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-   -   B737 Flt/Gnd switch CPCS (https://www.pprune.org/tech-log/277626-b737-flt-gnd-switch-cpcs.html)

G-Dawg 27th May 2007 00:55

B737 Flt/Gnd switch CPCS
 
Ok Guys

Just wrote this big question asking lots of interesting questions and witty observations, clicked the spell check option and lost it all....:mad:...so short and sweet, i thought the flt/gnd switch had air logic, hence if you forgot to put it to flight before t/o it wouldn't matter, however a few days ago i was shown this not to be the case, at least in the initial climb. I've looked for some info on it, can someone shed some light???

Cheers

G

IFixPlanes 27th May 2007 06:18

MLG in AIR and OK... :
http://img518.imageshack.us/img518/7...grd1ch9.th.jpg
http://img512.imageshack.us/img512/7...grd2jm5.th.jpg

Edit:
Typo correction and
Links removed due to lack of interest :mad:

InSoMnIaC 27th May 2007 17:35

G-Dawg: i thought the flt/gnd switch had air logic, hence if you forgot to put it to flight before t/o it wouldn't matter, however a few days ago i was shown this not to be the case, at least in the initial climb. I've looked for some info on it, can someone shed some light???

Yes it does have air logic in that if you place the flt/gnd switch to gnd while in pressurised flight then it won't depressurise. ie it is useless once airborne.

The point of the switch is to pressurise the cabin to an altitude of 200' below the departure airfied elevation before you take off. so placing the switch to flt before departure will pressurise the cabin slightly. this helps the aircraft structurally ie its a more solid object and also helps the passangers by providing a more comfortable transition to pressurised flight.

If the switch is left to gnd before departure it's not a big deal however the aircraft won't be pressurised slightly before the departure. soon as the air/gnd sensor detects flight mode then the cabin pressurises anyway and the flt/gnd switch is useless until touching down again.

the landing is pretty much the same.. the aircraft will remain slightly pressurised for the landing. if the flt/gnd switch is forgotten in GND then as soon as the aircraft touches down it'll depressurise. if it is in flt then it will remain in the slightly pressurised state until the flt/gnd switch is switched to gnd.

G-Dawg 29th May 2007 17:56

Yeah that's what i though, I have just had a running disagreement with one of the Captains here for the last week about it. Do you know at what differencial PSI the air logic will kick in as if you do leave it on grd, it will initially climb with the aircraft, up to a point then start to pressurize!

IFixPlanes 29th May 2007 18:11

Didnīt you see my Drawings? :confused:

G-Dawg 29th May 2007 18:36

yeah i did but the res on them is a bit low and its hard for me to make out, the exact schematic!!!

IFixPlanes 29th May 2007 18:44


...but the res on them is a bit low...
Sorry, then i can not help you :}

Rainboe 29th May 2007 20:04

That diagram is interesting. I once had a faulty Right Main Gear Ground/Flight switch give a Ground signal at 33,000'. Gentle depressurisation only noticed when the TOC/Cabin alt Warning went off. I do remember seeing the cabin alt on the cusp of 14,000' before we stopped it rising.

BOAC 30th May 2007 07:34

Conversely I had the air-ground switches fail in 'air mode' on landing which makes you scratch your head a bit!

As Rainboe says, we believe the activation signal is from the Rt main gear on the Classic, not the NLG.

NB: To improve the image res, rt click on the image, copy and paste into your favourite image handling prog - it might just work.

forget 30th May 2007 07:59

IFixPlanes, Allow me to thank you for the all the trouble you've gone to in posting your (hi-res) diagrams - as no b*gger else is.

I-2021 30th May 2007 11:04

Hello,

I remember somehow that if you forget the switch in GND position after T/O the system will work in standby mode... but maybe I am just making confusion with something else.

Byez.

G-Dawg 30th May 2007 17:26

Yes agree, IFP, thanks for posting the pics and have cut and pasted them into a doc and there great thanks, to prove to my capo all the points made above, took off with it in grd and watched as it pressurized nicely, thank you very much...thanks for all the info guys..

InSoMnIaC 30th May 2007 22:15

g-dawg

u have much to learn about captains my friend. if you get into a discussion with them concerning the systems and they say some thing that is incorrect it is very hard for some to admit that an FO could know more than them. u know the best way to handle them is just by agreeing with them. forget reasoning. and always remember you might be right but he is always left..

on a more serious note:
BOAC: if the flt/gnd switch is stuck in flight then can't u just set the pressurisation mode selector to manual and manually open the outflow valve after landing?

BOAC 31st May 2007 07:04

InSoMnIaC - of course, but that was just ONE of the head-scratches! The first clue was stick-shake on the landing roll 'cos we were below stall speed..............................:)


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