B737 pressurisation dumping

[downwind]

G'day Folks,

Quick Q, if in the simulator or real thing on the 737-300/500 what would happen is you placed the pressurisation selector to Manual AC mode and opened the pressurisation valve to full open, would the aircraft depress itself, or would the system logic realise sonething is up?? (Would the CAB ALT warning horn come on??????)

[cirrus01]

AC MAN would give you control of the outflow valve via the "close/open" toggle switch.....You will have to monitor the Cabin Alt......Get it all wrong ( or if the ac motor packs up ) and the cabin climbs ...... 10000 ' Cab alt horn goes, and then if uncorrected at 14625' Cab alt , the controller commands the outflow valve to close using the DC motor.

:ok: :ok:

[downwind]

cirrus01,

What I mean can you INTENTIONALLY depressurise the a/c using man ac or dc, just purely interested? (Never tried it in a sim??) Ie will you get the cabin altitude warning horn at 10000' if you don't catch the cabin pressure??? I think like the old DC9??

[LEM]

downwind, of course yes. yes and yes.

and then if uncorrected at 14625' Cab alt , the controller commands the outflow valve to close using the DC motor.
cirrus01, my ignorance knows no limits, but I've never heard about that feature.

[cirrus01]

Lem....

I should have made it clearer......... Cabin Alt of 14625 while in Stby or Auto mode will drive the outflow valve closed. (Pressure switch inside the Press. Controller) .
Failure of electrical power to either Man AC or DC circuits , then the outflow valve remains in its last position, held by the AC motor brake.
:ok: :ok:

[LEM]

cirrus01,
are you sure we are talking about classics?

I'd be grateful if you quote the manual reference for the 14625 ft feature, as I still can't find it.

Highly appreciated to get rid of my headache.




7p3i7lot,
what is the captain supposed to check when in the BEFORE START CKLIST he answeres "One pack, bleeds on ,set "?
;)


LEM

[cirrus01]

Lem.....

should be able to do that next week.....when I am back into work :* :*

[Flight Detent]

Hi all,
That pressurization recovery feature is the same in the Classic B747, but only whilst in one of the 'auto' modes.

If you were to operate the manual switch to 'open', the outflow valve WILL open, and you will depressurize, and the 10,000 foot warning horn will sound, (it's a 'dumb' horn, it always goes off at 10,000 feet, regardless of how you got there!).

Hope this helps,
Cheers

[alexban]

LEM I didn't find what cirrus was talking about,and I suspect it wrong,b
but I can tell you this: at 13875' the AUTO mode will fail and the system will go to STANDBY which,using the DC motor (unlike the AUTO which uses AC,that has already failed) ,tryes to close the outflow valve.
I found no mention about the 14625' feature .
Brgds
Alex

[LEM]

Exactly as you say, alex.
Thanks

LEM

[DDG]

LEM,ALEXBAN & Cirrus01,
Gents,
There are two different pressurisation config`s available for 737Classic depending upon build dates.

The latest uses dual digital cabin pressure controllers(evident by the cabin pressuization panel mode selector having "Auto"."ALT" and "MAN" selection available").The dual digital system is almost identical to the 737NG.

The NG System`s outflow valve`s two main electric actuators on the valve has a fail-safe aneroid switch which causes the outflow valve to fully close if the cabin pressure reaches 14,500ft.
This function overrides normal automatic control only.
It does not override manual mode of the valve.
Reference Boeing B737NG AMM part1 Chapter 21-30-00p40.

I know that this system is fitted to the AIR NEWZEALAND B737-300`s as they were some of the last of the Classic`s built.

This is yet another good reminder of how there can be drastic differences between aircraft models and series depending upon customer option`s and build dates.

Regards DDG

[LEM]

Roger that, thanks!

LEM

[cirrus01]

Lem……..( and the doubting alexban ! )
this straight from the horses mouth……..
Boeing 737 –300/400/500 Maintenance manual Ref. 21-31-00 (page 7)


“Pressurization control is automatically transferred from automatic mode to standby if the pressure rate of change exceeds 1.0 psi per minute or cabin altitude exceeds 13,895 feet. In the event of a power failure to the automatic system, control will automatically be transferred to the standby system provided power is available to that system. Upon transfer, the cabin pressure will change to the selected standby cabin altitude at the selected standby rate of change. During standby control the cabin altitude is limited to 14,625 feet by a pressure switch in the controller. This signals the DC actuator, which modulates the outflow valve closed. The Auto Fail and STANDBY lights come on when the automatic system has been transferred to the standby system and the mode selector is in AUTO. The automatic mode of operation may be reselected after an automatic transfer to standby. If any fault remains in the automatic mode, the system will automatically transfer back to the standby mode “.


Hope this makes it clearer
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[LEM]

Thanks, cirrus01, good to have people like you providing info which is only available in the maintenance manuals.

:ok: LEM