YOU’RE flying down to Malaga and you’ve just opened your curled up sandwich and your obligatory bottle of water, the water being quite important because we stopped humidifying cabins at least thirty years ago,
Then you hear over the PA ‘This is your Captain speaking, we are flying at 39,000 feet blah, blah, blah…’ You’re comfortable, at about the right temperature and breathing normally. You are in fact over seven miles up, the temperature out side is nearly -60 degrees (instantly turning brass monkeys into eunuchs), and the air so thin that not even a canary could live; so how’s it done.
The mean air pressure at sea level is 14.7 psi but at 39,000ft it is less than 3psi. We must pressurise the cabin and to do that we tap compressed air from the engines; this air is cleaned and controlled to allow the fuselage to be inflated and as the aircraft climbs to its cruise altitude more air that has been environmentally controlled is pumped into the cabin. At cruise altitude we have a maximum of 8.5psi in the cabin, not sea level pressure but adequate to keep passengers comfortable ; incidentally this puts 150 tons pressure against each door!.
Underneath the back of the aircraft is a hole with a door that modulates, called the outflow valve. The cabin air is changed by this device as it opens and closes to allow old air out and the engines pump new air in.
However some 20 years ago it was deemed that this door continually fluctuating into the airflow, as the aircraft cruised, caused excessive drag which in turn increased fuel consumption, so manufacturers decided to reprogram this valve so that it stuck out into the atmosphere less often causing less drag and saving fuel.
The lowering of the air change frequency results in more contagious ailments being picked up by passengers including severe head colds, and various strains of flu’. If it had been up to certain airlines I’m sure they would have devised a method to charge you for an air change!
Cabin pressure can be lost due to two possible malfunctions, vis. computers fail or explosive decompression. Apart from idiots putting bombs on aircraft, fuselage blow outs are due to poor maintenance and lack of correct inspection criteria.
When cabin pressure is lost your environment rapidly becomes that which is outside and you have only a few minutes before lapsing into unconsciousness, hypoxia. Pilots continuously practise for this eventuality and on receiving the bells and whistles warning, instinctively carry out the emergency procedure.
Quick Don Masks on, tell Air Controllers what you’re doing and quickly loose as much altitude per minute as is safely practical, until you reach about 12,000 feet were most people can breath normally; a slightly uncomfortable procedure for the nervous or elderly. The oxygen masks will have dropped down so that whilst your crew are saving your life you will be able to breath the oxygen from those masks, but, remember give it a slight tug which opens the valve.
Geoff spent over 40 years working in the aviation industry. Trained by the Royal Air Force Engineering College at Halton and after working on military aircraft went on to work in the civil aviation industry.