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So this was from a barrage of History Channel documentaries celebrating the century of flight, some 13 years ago. In one of the shows, I recall it being said that an airliner has a time limit for a line-up and wait, else an asphalt runway would start melting behind the jet engines.
Update:
Maybe, it was the case in the early jet age. For those good at finding historic regulations, etc.
We can get some idea of what temperature the pavement would be subjected to by looking at some data on different jet aircraft.
Older low-bypass turbofans produced higher exhaust temperatures over a larger area than new high-bypass turbofans. This is due to the high-bypass engines having more of the cooler bypass air mixing with the hotter core exhaust.
Manufacturers provide various data about their aircraft that can be useful for planning operations at airports. Included in the data are figures showing exhaust velocity and temperature for different throttle settings.
It should be noted that aircraft holding for takeoff clearance will be at idle power. The power will be increased only when cleared for takeoff, so the takeoff temperatures will be experienced only briefly.
Although hot air tends to rise, the 737 analysis notes that:
The buoyancy effects are considered to be small relative to the exhaust velocity and therefore are not included.
The figures for the 707 show the exhaust contours starting at the engines. The highest temperature labeled is 200°F (93°C), though higher temperatures would be expected closer to the engines. At idle power the 150°F (66°C) contour barely reaches the tail and stays above the ground. At takeoff power, the 200°F contour reaches bast the tail, but the highest temperature in contact with the ground would be the 150°F. These figures do note "data not verified by test".
The figures for the 737 note that the contours were produced by computer analysis. These contours only start at the aft end of the aircraft. The engines on the 737 are located closer to the ground than the 707. The original 737-100 and -200 models have the highest temperatures. At idle thrust, the 150°F contour barely reaches the tail of the plane, though it may reach the ground closer to the engines. At takeoff, the 200°F contour extends past the tail, and definitely reaches the ground.
The DC-8 is similar to the 707. At idle power, only small contours are shown for 125°F (52°C). At takeoff power, temperature contours reach 200°F but only the 150°F contour reaches the ground.
Based on these temperatures, it does not seem that an aircraft sitting on the runway with engines at idle would cause issues with the pavement. On a hot sunny day, asphalt could easily reach 150°F or more.
The exhaust temperature of a jet engine is more than adequate to melt asphalt. But the real answer to this depends on several factors
If the engine is mounted high up on the fuselage, this is essentially impossible. The heat rises away from the engine. For example the MD80:
The engines are mounted high up on the fuselage. The exhaust gas exits parallel to the ground and rises away. It will not appreciably heat the runway.
The engine idle speed determines the volume of fuel and gas consumed. For reasons of economy, this is kept as low as possible. However, the engines also provide electrical power and "bleed air" (pneumatic power essentially). Therefore they must run at a speed adequate to power the aircraft. The exception to this would be if the plane has an APU, a small engine which can provide power even with the main engines off. An airline might choose to use an APU (if equipped) in the circumstance of an extended delay on the runway.
The bypass of a turbofan engine on an airliner plays a large part in dictating the temperature of the exhaust gas. The bypass ratio is the percentage of air that is simply accelerated by the engine, not combusted. If the bypass ratio for example 90%, 10% of the air is heated dramatically by the combustion in the engine's core. But in the exhaust stream that air is recombined with uncombusted air. Therefore, it cools quickly after exiting the engine. This only applies to a turbofan engine, but most airliners use a turbofan engine.
You are unlikely to find much information about the bypass ratio of a turbofan at idle. No one cares, since the engine is not providing thrust in the first place.
Most airliners do not have downward facing exhaust outlets. There is no reason to have them.
On the other hand, you don't need the exhaust to melt the asphalt in a measurable way at all! Consider this report :
http://www.theatlantic.com/technology/archive/2012/07/wait-tarmac-can-melt/259565/
The runway "melted" just by atmospheric conditions! Saying it melted is a very relative term. The article specifies an ambient temperature of 100 F. If a person had walked over the runway, there would have been no noticeable change. But with the ground pressure exerted by the airliner's tires, the asphalt deformed.
There is no time limit to a line up and wait clearance, but it behooves both ATC, the flight crew of the aircraft waiting to take off and the runway transgressors to expidite the process, particularly at a high traffic airport, which is really the only reason a line up and wait clearance is issued.
As for melting the runway with your jet exhaust, as long as you are not in a Harrier or F-35B running at near maximum power prior to hover, you'll be fine. Now as that Atlantic article above points out, hot asphalt is prone to deformation with a heavy object resting on it for prolonged periods, so it is advisable not to park a 747 at or near gross takeoff weight on hot asphalt for 12 or more hours to avoid tire indentations forming in the asphalt. But a loaded 747 waiting 35 seconds for an A320 to clear the active prior to his takeoff roll should be fine.
