Science: Airline Passengers under Pressure

Most people who travel by air know that there are pressure changes when the aircraft climbs and descends. This is often reflected in their ears “popping” as the ear attempts to equalise the pressure changes. But few of them understand what goes on during these phases of a flight.

    As you gain altitude, air pressure decreases and so does the temperature. In fact at a normal cruising altitude of 36,000 feet, the outside air temperature is about minus 60 degrees Celsius. If you were to be able to step outside, you would not last very long. So it is important that a more normal pressure is maintained within the aircraft. But if you think that the pressure will be at sea-level, you would be wrong. And here’s why.

    At cruising altitude of say 36,000 feet, if the cabin pressure was set to sea-level, then the pressure difference would be 36,000 feet. The insides of the fuselage would be pushing outwards at great force. Therefore, the fuselage would have to be very strong and that means the aircraft becomes heavier. Heavy aircraft consume more fuel thus making the flight more expensive and your ticket.

    The answer is to reduce the pressure difference and so most aircraft at cruising altitude have a cabin pressure equal to being on a mountain 6,000 feet up. The pressure difference now is 36,000 minus 6,000 = a difference of 30,000 feet. This means the forces are now reduced making the aircraft lighter and more economic to fly. This induced height does no harm to passengers, although the air tends to be drier and so fluids such as water should be taken regularly.

    The change in pressure inside the aircraft should be in proportion to the change outside the aircraft. The rate of change in pressure outside is greater than inside, so the rate of change inside is gradual at about 300 feet per minute. So when the aircraft reaches its cruising height of say 36,000 feet, the inside pressure reaches 6,000 feet. For most passengers, this is hardly noticeable.

    This now brings up an interesting point. Did you know that the doors in an airliner are not locked in flight? Does this mean that some silly person might accidentally open the door? The answer is no. If you look carefully when one of the crew opens the door after landing, they first have to pull the door inwards before it will open outwards. Therefore they would have to be able to fight the pressure difference keeping the door closed. There is no human capable of coming close to having that strength. You can now breathe a sigh of relief.

    Finally, a quirky bit of information; which do you think tastes better in an airliner, tea or coffee. Most tea drinkers would agree that the water to make a good pot of tea needs to be boiling. But coffee less so. Since the aircraft is pressurised to 6,000 feet, water boils at a lower temperature. In fact at 6,000 feet it boils at about 90 degrees Celsius instead of 100 degrees. So coffee usually tastes better in the air.

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