Naked Science Forum
General Science => General Science => Topic started by: AndyW on 02/03/2009 09:30:02
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AndyW asked the Naked Scientists:
Aircraft at cruising speed and altitude have a huge excess of kinetic and
potential energy to lose before landing; so why do they crash when engines fail on approach? Could a steeper glide slope not be chosen that they can simply glide down if necessary?
What do you think?
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On finals it is normal and safest to try to lose as much kinetic energy as possible before meeting the runway. Aircraft deploy flaps to lower their stalling speed, effectively adding wing area but increasing drag. They have a certain descent angle to maintain speed and, after crossing the runway threshold, the pilot will level the aircraft to hold off the craft a few feet above the runway until drag causes it to descend in a semi-stalled state onto the runway. He will then use conventional braking to stop the aircraft, and on large passenger aircraft, reverse thrust.
Because the approach to the runway involve flying close to the stalling speed, the craft is susceptible to changes in the direction of the wind. It is normal to approach runways into the wind. This reduces the ground speed whilst maintaining airspeed. As you get closer to the ground, however, the wind will drop (this is normal wind shear due to the ground proximity) which can reduce your airspeed rapidly close to the ground and cause the craft to drop quickly near the ground. Pilots have to take this into account but is often responsible for the occasional heavy landing. Landing downwind is more difficult because the airspeed increases as the craft gets close to the ground and can result in a long hold off time. It's surprising how fast you can run out of runway in such circumstances.
Wind shear can also occur as an atmospheric condition and, at altitude or in clouds (especially cumulo-nimbus which are avoided) can be very large and dangerous. If they occur on an aircraft approach it can obviously be dangerous and can result in a stall. This is rare though, and pilots ensure there is a safety margin above the stall speed on approach. There can also be turbulence which can have the same effect. This can be caused by a previous aircraft and the swirling air-pattern can remain for many minutes in otherwise still conditions or drift gently in the breeze.
During WWII, if an aircraft that had been damaged in an unknown way it was good policy to come in at well above normal stalling speed and then fly the aircraft into the ground at above the stall speed to avoid a dangerously long hold off. This takes practice but is safer because the pilot would not be sure of the actual stall speed. This technique is used on aircraft carriers too although they also have an arrestor system for stopping the craft on the short runway. The technique can be employed on passenger aircraft too but is not without its own dangers.
Flocks of birds are also a greater danger on approach as they can take out an engine. The engines are used to control the steepness of the descent and maintain the speed of the aircraft. Because the descent is close to a glide path, loss of an engine can often be managed. This is more of a problem on take-off where all engines are required to climb. Loss of an engine can require a quick re-orientation of the craft to keep well above stall speed. Loss of multiple engines is usually disastrous. Again this is quite rare.
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Graham - You seem very well-informed. Do you hold a pilot's licence?
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I used to fly when I was young but then got married and bought a house instead! Also my wife's late father was in the RAF and saw a lot of action in WWII as a pilot, so had quite a few stories to tell.
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While I am sure that Graham's explanation is correct I can't help wondering if the answer to "Could a steeper glide slope not be chosen that they can simply glide down if necessary?" is that a steep enough glide path is called a crash.
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BC, a steeper glide path results in a higher speed until drag limits it, The glide path is determined by the flap settings and a speed which is safely above the stalling speed. As a matter of interest the slowest landing speed is with the engines on full power with the nose up and the aircraft in a semi-stalled state. I doubt this is practiced by commercial aircraft but is what is used in light aircraft making a forced landing on rough terrain. I remember having to practice this and it was quite tricky and quite worrying with the stall warning indicator screaming away.