Airplane Tech Talk for the Non-Technical: Cabin Pressurization

Another in my occasional series demystifying the inner workings of airplanes. This week – cabin pressurization.

As most people know, modern passenger planes are usually propelled by turbine engines, and usually fly at around 35,000 feet. As we go up in altitude the air becomes progressively less dense. What we Earthlings call “air” is about 78% nitrogen, 21% oxygen and 1% other stuff. As the air becomes less dense there simply aren’t enough oxygen molecules per square inch of air to support life. So, the air inside the plane needs to be pressurized.

So, what does this all mean? Earth’s atmosphere always contains roughly the same ratio of nitrogen, oxygen and trace other elements. At higher altitudes there is just less of everything for a given volume. So, if you take the very un-dense air that’s at 35,000 feet and squeeeeeze it together, you get – air that’s very similar to the air at lower altitudes. Pressurization allows us to fly at high altitudes, breathing the air that’s up there – it has simply been squeezed together. Unless something goes wrong, we don’t need supplemental oxygen (although, to reassure everyone, EASA and the FAA have very specific rules about provisions for supplemental oxygen for passengers and crew – exact requirements depend on how high the plane will be flown, but there will always be supplemental oxygen if it’s needed).

Back in the 1920s the US Army designed a pressurized vessel made to go inside the cockpit of a reciprocating engine airplane. The idea was that the pilot would be inside this small oval steel tank and would look out through a glass port. The controls were inside the tank and there was a manual exhaust valve so pressure could be controlled. It sounds very cumbersome, and while the whole setup flew once, it never caught on, but it was the forerunner to today’s pressurized airplanes.

By 1940 the Boeing 307B was put into service with the first fully pressurized cabin. Today all turbine aircraft and some reciprocating engine aircraft have pressurized cabins.

How does it work? First of all, most of the space inside the airplane is contained within the pressure vessel. At the fore and aft ends of the plane there are bulkheads that are strong enough to withstand the pressure differential between the outside and inside when the plane is pressurized.

Here’s a picture of the aft pressure bulkhead, so you can get a sense of what it looks like and what it does. Note that the stuffed spider is an anomaly!

Apologies, arachnophobes!

Once the interior is sealed off, we need a source of pressurized air. On turbine-engine aircraft, this comes from the engine compressors – it’s called bleed air and it’s bled off the compressors for a variety of purposes, including pressurization. On some planes, the air comes straight from the engine into the cabin and on some it is then routed to a specialized compressor which then compresses the air even more for the cabin.

But wait, what about that pressure vessel? Can it contain infinite pressure? No, it can’t. For this reason, all aircraft have a maximum ratio of inside pressure to outside pressure. The cabin is pressurized to a predetermined altitude, then maintained by an outflow valve that opens and closes as needed to maintain that altitude. If it reaches the maximum inside/outside pressure ratio the outflow valve will open and close to keep it at or below that ratio and mitigate stress on the fuselage.

The pressure is measured as cabin altitude because the air is pressurized to the density of a lower altitude – for most aircraft it’s about 8,000 feet, but the 787 Dreamliner can maintain a density altitude of about 6,000 feet.

But, but … what happens when the sealed, pressurized plane lands at sea level? Won’t the higher pressure outside crush the fuselage? It would if that were allowed to happen, but modern aircraft have automatic pressure management systems that lower the pressure inside the pressure vessel as the plane descends. And if that system were to fail, all pressurized planes have a negative pressure relief valve that’s forced open if outside pressure is greater than the inside pressure. Once it’s opened, pressure equalizes between the inside and outside.

And, lastly, there’s a dump valve that can be used in flight to clear the cabin of smoke or fumes. This valve is also activated on the ground by a switch on the landing gear, called the squat switch, to keep the cabin from accidentally being pressurized while on the ground, since this would cause unnecessary stress on the fuselage.