View Full Version : B737 oxygen valve: FCOM ERROR!!!!

31st Jul 2007, 10:51
Hi, I'm aware there are numerous variations of our manuals, maybe customized by Boeing for each airline (?)...

I noticed a very DANGEROUS mistake in ours, despite the Helios accident, Boeing still has it WRONG!

In the Normal procedures - I'm talking 737 classic but probably NG is the same -, where they tell you how to perform the mask test, I read:

If the oxygen cylinder is not in the full open position, pressure can:
.decrease rapidly. or
.decrease more than 100 psig, or
.increase slowly back to normal.


Actually, the sensor is located BEFORE the crew shutoff valve, not after!

So, if the valve is closed, dumping the pressure with the EMERGENCY selector of the mask, will have absolutely no effect on the gauge reading.

Try it.

Close the valve. Dump the pressure using the mask.

Observe the reading of the bottle pressure.

You will se no movement.

This error is absolutely crazy, from the part of Boeing and airlines.

Or have I missed something? :D

31st Jul 2007, 11:24
Oxygen cylinder valve, not the flightdeck shut-off valve.

The cylinder valve is the one that has been plagued by maintenance errors and should, IMHO be removed and replaced by something that can only be on or off and nothing in between. When this imaginary tap is off it would put a warning light on in the flight deck.

31st Jul 2007, 11:28
The B737NG crew oxy bottle shut off valve is located on the bottle itself (in the E & E bay accessed via the fwd cargo hold by removing the lwr centre bulkhead panel)it is upstream of the pressure transducers so this Boeing FCOM is correct for the B737NG aircraft.We use it as a test after servicing of crew oxy to ensure that the bottle shut off valve is fully open.This test does work for a NG.
The B737 CLASSIC is different (this a/c has options for different configurations depending on who ordered the airframe so do be carefull),the crew shut off valve is located in the flight deck physically and between the pressure regulator and the masks/demand regulators in the system,the pressure transducer is on the bottle side of the S.O.V so this test will not show if the bottle valve is closed as you said.
But the big difference is you have a valve that is accessable in flight on the classic`s.
Sorry i don`t have diagram`s available on a computer to help explain.

31st Jul 2007, 11:37
As far as I understand following the line from the cylinder to the mask, you have one pressure gauge (indicator on starboard side of outside fuselage...the green thing, verifies positive pressure), then the shut-off-valve, then the pressure gauge (to the cockpit indicator), then the pressure reducer, then the mask with regulator.
When checking the regulator you also select emergency flow, and hold the test/reset button for 5 sec, to check that the pressure doesn't drop more than max 100 psig to verify that the valve is not partly or fully shut. If it is you will simply empty whatever pressure you might have in the line between the valve and the mask. Just my two pennies worth.

31st Jul 2007, 20:44
You are talking about the "CREW OXYGEN SHUTOFF VALVE" at the P6 Panel.

FCOM says:
"If the oxygen cylinder is not in the full open position..."
so Terraplaneblues is absolutly right.

...Sorry i don`t have diagram`s available on a computer to help explain.Here they are:
http://img513.imageshack.us/img513/7364/737clata35pv9.th.jpg (http://img513.imageshack.us/my.php?image=737clata35pv9.jpg)...http://img513.imageshack.us/img513/9929/737ngata35pd2.th.jpg (http://img513.imageshack.us/my.php?image=737ngata35pd2.jpg)

@RYR-738-JOCKEY (86 years old???)
"...the green thing" starboard side indicates thought its existence only that the bottle has not discharge the O2 due to overpressure.

1st Aug 2007, 17:11
The O2 bottle Valve is lockwired in the open position.

1st Aug 2007, 18:21
This FCOM Procedure comes from the AMM.
Normally you should not let the valve stay in its fully opened position (end stop). This prevents the valve from locking in the open position.

The AMM tells you during installation that:
To open the shutoff valves on the oxygen cylinders, do the steps that follow:
(a) Open the shutoff valve on the crew oxygen cylinder slowly and fully.
(b) Do not tighten the valve more than 25 inch-pounds (Hand tighten).
(c) Close the valve one-fourth of a turn.
(d) When you put the system back to its initial condition, you must use an inspection wire to keep the valve in this position.

And here is the problem. If you mess up the open/close direction, it is possible that you leave the system in a slightly open (one-fourth of a turn) position.
This can fill up the system (slowly) to the bottle pressure and the indicator in the cockpit shows a proper value.
But if you try to use O2, the flow rate is to low to supply a normal breathing person and you get this scenario described in the FCOM.

BTW: Firefighter open their bottles nearly the same way here in germany (half turn back)

1st Aug 2007, 18:35
The machine is always right...

1st Aug 2007, 18:57
ASFKAP, for info...
did you read the title? :ok:

what do you try to tell us? :confused: