4 ms·

How can a pilot depressurize a plane? It seems crazy that this would be allowed while in flight unless there is a fire or some other emergency.

dandelany 13y ago
I am certainly not an expert but I'm basing this off of a discussion in the airliners.net forum among users who generally know what they're talking about: "Quoting nupogodi (Reply 178): Now, mandala, I have another question for you: I have heard that the outflow valves will not allow cabin altitude to drop below 14000ft as long as bleed air is working. Can you confirm or deny that? Is it possible to completely depressurize the aircraft without turning off in the "inflow" (don't know the jargon) ? If so, wouldn't the flight deck and cabin quickly grow incredibly cold? Or do you think intentional decompression is a valid hypothesis?" "Turn pressurization to manual, open the valves... it will depressurize... Wanna make it quicker, just turn off the bleeds too (inflow)... Just pressing a few buttons... the longer button is the outflow valve in manual... Mandala499" If anyone has information proving otherwise, I'd definitely be interested to hear it. Certainly a bit of a scary thought.
- alexeisadeski3 13y ago
  Your sources are correct, it is easy. Also, it neither problematic nor scary nor weird. The pilot can kill everyone on board in any number of ways. You're putting your life in his (or her) hands every time you step on the aircraft. Same goes for your city bus and taxis...
- userbinator 13y ago
  To me this looks like it's based on the principle that the pilot is always in control of the aircraft as much as possible, to allow recovering from unusual failures. Just as communications equipment can be turned off if they pose a fire risk, it should be possible to vent the plane (after descending to a breathable altitude) if the system fails and the cabin begins to overpressurise. It's probably worse than depressurising since it could the plane to explode...
sokoloff 13y ago
That's the thing. Your "unless..." clause is the key element. As background, I fly a twin engine piston airplane that's pressurized (Beech 58P). Different level of responsibility from a 777-ER (I have only 6 seats, including my own), but many of the same basic design principles apply. My cabin is pressurized by air coming off the turbos (the turbos "make" higher than ambient pressure air, and that is fed into the cabin after passing through a restrictor and an intercooler). Turbines (jets and turbo-props) pressurize from bleed air (also high pressure air created in the engine, but via a different physical mechanism.) If I have an engine or turbo failure, I need to be able to isolate the cabin from the smoke/fumes/fire that may be present on that engine. So, I have a red knob that can isolate the cabin from the feed from that side (and another knob that will isolate the other side). Similarly, I have a cabin pressure controller, and on that a cabin dump switch. The controller controls a single outflow valve at the rear of the cabin, about the cross-section of a roll of toilet paper, and there is a second, spring-loaded valve of the same size that's designed to automatically open if the cabin were at risk of over-pressurization. It would be trivial for me to dump the cabin quickly with a single switch. This results in a cabin rate of climb of over 3K feet per minute, which is painful on the ears, and within minutes the cabin will climb to the aircraft altitude. (I test virtually all of the systems annually prior to the annual major maintenance event, such that if a system has failed, I can have it addressed during major maintenance.) Why is there a dump switch at all? Dumping the cabin is a great way to clear the cabin of smoke, so in the event of an electrical fire or smoke, once the situation is contained, I can dump the cabin, venting all the toxic smoke inside to the outside, with a supply of "clean" higher-pressure air entering at the front. Once the smoke has cleared, I close the outflow valve to re-pressurize the cabin from the engines. The point of this wall of text is that aircraft are designed assuming that someday something will fail. When it fails, the pilots, assumed to be the "good guy", need to have the best chance possible to react. All common, and many uncommon, failures have a specific checklist response, sometimes with initial items from memory. (With an engine on fire, you want to perform the initial response from memory, backing it up later with the written checklist.) No plane is designed to protect the occupants from a pilot intent on killing everyone on board. The pilot has any number of ways to do that, so it's counter-productive to prevent the 99.99% emergencies from being responded to by a well-intentioned pilot in order to prevent the occasional madman from killing all aboard.

dandelany 13y ago

I am certainly not an expert but I'm basing this off of a discussion in the airliners.net forum among users who generally know what they're talking about: "Quoting nupogodi (Reply 178): Now, mandala, I have another question for you: I have heard that the outflow valves will not allow cabin altitude to drop below 14000ft as long as bleed air is working. Can you confirm or deny that? Is it possible to completely depressurize the aircraft without turning off in the "inflow" (don't know the jargon) ? If so, wouldn't the flight deck and cabin quickly grow incredibly cold? Or do you think intentional decompression is a valid hypothesis?" "Turn pressurization to manual, open the valves... it will depressurize... Wanna make it quicker, just turn off the bleeds too (inflow)... Just pressing a few buttons... the longer button is the outflow valve in manual... Mandala499" If anyone has information proving otherwise, I'd definitely be interested to hear it. Certainly a bit of a scary thought.

alexeisadeski3 13y ago
Your sources are correct, it is easy. Also, it neither problematic nor scary nor weird. The pilot can kill everyone on board in any number of ways. You're putting your life in his (or her) hands every time you step on the aircraft. Same goes for your city bus and taxis...
userbinator 13y ago
To me this looks like it's based on the principle that the pilot is always in control of the aircraft as much as possible, to allow recovering from unusual failures. Just as communications equipment can be turned off if they pose a fire risk, it should be possible to vent the plane (after descending to a breathable altitude) if the system fails and the cabin begins to overpressurise. It's probably worse than depressurising since it could the plane to explode...

sokoloff 13y ago

That's the thing. Your "unless..." clause is the key element. As background, I fly a twin engine piston airplane that's pressurized (Beech 58P). Different level of responsibility from a 777-ER (I have only 6 seats, including my own), but many of the same basic design principles apply. My cabin is pressurized by air coming off the turbos (the turbos "make" higher than ambient pressure air, and that is fed into the cabin after passing through a restrictor and an intercooler). Turbines (jets and turbo-props) pressurize from bleed air (also high pressure air created in the engine, but via a different physical mechanism.) If I have an engine or turbo failure, I need to be able to isolate the cabin from the smoke/fumes/fire that may be present on that engine. So, I have a red knob that can isolate the cabin from the feed from that side (and another knob that will isolate the other side). Similarly, I have a cabin pressure controller, and on that a cabin dump switch. The controller controls a single outflow valve at the rear of the cabin, about the cross-section of a roll of toilet paper, and there is a second, spring-loaded valve of the same size that's designed to automatically open if the cabin were at risk of over-pressurization. It would be trivial for me to dump the cabin quickly with a single switch. This results in a cabin rate of climb of over 3K feet per minute, which is painful on the ears, and within minutes the cabin will climb to the aircraft altitude. (I test virtually all of the systems annually prior to the annual major maintenance event, such that if a system has failed, I can have it addressed during major maintenance.) Why is there a dump switch at all? Dumping the cabin is a great way to clear the cabin of smoke, so in the event of an electrical fire or smoke, once the situation is contained, I can dump the cabin, venting all the toxic smoke inside to the outside, with a supply of "clean" higher-pressure air entering at the front. Once the smoke has cleared, I close the outflow valve to re-pressurize the cabin from the engines. The point of this wall of text is that aircraft are designed assuming that someday something will fail. When it fails, the pilots, assumed to be the "good guy", need to have the best chance possible to react. All common, and many uncommon, failures have a specific checklist response, sometimes with initial items from memory. (With an engine on fire, you want to perform the initial response from memory, backing it up later with the written checklist.) No plane is designed to protect the occupants from a pilot intent on killing everyone on board. The pilot has any number of ways to do that, so it's counter-productive to prevent the 99.99% emergencies from being responded to by a well-intentioned pilot in order to prevent the occasional madman from killing all aboard.