the same manner.

Isolation valve permits, in certain conditions, that one bleed source feeds the opposite side - must be for eng start, operate a pack or give wing anti-ice.
CLOSE and OPEN positions are very easy to understand, don't you think? ;)
About AUTO, to remember how will the valve stay, take its name : ISOLATION. So, this means that, in normal conditions, AUTO position will maintain ISOLATION isolating both sides (closed). The ISOLATION VALVE will open if any pack or any eng bleed switch is positioned to OFF.
One good thing to remember is which switches of this panel are AC operated. Note that we have six switches (this order, from left to right, up to down): RH PACK, ISOLATION, LH PACK, LH ENG BLEED, APU BLEED and RH ENG BLEED. Now, imagine an "A" made in the panel: we will have LH ENG BLEED, ISOLATION and RH ENG BLEED, don't we? The "A" will show you the AC operated switches (the other ones, which make a "V", are DC operated).
Now you my ask why is important for you to remember it. It's because you must never forget to check "bleeds on" before shutting down the engines: in the event you lose APU and don't have 115VAC ground cart, you won't be able to start your engine ("battery start"), because you cannot open the engine bleeds.
I hope this helps you o understand a little bit more of the system.
Good flights!

PP

[Cross Check]

Pif now I’m confused :confused: …

The –200 is in question, but you said make sure that the text and figures are for –500/-400? := And since you’re flying –200’s, and if –200’s don’t have the AUTO position (I dunno coz I’m on –300’s, never seen a –200 up close and personal) why the description if it’s not applicable?

I gather your pneumatics panel/switch/valve description is peculiar to the –200’s coz on the –300’s the engine bleed valves are on their respective DC buses, only the isolation valve is AC (#1 TRFR Bus). Additionally, on engine start, the corresponding engine bleed valve closes when the start switch is turned to GRD and does not open until starter cutout.

So what’s the big drama with closed bleeds on the –200 for engine start? :uhoh: … curious how the other half lives :ok:

[Cough]

Cross Check.

The isolation valve on the -2 still has the auto position, its the pack switches that don't have an auto position, simply off/on (-2) as opposed to off/auto/high(-345etc).

Crossbleed starts on the -2 aren't much different from the rest, but most of the time you don't have to do them coz the ground crew frequently will let you start both from the air cart. Smaller intakes...further out...safer!

[Cross Check]

Oh yeah ... RTFQ ... := missed the word "packs".

But still (and RTFQ yourself :p ) it was closed bleeds, no normal AC available => can't start either donk which puzzles me :confused: Wot ... is there too much puff in the spooling N2 high pressure section to light off with the lower bypass engines or something?

[PifPaf]

Cross Check,

Air must flow through bleed valve to reach the starter, thus enabling the engine to begin running.
This air could be from APU, gnd cart or even the other engine.

I don't know the -300 system, but in the -200 series the engine bleeds must be ON for start - and it is AC powered, as I said before. That's why you'll be in trouble if you don't have 115VAC (from APU or a gnd cart).

If you have any further question or if my answer wasn't so good (my english is poor, I know, I know...), don't hesitate to ask!

Cheers,

PP

[Cross Check]

Hey Pif :cool: No wuckin furries mate your English is fine - you should try out the English down here :ugh:

Maybe I'm missing something here or there is something substantially different about the -200? Even on the -300's ENG BLEEDS ON is required in the shutdown C/L, but it doesn't prevent a subsequent start in the event of no AC being available as far as I know. Plus if you were doing a crossbleed start you would have the opposite Generator on bus so there's your AC source. Bear in mind the 737's (well at least the original generation) were designed and built with crappy aerodrome services in mind! :}

If you are doing a Battery Start (with APU bleed air, but no AC) then you won't be able to start ENG #2 first because the Isolation Valve will be closed (TRFR BUS #1 => AC) ... so start ENG #1 (recommended anyway). You will have bleed air to the #1 starter (APU Bleed Valve is on BAT BUS) and you can engage the starter (both Starter Valves are on BAT BUS) ... clean start, GEN #1 on bus, AC available, the Isolation Valve will open, start ENG #2 ... ya laughing :cool: Or if you were starting with a GND cart for air you'd just have to start ENG #2 first (after the F/O hangs out the windows gesticulating wildly to the fool walking by the inlet face :eek: ) Okay, if for some screwed up twist of fate neither Generator would come online, the APU GEN is fritzed and the GND staff can't get the cart started coz somebody borrowed the battery and didn't bring it back ... well ... I wouldn't be laughing then :mad: ... but then ... we wouldn't be going anyway :ok:

A very learned ground instructor once told me that it is possible to get to the Isolation Valve and manually manipulate it, as ground engineers do on occasion. It's buried somewhere high up in the main gear well I think he said, but I've never had chance to find it - there's never enough time and it's always too noisy when I'm there :bored:

[PifPaf]

Hi X-Check;

If you are doing a Battery Start (with APU bleed air, but no AC) then you won't be able to start ENG #2 first because the Isolation Valve will be closed (TRFR BUS #1 => AC) ... so start ENG #1 (recommended anyway). You will have bleed air to the #1 starter (APU Bleed Valve is on BAT BUS) and you can engage the starter (both Starter Valves are on BAT BUS)

No, you will only be able to engage the starter if the eng bleed is open, permitting air to flow to the starter.
Doing Bat start, no matter using APU bleed or gnd cart bleed air, if you don't have AC you can't move eng bleeds (so, you must left it open!). In the system schematic you'll see that, to reach the start valve air must flow thru eng bleed (going into the engine). That's why you couldn't start your "first" engine if the respective eng bleed was off.
After starting the first one, generator on bus, AC available and no problems to start the other engine (and now, with AC, no more BAT start - will have oil press, fuel flow etc).
Hope this clarifies, friend!
Cheers,

PP

[Cross Check]

Hmmm ... :hmm: ... Kinda defies the logic of a battery start doesn't it?

That's gotta be something queer with the -200's then coz on my system schematic for the -300/500's the Starter Valve off-take is downstream of the Engine Bleed Valves. Mr Boeing sure screwed that up then if yours is the case. :ugh:

I don't envy you then, stickin' to my -300 till something better comes along :cool:

May all your starts be AC ;)

[eng123]

Cross check,sorry,you are wrong.The starter pneumatic off-take is before the bleed valve on the classic [-3/4/500 as well as NG] but it is after the bleed valve on -200.That's why PifPaf is correct in saying you need bleeds on for engine start on -200's.You can start engines with the bleed switch off on -300 and beyond.
Also,the Isolation valve is buried in the keel beam within the air con bay,not the MLG wheel well.
Being a 'heavy',I would have to look at the wiring diagrams for the electrical side of this debate.

[Cross Check]

Errmm ... but by your description you've just described I'm right ... :confused:

I think we have an apparent "difference" of opinion due to duct location relative to airflow, but are visualising the correct picture.

You have just confirmed what I suspected Pif was alluding to - namely the starter duct is isolated from it's respective main (L/R) manifold by its respective ENG BLEED valve for the -200's whereas the same duct on the later variants can be "considered" to branch off the respective pneumatic manifold directly. Are we in agreement now? :) Pretty please? :D

Yeah you're right, I remember now - ta - the ISOL valve is tucked away between the packs in the A/C bay. It can be gotten to, but it's a bit of a chore.

[eng123]

Crosscheck,Yes we are in agreement.The confusion arose when in your previous post you said the starter off-take was downstream of the bleed valve on the -300,whereas it is,in fact, upstream.[when talking about air being supplied by APU/ground cart] It is,of course downstream when air is being supplied by the engine,in which case this debate would be irrelevant as the engine is already running!

[Cross Check]

Yeah sorry eng123 - always thinking engines as point of reference. Should have used inboard/outboard of such-n-such. :8

Hey PifPaf (and eng123 can you confirm this):

Couldn't believe Boeing would be so dumb to have AC ENG BLEEDs that couldn't be opened if there was zip AC power available thereby making the first start impossible (I'm speaking -200's of course). Spoke to an old dog on -200's and the ENG BLEED valves are powered by the AC STBY BUS so unless you've cooked (or lost :eek: ) your ships battery - switch the STBY POWER switch to BATT and voila ... start away! So it doesn't necessarily matter what position the ENG BLEEDs were on shutdown coz you can always open them again on STBY PWR afterwards.

:ok:

[PifPaf]

Hi Cross Check,

Sorry for taking so much time to answer. First I was traveling, and when I returned I couldn't access the forum (and by now it's still slow, despite using DSL connection - the hackers...)

Regarding your last message, you said
Couldn't believe Boeing would be so dumb
Well, if you think that would make Boeing dumb...I'd have to tell you that your "-200 old dog" was wrong. When you select STBY BUS, you will power only AC STBY BUS (no big deal, hummm). The matter is that L ENG BLEED is connected to BUS TRANSFER 1 (and so is ISOLATION VALVE), and R ENG BLEED is on BUS TRANSFER 2.
So, even placing the switch to STBY BUS you wouldn't be able to operate ENG BLEEDS, as transfer busses 1 & 2 wouldn't be powered - and we return to the starting point, when I said you couldn't start you engine if bleeds were off and you haven't AC power!
That's it, friend!

PP

[Cross Check]

:uhoh:

:hmm: Standby One ... cross checking now ...

:8

[timzsta]

Tell you what, next time you get back on stand, wack the bleeds off before you shut down the hairdryers:eek: Then get the FO to go hide the GPU:O Find a friendly engineer to disable the APU:} Then see if your oppo's who are doing the next sector can get the old girl started again:mad:

Report findings:cool: