Not sure the best forum for this question, and the search feature didn’t turn up anything related.
How are planes pressurized? Is the air in the plane just pressurized air from outside the plane (i.e., not supplemented with anything from tanks, etc.)?
Also, is oxygen (for the oxygen masks) carried in tanks in most modern jets? If so, doesn’t that oxygen reservoir dramatically increase the flammability of the plane in a crash?
Just some things I was thinking about while en route last week and playing with the barometer/altimeter on my GPS.
Oxygen generators are only used for passenger O2, even on airliners. The crew has their own O2 bottle. Most (all?) bizjets have O2 tanks (separate or combined) for crew and pax.
The pressurization system is also part of the heating/cooling system. Basically the only pressurization-specific equipment is the outflow valves. Think of it as a soda bottle with a hole in it. You can blow air into it (evironmental control system) and leave the hole uncovered (aircraft on ground) and the bottle stays at ambient pressure. When you start to cover the hole with your finger the bottle pressurizes. By varying the size of the opening (outflow valves) you can control how much air pressure stays in the bottle (airplane.)
Interesting side point…many people misunderstand “pressurization”. When you’re in an airliner, the cabin pressure is still a much higher altitude (lower pressure) than you were when sitting on the ground. Anywhere from 6 to 8 thousand feet. So you’re not experiencing more pressure, it’s less pressure than normal. sidebar, techincally, you can leave denver’s 5000ft and cruise in an airliner at 4000ft cabin pressure, so that makes a liar out of me, but you get the idea. ] It is, though, a much higher pressure than the outside air. (6 thousand feet pressure versus 35000 feet pressure). I dont know what the numbers are for airliners, but they’re probably somewhere between 6 and 9 psi maximum pressure differential. The PC12 was 6.75 which is good for 10,000ft cabin at FL300 (or maintain sea level pressure up to 14000ft) and the P180 is 9psi, on the high scale for aircraft of it’s size, at our maximum service ceiling of FL410 the cabin stays at a comfy 6200ft and we can maintain sea level cabin up to FL240. 8)
I forgot one thing.
Yes, pressurization usually comes from “bleed air” from the engines. (turbocharged piston engined airplanes get their pressurized air from the turbochargers)
What is bleed air? Well a turbine engine operates on pretty much the same principle as any other internal combustion engine. You know that old saying suck, squeeze, bang, blow. (aka, intake, compression, combustion, exhaust)
The only difference is that it all happens at once.
A turbine engine is as close to a perpetual motion machine as you can get.
Step one. Suck.
This part is easy. During start, the starter motor (or pressurized air) starts the whole system spinning. Once the engine gets going, it happens automatically as the first part of the compressor section. See step two.
Step two. squeeze.
The first stages of the engine are the compressor turbines. These turbines accelerate the air and then squeeze the air with a centrifugal compressor. This is where the bleed air comes from. Even before the fuel is added the air has been compressed so much and hot enough to be used for pressurization and heating of anti-ice devices (leading edge wing heat, boots etc.) Hell, it’s even used as part of the engine fuel system.
Step three. Bang.
During start (and during other times as neccessary like heavy rain/ice/turbulence) the electrical ignitors provide the spark to ingite the fuel. Once running, the fuel ignites automatically as a continuous flame.
Step four. Blow.
The exhaust gasses acclerate through another series of turbines, finally providing the power to make the airplane “go”. Either through turning the propellors or exhausting out the rear to provide thrust. These turbines also provide the rotational energy to the first half of the system. Remember those first compressor turbines? They get their power to do all that compressing from the back half of the turbine, from the expanding exhaust gasses. Cool, huh?
True, but the info on wikipedia was very general, gave a few different possibilities (bleed air extraction, electric air compressors, etc.), and I was curious what folks with first-hand experience may have to say about it Google is always my first stop, though.
Cool explanation of turbines, cfijames. Thanks. I’ve frequently wondered how a jet engine differs from a piston-driven engine. And I understand that planes are only pressurized to a few thousand feet-- recently, I was trying to “feel” how the cabin pressure was changing on a flight out of UIO (at nearly 10,000 asl).
It often feels like pressure in the cabin increases briefly (but rapidly) during take-off. But I can’t use my GPS (with barometer) below 10,000 feet, so I can’t tell for sure Anyone know if this actually happens? If so, is it because the outflow valves don’t quite keep up with the rapid influx of air during initial acceleration & climb, or something entirely different?
Seems that exothermic oxygen generators aren’t necessarily the safest thing to have in aircraft. I’m still curious if the oxygen generated by these systems play a role in post-crash fires. Unless there’s some mechanism to prevent them from accidentally being activated upon impact?
Perhaps.
We usually set field elevation plus a few hundred feet on the ground and before landing. This allows the airplane to be unpressurized on the ground. (kinda important if you need to get the doors open.) So the airplane is unpressurized for the first 2 or 3 hundred feet after liftoff, and then the outflow valves move towards the closed position and begin metering the amount of flow to whatever cabin-rate-of-climb that is set.
Some aircraft are restricted from taking off and landing pressurized. However, most airliners do takeoff and land with a very slight amount of PSID in an effort to reduce the pressure bump. Most likely what you are feeling is the inability of the system to deal with the sudden inflow when the engines are spooled up for take off.
Most aircraft have a weight on wheels switch that keeps the safety valve open while on the ground to prevent the cabin from pressurizing. Once you lift off, the safety valve will close, and the pressure controller starts to regulate cabin pressure with the outflow valve. The valves work off of vacuum, and it takes a few seconds for the vacuum to build up and the outflow valve to start opening and regulating. That can cause the sensation of a pressure build up.
We just had an aircraft with an outflow valve that wasnt working, and when you took off, the cabin would start pressurizing to below sea level, really terrible on the ears and tummy. Hope this helps and im sure someone has more info on how other systems are set up.
GPS device is acceptable electronic device for SOME airlines once they approve it’s ok to use a laptop, gameboys and so on. It’s clearly marked in the approved device list of their magazines.
I don’t know of any airlines that allow devices that SEND a signal.
This may be getting a bit off-topic (sorry!), but just wanted to follow-up on this: I’ve been told that it’s actually the pilot’s discretion if GPS units can be used (above 10,000 feet of course), even if the in-flight magazine says they can’t. If I have any doubt, I usually ask the flight attendant at the front of the plane, “What is the pilot’s policy on the use of GPS units during flight?” and I’ve only had one pilot tell me that I couldn’t use it. Geology is fascinating from the air, so I like to keep track of where I am when I see cool features
Cool, thanks for the explanation. It’d be interesting to make a graph of actual cabin pressure versus time for an average ascent & descent (but I’d have to invest in an altimeter watch like G4Driver mentioned).
Reading a bit about cabin pressurization, I found some interesting info on the flight of Helios 522. I forgot about that incident (and how creepy it was)!
Barometer in the GPS is, in MHO, useless in a pressurized A/C.
As far as if we can or not use it, I simply go ahead. Most of the time, flight attendants either don’t see me or don’t mind. Some are even interested by it. Only once have I been told by a F/A not to use it [Air Wisconsin CRJ] so I just turned it off and put it away, no question asked.
I got one anecdote [It wasn’t me, I swear ] a guy was tracking the flight with a unit, and for some reason, we made a detour around some city. I don’t know if he was trying to get some conversation going with the F/A, told her in a know-it-all way that he KNEW the flight has gone off-course. The F/A checked with the captain, next thing was him warning everyone on the PA that navigation equipment on board is a no-no. Bummer.
Not sure why you say that. The barometric altimeter in my GPS reads the cabin pressure altitude while the GPS altimeter reads the actual aircraft altitude. Both give useful information.
Google is always my first stop, though.
] a guy was tracking the flight with a unit, and for some reason, we made a detour around some city. I don’t know if he was trying to get some conversation going with the F/A, told her in a know-it-all way that he KNEW the flight has gone off-course. The F/A checked with the captain, next thing was him warning everyone on the PA that navigation equipment on board is a no-no. Bummer.