It would be reasonable to wager that the majority of air passengers have never questioned why the traditional flight altitude is 35,000 feet (10,600 metres).
The number has entered aviation lexicon as the catch-all height at which most aircraft travel from A to B. But do all planes fly this high – and if so why?
Why do planes fly so high?
The higher the better
One of the central reasons behind aircraft altitude is that, as the air gets thinner with every foot climbed, planes can travel more easily and therefore move faster and burn less fuel, saving money.
The "sweet spot" of flying is regarded as between 35,000 and 42,000 feet (the airline industry still uses feet and inches as its standard measurements) – too high and the oxygen becomes too sparse to fuel the engines, too low and the air resistance is greater. This optimum height is linked to the usual weight of a commercial jet – that is, heavier planes would fly lower, and lighter higher.
It would actually be most efficient to be forever climbing, rather than plateau when reaching say, 35,000 feet, cruising altitude, as the weight of the aircraft decreases due to fuel usage and the air thins.
"Each individual aircraft has an optimum altitude (for minimum cost or minimum fuel burn) which will be based on its individual weight," explains Peter Terry, a commercial airline pilot of 30 years.
"Concorde flew at much higher altitudes – 50,000/60,000 feet – where there were no other aircraft and so were able to cruise climb [that is keep climbing]."
Doug Morris, a captain with Air Canada, explains that the general rule is the higher the better "because the thinner air imposes less drag".
"There is a trade-off between fuel efficiency and power," he said.
How's the weather up there?
Flying thousands of feet above the ground also means aircraft avoid much of the bad weather people on the ground are subjected to. You know the feeling when you see nothing but bluebird skies from your window seat, only to descend into your destination airport to dreary drizzle.
The troposphere – that is the atmospheric layer closest to the ground – is home to most of the world's weather phenomenons. Usually measured up to 36,000 feet, this is where clouds are most likely, as well as heavy rains and high winds. Aircraft prefer life in the stratosphere, which means less turbulence.
Avoiding heavy traffic
Flying so high also means that aircraft are able to avoid other airborne traffic, such as light aircraft or helicopters, which fly lower, as well as insects and birds.
Light aircraft do not have pressurised cabins, therefore stick below 10,000 feet. Any higher and the pilot is required to don an oxygen mask to keep conscious.
In the event of an emergency
Should something bad happen to an aircraft at 35,000 feet, like losing power in its engines, the pilot has much longer to deal with the situation, than if the aircraft was just at 10,000 feet. This may sound silly, but remember that planes can still land safely even if both engines fail – so having more time to get your ducks in a row before attempting such a manoeuvre could save lives.
See also: The truth about oxygen masks on planes
How cold is it up there?
The higher you get, the colder it gets, up until 40,000 feet. If the temperature at ground level was 20C, at 40,000 feet it would be -57C. At 35,000 feet the air temperature is about -54C.
Is there a minimum flying height?
Has anyone seen comedian Eddie Izzard's Glorious, where he does a sketch about taking a flight in a very small aircraft with only a handful of passengers? He jokes that the pilot's announcement is thus: "Welcome to Flight One from here to there. We'll be flying at a height of ten feet, going up to 12 and a half feet if we see anything big."
Well, why not fly at the minimum height required to clear any obstacles?
Because there are laws that govern just that.
Known as the lowest safe altitude (LSALT), the value has been applied by aviation bodies around the world, including Australia's Civil Aviation Safety Authority (CASA), the British Civil Aviation Authority (CAA) and the US Federal Aviation Administration.
"This applies much more to light aircraft than airliners, but it is illegal to fly below 1,000 feet when over a built-up area, or 500 feet over any person, vehicle or structure," said Richard Taylor of the CAA.
"That is as much for environmental reasons such as noise as it is for safety."
Aircraft must not fly less than 1000 feet above the "highest fixed object" beneath them and pilots must make sure that they are high enough to clear the congested area below in the event of engine failure. Obviously, these don't apply to take-off and landing when aircraft are in controlled flight paths.
What about a maximum?
There is no maximum altitude for flying, however, engines will struggle as the oxygen levels fall, and communication with the ground will become a greater challenge.
The record altitude for a jet plane is 123,520 feet, set by Alexandr Fedotov in 1997 flying a military Soviet MiG-25M.
Incidentally, the record for a paper plane is 89,590 feet, which did not fly to that height but was released by a helium balloon.
The Telegraph, London
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