Pop!

CCLXXI

The cabin pressurization system on even the most modern jet aircraft is decidedly low-tech. Essentially, all that is required for its operation is for the Flight Engineer to switch it to “On” and set the desired pressure for the cabin. The movement of the plane through the sky drives the outside air into the compressor where it is released into the cabin by a series of simple valves and vents at the front of the passenger cabin. In level flight, the air circulates about once every one to two minutes.

A pristine outflow valve

As the plane gains or loses altitude the pressure changes outside, which means that the pressure changes inside too. Excess pressure is vented out through a simple outflow valve with a spring-hinged door. When the interior air pressure grows too great the door pops open, releasing air until the pressure is normalized. The outflow valve door then snaps shut until the next time it needs to operate.

In the Good Old Days when smoking was permitted on commercial aircraft smokers were almost always seated at the rear of the cabin. Since fresh air flowed from front to back and since the outflow valve was usually toward the tail of the aircraft, this meant that smokers existed in their own little blue-tinged biosphere. It also meant that the air in the outflow was particularly thick with a gunk of tars and nicotine and resins. Most planes had ugly long brown stains leading backward from the outflow valve toward the tail. Such stains could be several feet long. 

An outflow valve with normal use

They were difficult to remove, and unsightly, but they also caused a technical problem. The gunk was sticky. It would clog the valve, reducing the system’s overall efficiency and it would frequently glue the outflow valve door shut; the door would only snap open when the pressure differential became extreme. The sudden change caused passengers’ ears to pop, sometimes quite painfully. Passengers often complained, but there was little the airlines could do other than subject the outflow valve to expensive and intensive maintenance. There was also the issue of inadvertent damage in the cabin. Errant sparks would burn holes in seat cushions, carpeting, and blankets. The smell of stale cigarettes was nearly impossible to remove except with costly regular shampooing. And even tray tables and armrests would suffer burn marks through passenger carelessness. 

A sticky mess

Aircraft damage by cigarette smoke cost the airlines many thousands of dollars a year to correct. Thus, the 1990 decision by the Federal Aviation Administration to ban in-flight smoking was, unsurprisingly, greeted with joy by the airlines.

As with most systems on modern aircraft, the pressurization system has built-in redundancies. In the event that the outflow valve is overwhelmed or fails, the aircraft is equipped with both Positive Pressure Relief Valves --- small round butterfly valves that open automatically when the pressure inside the fuselage rises too high --- and Negative Pressure Relief Valves --- rectangular doors that are pushed open by outside atmospheric pressure when the cabin pressure drops too low.

Positive (top) and Negative (bottom) Pressure Relief Valves in the open position

Without these systems, the cabin walls would be subject to extreme flexing through expansion and contraction leading to a catastrophic structural failure of the airplane. Or, just as bad, the cabin windows could be pushed out of their frames --- inward or outward depending on the pressure differentials --- which would lead to explosive decompression, the loss of the aircraft, and the deaths of the passengers and crew. 

“Bleed holes” are small but crucial, and every airline passenger has at one time or another wondered about them

This last is why there is a small “bleed hole” in aircraft windows --- as a matter of course, any aircraft window is a double window, with a small air gap between the inner window (next to which you sit) and the outer window (flush with the exterior of the fuselage). The bleed hole allows the air pressure between the two panes to be equalized at all times, minimizing the chance of a pressure-induced failure of the window system. And it needs to be stressed that any openings in a pressurized container are high-risk areas where the pressure system is more likely to collapse, if adequate precautions are not taken.