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Captain's Decision To Depressurise In Flight  
User currently offlineJulesmusician From , joined Dec 1969, posts, RR:
Posted (8 years 11 months 1 week 4 days 11 hours ago) and read 4228 times:

I have heard that a pilot can depressurise the cabin by choice in modern aircraft. When would that be of use? What condition's/events would ever involve this choice? Would it ever be used in the event of a fire to starve it of oxygen?

J

36 replies: All unread, showing first 25:
 
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17068 posts, RR: 66
Reply 1, posted (8 years 11 months 1 week 4 days 11 hours ago) and read 4227 times:

Fire?
.
.
.
.
.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinePilotpip From United States of America, joined Sep 2003, 3150 posts, RR: 11
Reply 2, posted (8 years 11 months 1 week 4 days 11 hours ago) and read 4221 times:

Outflow valve being stuck completely closed resulting in a dangerously high pressure differential.


DMI
User currently offlineFr8mech From United States of America, joined Sep 2005, 5491 posts, RR: 14
Reply 3, posted (8 years 11 months 1 week 4 days 10 hours ago) and read 4194 times:

Quoting Pilotpip (Reply 2):
Outflow valve being stuck completely closed resulting in a dangerously high pressure differential.

How would the flight crew depressurize the aircraft?

The only valid reason I could think of is smoke in the cabin. But, I think it would have to be a lot of smoke. But what about the fire causing the smoke? I'm thinking depressurization could starve the fire of oxygen...and all the passengers. But wait, the passengers will have oxygen. So, we will be feeding 100% pure oxygen into the masks, some of which will not be worn because there are extras. Will this feed the fire after the pressure equalizes? Who knows.

Maybe, a terrorist attempt to take the aircraft? Dump the pressure, pull a couple of quick, zero g manuvers and hope that in the confusion, the terrorist looses the upper hand.

Jules, the flight crew does have the ability to dump the cabin, but that ability exists because of the design of the system and not as an option for anything but the most extreme of situations.



When seconds count...the police are minutes away.
User currently offlineEMBQA From United States of America, joined exactly 11 years ago today! , 9364 posts, RR: 11
Reply 4, posted (8 years 11 months 1 week 4 days 10 hours ago) and read 4185 times:

Quoting Fr8mech (Reply 3):
How would the flight crew depressurize the aircraft?

Easy... The Emergency relief valve. Every aircraft I've even dealt with have two outflow valves. One is normally controlled, the other is controlled only by the dump switch.



"It's not the size of the dog in the fight, but the size of the fight in the dog"
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 5, posted (8 years 11 months 1 week 4 days 9 hours ago) and read 4173 times:

Quoting Pilotpip (Reply 2):
Outflow valve being stuck completely closed resulting in a dangerously high pressure differential.



Quoting Fr8mech (Reply 3):
How would the flight crew depressurize the aircraft?

Turn off the packs.

Would not want to be aboard.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 6, posted (8 years 11 months 1 week 4 days 6 hours ago) and read 4128 times:

Why would the Crew want to do that.

Quoting Pilotpip (Reply 2):
Outflow valve being stuck completely closed resulting in a dangerously high pressure differential.

Modern Aircraft have built in backup systems to ensure it never occurs.
If it however does occur the Safety relief valves will Dump open.

Quoting EMBQA (Reply 4):
Easy... The Emergency relief valve. Every aircraft I've even dealt with have two outflow valves. One is normally controlled, the other is controlled only by the dump switch.

Are you refering to the MOFV & Fwd OFV.
regds
MEL



Think of the brighter side!
User currently offlineFr8Mech From United States of America, joined Sep 2005, 5491 posts, RR: 14
Reply 7, posted (8 years 11 months 1 week 4 days 6 hours ago) and read 4123 times:

Quoting EMBQA (Reply 4):
Easy... The Emergency relief valve

Enlighten me. I've never heard of one. I know about pressure relief valves, but have never heard of an emergency relief valve that is controlled by a dump switch.



When seconds count...the police are minutes away.
User currently offlinePhilSquares From , joined Dec 1969, posts, RR:
Reply 8, posted (8 years 11 months 1 week 4 days 3 hours ago) and read 4093 times:

There is no real reason to depressurize a passenger flight under "normal" circumstances. However, in the 744F the procedure for a main deck cargo fire is to depressurize the aircraft. Thus, no oxygen, no fire.

User currently offlineJush From Germany, joined Apr 2005, 1636 posts, RR: 3
Reply 9, posted (8 years 11 months 1 week 3 days 23 hours ago) and read 4066 times:

Quoting Fr8mech (Reply 3):
Maybe, a terrorist attempt to take the aircraft? Dump the pressure, pull a couple of quick, zero g manuvers and hope that in the confusion, the terrorist looses the upper hand.

Well that's sounds like a kick ass maneuver, doesn't it.
That would be something for the airport movies i reckon.

Regds
jush



There is one problem with airbus. Though their products are engineering marvels they lack passion, completely.
User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 10, posted (8 years 11 months 1 week 3 days 20 hours ago) and read 4027 times:

Quoting PhilSquares (Reply 8):
There is no real reason to depressurize a passenger flight under "normal" circumstances. However, in the 744F the procedure for a main deck cargo fire is to depressurize the aircraft. Thus, no oxygen, no fire.

Shouldn't there be a Main deck Isolation valve that shuts off packflow to the Main deck in case of Fire,rather than Depressurizing the Entire Aircraft.I thought that was an Important Equipment on Freighters.Which operators B747 Freighter are you referring to.Any details.
regds
MEL



Think of the brighter side!
User currently offlineFr8Mech From United States of America, joined Sep 2005, 5491 posts, RR: 14
Reply 11, posted (8 years 11 months 1 week 3 days 18 hours ago) and read 4008 times:

Quoting HAWK21M (Reply 10):
Shouldn't there be a Main deck Isolation valve that shuts off packflow to the Main deck in case of Fire,rather than Depressurizing the Entire Aircraft.I thought that was an Important Equipment on Freighters.Which operators B747 Freighter are you referring to.Any details.

You would then need 2 pressure vessels, or more to the point, a pressure bulkhead, with a door, between the main deck and the upperdeck. Expensive and heavy. When you are dealing with trained crew members and a handful of untrained (maybe), just open the outflow valves and let it go.

Quoting Jush (Reply 9):
Well that's sounds like a kick ass maneuver, doesn't it.
That would be something for the airport movies i reckon.

Actually, I was in Italy shortly after 9-11 and was reading the International Herald Tribune and a pilot (he was named as was the airline, but both escape me) suggested that the rules are now changed. A hijack will not be treated the same. The goal will not be to get the aircraft on the ground safely, it will be to deny the hijackers control of the aircraft. He just mentioned the use of negative g maneuvers and depressurization as an option.



When seconds count...the police are minutes away.
User currently offlinePhilSquares From , joined Dec 1969, posts, RR:
Reply 12, posted (8 years 11 months 1 week 3 days 12 hours ago) and read 3944 times:

Quoting HAWK21M (Reply 10):
Which operators B747 Freighter are you referring to.Any details.

All B744 built are like that. As explained, you'd have to have two separate pressure compartments to accomplish what you're talking about.


User currently offlineEMBQA From United States of America, joined exactly 11 years ago today! , 9364 posts, RR: 11
Reply 13, posted (8 years 11 months 1 week 3 days 9 hours ago) and read 3918 times:

Quoting Fr8Mech (Reply 7):
Enlighten me. I've never heard of one. I know about pressure relief valves, but have never heard of an emergency relief valve that is controlled by a dump switch.

Well, I don't know about what you have worked with as it may be different, but all the regional aircraft I've worked with have two valves. One is the pneumatic controlled valve and is used under normal conditions. The emergency relief valve is electronically controlled by the dump switch.

[Edited 2005-11-20 01:52:51]


"It's not the size of the dog in the fight, but the size of the fight in the dog"
User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 14, posted (8 years 11 months 1 week 3 days 1 hour ago) and read 3870 times:

Quoting Fr8Mech (Reply 11):
You would then need 2 pressure vessels, or more to the point, a pressure bulkhead, with a door, between the main deck and the upperdeck.

Are you saying there is no Pressure Bulkhead on the B747 Freighter like the B737 Freighters.
regds
MEL



Think of the brighter side!
User currently offlinePhilSquares From , joined Dec 1969, posts, RR:
Reply 15, posted (8 years 11 months 1 week 2 days 22 hours ago) and read 3858 times:

Quoting HAWK21M (Reply 14):
Are you saying there is no Pressure Bulkhead on the B747 Freighter like the B737 Freighters.
regds

No, not at all. However, how are the lower cargo holds pressurized? The main deck, upper deck and the lower fad and aft cargo holds are all in the same pressure structure. So, if you depressurize the main deck, you do in effect depressurize the entire aircraft. There is no separate pressurization system for the lower cargo compartments.


User currently offlineVC10 From United Kingdom, joined Nov 2001, 1412 posts, RR: 16
Reply 16, posted (8 years 11 months 1 week 2 days 22 hours ago) and read 3856 times:

I always thought that freighters only had a "strengthened" flight deck bulkhead but not a pressure bulkhead. Many freighters that carry animals also carry people in the back to care for them, and they might not be too pleased to be denied pressurization.

When talking about de-pressurizing an aircraft for fire reasons unless the memory is failing me [ and that could be the case ] were not the crew first supposed to descend the aircraft to a safe height before depressurizing the aircraft. To do this the air input has to be denied and then the remaining pressure dissipated by the override system [whether this be mechanical or otherwise]

Best avoid it little vc10


User currently offlinePhilSquares From , joined Dec 1969, posts, RR:
Reply 17, posted (8 years 11 months 1 week 2 days 20 hours ago) and read 3840 times:

Quoting VC10 (Reply 16):
I always thought that freighters only had a "strengthened" flight deck bulkhead

I can only speak of the 744. If you travel with grooms, there are extra O2 walkaround bottles put on the aircraft. If the groom(s) go to the main deck, then they have to take the O2 with them since there isn't any provision for that on the main deck. Remember, on the 744, they are seated in the upper deck.

As far as descending to a "safe" altitude, looking at the 747-400 QRH, is says "Climb of descent to 25,000 feet when conditons and terrain permit."


User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 18, posted (8 years 11 months 1 week 2 days 18 hours ago) and read 3820 times:

Quoting PhilSquares (Reply 15):
The main deck, upper deck and the lower fad and aft cargo holds are all in the same pressure structure. So, if you depressurize the main deck, you do in effect depressurize the entire aircraft. There is no separate pressurization system for the lower cargo compartments

What about the Cockpit & the Area between the Cockpit & the 9G bulkhead [The Supernumery Area].Isn't that zone Isolated by a Fire proof 9G bulkhead as on the B737 Freighters.
regds
MEL



Think of the brighter side!
User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 19, posted (8 years 11 months 1 week 2 days 18 hours ago) and read 3812 times:

Quoting HAWK21M (Reply 18):
What about the Cockpit & the Area between the Cockpit & the 9G bulkhead [The Supernumery Area

This depends on the a/c config. At Fedex we have some planes (only MD-10s I think) that have the rigid bulkhead and the rest (all MD-11s I believe) that just have the 9G net. However if you get a main cabin smoke alert the "SMOKE/OFF" switch on the AIR Panel is pushed shutting off air to the main cabin while still providing air to the cockpit creating a pressure barrier of sorts to keep smoke out of the cockpit. The procedure still calls for O2 100% and bringing the cabin alt to 25,000ft to starve any fire.
We had a crew which experienced one such main cabin cargo fire and put this checklist to the test. Everything went perfect for them but after ldg. the fire re-ignited when the cargo door was opened and the a/c burned to the ground.
This was Stewart NY which pix have been posted here.


User currently offlineZeke From Hong Kong, joined Dec 2006, 9155 posts, RR: 76
Reply 20, posted (8 years 11 months 1 week 2 days 17 hours ago) and read 3803 times:

In a previous life I used to fly mentally unstable people for medical reason for them to get better help. They were chemically and physically restrained with a nurse and security person onboard. Despite these measures, and due to the high tolerance a lot of these people had to the chemicals one of them had a episode airborne where they removed themselves from the restraints and knocked out the nurse and security person out, and was choking the pilot in a head lock.

The nurse regained consciousness enough to inject the person to stop the attack.

Many times after that when transport similar persons I had a plan to dump the cabin at FL350, go on O2, and let everyone in the back black out.

In passenger jets, only reason would be for smoke or electrical fire to remove smoke from the cabin.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlineJulesmusician From , joined Dec 1969, posts, RR:
Reply 21, posted (8 years 11 months 1 week 2 days 17 hours ago) and read 3796 times:

It would be interesting to know medically what happens when you depressurise and how long it takes to black out - we now know this was the cause of the Helios airways crash in Greece. The captain didn't realise his controls were not set to pressurise automatically as they had been changed for maintenance on the system.. Warnings sounded in the cockpit and he thought it was for something else before blacking out along with the co-pilot. However on that flight there were people still concious and trying to control the aircraft. I believe that when you black out from no oxygen you reawake quickly when oxygen is restored?

User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 22, posted (8 years 11 months 1 week 2 days 16 hours ago) and read 3788 times:

Quoting Julesmusician (Reply 21):
The captain didn't realise his controls were not set to pressurise automatically as they had been changed for maintenance on the system.. Warnings sounded in the cockpit and he thought it was for something else before blacking out along with the co-pilot.

Not only is that so outrageous and unbelieveable but from some tech folks at work I've heard there were even more unbelieveable stuff that went on in that a/c that I won't post here. No reason whatsoever that this accident occurred.


User currently offlineFr8mech From United States of America, joined Sep 2005, 5491 posts, RR: 14
Reply 23, posted (8 years 11 months 1 week 2 days 12 hours ago) and read 3747 times:

Quoting HAWK21M (Reply 18):
What about the Cockpit & the Area between the Cockpit & the 9G bulkhead [The Supernumery Area].Isn't that zone Isolated by a Fire proof 9G bulkhead as on the B737 Freighters.
regds

A 9g bulkhead is still not a pressure bulkhead. It can be designed as such, but again, why add the cost and weight?

I'd also be careful using the term fireproof. The rigid 9g barrier is a smoke barrier, the heat of a fire impinging on it would quickly melt the seals around any pass through and render it ineffective.



When seconds count...the police are minutes away.
User currently offlinePhilSquares From , joined Dec 1969, posts, RR:
Reply 24, posted (8 years 11 months 1 week 2 days 12 hours ago) and read 3740 times:

Quoting HAWK21M (Reply 18):
What about the Cockpit & the Area between the Cockpit & the 9G bulkhead [The Supernumery Area].Isn't that zone Isolated by a Fire proof 9G bulkhead as on the B737 Freighters.

Again, we're talking about a 744F. There is no cargo in the upper deck, so no need for any type of 9G barrier.


25 HAWK21M : Again the Underline Statement:- "Rules are to be followed". What about the B748.I know its too early.But Is it going to be a Three Layer cargo hold o
26 Post contains links Zeke : Altitude Time of Useful Consciousness FL 180 20 to 30 Min FL 220 10 Min FL 250 3 to 5 Min FL 280 2.5 to 3 Min FL 300 1 to 2 Min FL 350 0.5 to 1 Min F
27 XFSUgimpLB41X : We have an emergency depress button.... if you're at altitude it will dump the airplane to 14,250 +/- 750 feet or current altitude if you are below th
28 PhilSquares : I wouldn't forsee any drastic change to the basic structure at all. It appears as if every operator is very satisfied with the current configuration.
29 HAWK21M : I presume the Dump valve is actually your MOV moved to Full open.Right or is there an Added Component. regds MEL
30 Julesmusician : What cabin altitiude do the oxygen masks drop down at automatically?
31 HAWK21M : If Im not mistaken 14,000 +/- 200 ft on a B737. regds MEL
32 HAWK21M : If Im not mistaken 14,000 +/- 200 ft on a B737. regds MEL
33 Tod : On the 742SF that I've worked on the smoke event procedures included shutting off packs, opening outflow valves and also opening a small air inlet fo
34 HAWK21M : I think the Upper deck might be for Bulk. regds MEL
35 David L : Is this to do with the reduced ceiling height on the main deck immediately below the upper deck - shorter hump, more full height available on the mai
36 Tod : Yes, but you could still get the full height containers under the back of the longer upperdeck if it has supports replacing the upperdeck floorbeams
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