View Full Version : 737 Oxy-mask deployment
lineboy_nz
7th Apr 2004, 06:31
What causes pax masks to deploy in the 733? i just heard a story about a hard landing at Windy Wellington, NZ. The masks deployed and the plane went around. I always thought the cabin pressure was the trigger?
thanks in advance.
stalling attitude
7th Apr 2004, 08:34
The masks on a 733 are supposed to drop when then cabin altitude exceeds 14000' or a switch in the flight deck is operated but a good hard landing has been known to dislodge the panels and release the rubber jungle.
Notso Fantastic
8th Apr 2004, 10:35
It ws not unknown for a variable numbers of 747 masks to drop on heavy landings, right up to nearly all of them.
on a 747, each individual PSU has a pressure switch that is activated by high oxygen pressure in the ringmain. The ringmain is pressurised by two things:
1. high cabin altitude - cabin altitude controller sending signal to the main oxygen flow control valve (in fwd freights) where all passenger oxygen bottles are located. The bottles are individually lockwired open and are all bussed together so that the pipework that comes from each of them exits to the cabin ringmain at one point. The flow control valve is located at that point.
2. A guarded switch (when activated) in the flight deck sends the electrical signal to the valve should the captain feel the need to release the masks.
These two actions automatically pause all your video and through a PRAM recorder at the pursers workstation, activates an "ATTENTION ATTENTION, THIS IS AN EMERGENCY ANNOUNCEMENT..........." over the PA at max volume
NB. when the ringmain becomes pressurised, the pressure switch at each PSU opens the oxy box door. When the masks fall, you still need to pull the lanyard (annotated 'PULL'!) to release the pin to allow oxygen flow to each individual mask
A heavy landing can in some cases jar the lock on the oxy box door and release the masks - (this lock can be opened manually with the correct tool)
hope that helps :ok:
lineboy_nz
9th Apr 2004, 15:05
thanks guys. i think the lady was telling fibbies.
q2: how long does it take to put them all away again in say a 744?
a looooooooonnng time!!!! (believe me, i've done it!!) across shifts, a couple of days. Each one has to be packed in a special way so its not a question of just shoving them all back in.