Why Are Aeroplane Runways A Cement Based Surface Instead of Tar Based Like Roads?
Answers:
Lots of reasons. But basically it has to do with weight and shock.
Aircraft are very heavy and there have been many cases of them sinking through lighter constructed taxi ways! When the aircraft lands it exerts a sudden tremendous weight at speed onto the surface giving momentary stress far in excess of it's actual weight. A lot of this stress is directional from the push of the direction of the force but also sideways in reaction to the weight.
Concrete takes both shock and weight in it's stride. Tarmac on the other hand is much weaker and would therefore give and move in response. Ever noticed those shallow ruts in tarmacadam roads where the big trucks tires have been and they are a fraction of the weight of an aircraft.
Most runways have four feet or more of reinforced concrete topped by a friction coat of tarmacadam. The concrete gives strength whilst the tarmac gives grip and an easily replaceable top surface. The tarmac also acts as a weather coat to protect the runway from breaking up due to the effects of rain, wind and ice.
Tar is softer, planes are heavier, bingbangboom
Concrete is a rigid material, asphalt is malleable and will deform quickly under the heavy landings of aircraft, leaving ruts and unevenness.
Because cement can stand the heavier weight of aircraft crashing down on them at 100mph plus.
Also, tar goes soft in hot weather, unlike cement.
Could be something to do with the fact that tar gets really soft in hot weather and becomes really loose, I guess with a runway even the smallest bumps can be pretty dangerous when a plane is speeding up? And also the friction when the plane lands/brakes etc would heat it up loads?
Tar doesn't have the strength to take in the impact of the landing of that heavy and massive airplane while cement has!
answers as above.
and the reason that cement isn't usually used on roads is because it causes more road noise. there was uproar when they opened the new strecth of the A30 in devon with a cement surface beacuse the road noise could be heard -literally - miles away (see: news.bbc.co.uk/2/low/uk_news/e. )
Because cars don't land on roads. Tarmac is soft and, in the hotter months is a little bit sticky and malleable. If an aircraft were to land on tarmac, then the runway would be pitted with "dents" where the tyres touched down, making it an uneven surface for the next approaching aircraft. The runway would have to be resurfaced nearly every day! Also, the impact of an aircraft landing on soft ground would be damaging to the aircraft's undercarriage and an uncomfortable experience for the passengers.
you've seen how motorways get in hot weather sticky and tacky.i wouldn't like to be on a plane and land on that plus theirs to much expansion in tarmac. the concrete is laid in bays with rubberised expansion joints.its normally a 1ft thick and over to take the weight of the plane.
Cement surface can grip better on plane tires and when is wet it would get slippy.
Concrete (or asphalt if you prefer) is harder than Tar and will therefore last longer before it needs maintenance.
Tarmac is soft and, in the hotter months is a little bit sticky and malleable. If an aircraft were to land on tarmac, then the runway would be pitted with "dents" where the tyres touched down, making it an uneven surface for the next approaching aircraft. The runway would have to be resurfaced nearly every day! Also, the impact of an aircraft landing on soft ground would be damaging to the aircraft's undercarriage and an uncomfortable experience for the passengers.
also the extreme heat from the engines would cause alot of damage even if the hot weather didnt
Concrete is less malleable than tar, and is consistent over a wider range of temperatures. Look at bus stops, if a 15ton buses can deform tar over a period of time what would a 200ton plane do?
there is a airplane runway made with tar by me,its called the M42, also did you know that coventry airport is the only one in the world with a cattle grid going across it.
Concrete can be reinforced, for one thing. Secondly it lasts much longer. Thirdly. it melts in extreme temps, which a crash would produce.
The have used tar runways on airports to my knowledge from 1973 at Heathrow and Gatwick airports its not tarmac as normal roads and sets very hard.
Everyone's answers are essentially correct, however tar-based runways are actually very common. Many GA airports (where extreme weight tolerances are not an issue) do use asphalt because its cheaper.
Runways are constructed out of asphalt and concrete. Both have advantages and disadvantages. Ashpalt is more flexible and can bend with stress. Concrete is harder but doesn't bend, it cracks when overstressed.
If you're really into this stuff, I suggest you check out the FAA Runway Pavement Test Facilty, at the FAA Technical Center in Atlantic City, NJ. This facility, the only one of its kind in the world, is a quarter-mile-long building inside of which they build sections of runway, usually one of concrete and one of asphalt. They have a HUGE machine that runs aircraft landing gear components up and down the test runways until the runways fail. The runway sections are built with a series of sensors in them down to 12 feet below the surface, so they can measure the stress that aircraft would exert on the runway. You can find out more about this facility at their website: http://www.airtech.tc.faa.gov/naptf/.
Incidentally, the answers you got about the high loads placed on the runway by landing aircraft are incorrect. Aircraft taxiing and taking off place a far larger load on the runway surface than do landing aircraft. This is due to the fact that aircraft taking off are much heavier since they're full of fuel, they're moving slowly over the runway or taxiway surface and none of their weight is supported by their wings. Landing aircraft have a large amount of their weight supported by their wings until after they've landed and slowed down, they're moving faster and they're lighter due to containing less fuel.
Aircraft landing gear configuration is also very important in determining runway stress. While the 747 has 4 main landing gear carriages, most other aircraft have only 2, with a varying number of wheels per carriage. The new Airbus A-380 will have 2 main landing gear carriages but with 10 wheels per carriage, 5 on each side. The distance between the main landing gear carriages is also important as the loading that they place on the runway spreads out as it moves down beneath the runway surface. At some point, these loads could actually intersect deep beneath the runway surface and cause a failure of the subsurface material.
This is probably more than you wanted to know, but hope it's helpful.
Reinforced concrete is stronger. Airplane tires are at very high pressure (well over 100 psi on a large plane), and that would dig a hole in an asphalt runway.
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