When the reverse thrust is engaged upon touchdown, is the engine turning at 100% N1and pushing the thrust in the opposite direction?
I have also wondered why airplane tires operating in snow covered runways are not outfitted with more grooves and threads to give better friction index.
When a pilot applies maximum reverse thrust on landing to slow the airplane down, he does not get 100 % thrust (N1). For instance on an Airbus A330, the maximum reverse thrust is obtained at between 70 % and 85 % (N1) and is controlled by a computer (FADEC).
In some airports, such as London Heathrow, night curfew (between 23:00 and 06:00 hours local) would prohibit a pilot from using maximum reversers thrust unless in an emergency due to the tremendous noise it generates after landing.
The thrust reversers efficiency is proportional to the square of the speed. So the pilot usually uses reverse thrust at high speeds after touchdown and cancels them around 70 knots. The effect of the reverse thrust in assisting the stopping of the plane is not that significant as compared to the aircraft braking system.
In fact it is not taken into consideration for performance calculations. It is also interesting to know that on the 4-engine Airbus A380, the wheel brakes are so efficient that only reversers are fitted on the 2 inboard engines and none on the outboard engines!
Aircraft tires tread patterns are designed to facilitate stability in high crosswind conditions for take-off or landings - basically to channel water away to prevent hydroplaning (skidding) and for the braking effect. So it does that have those elaborate grooves as found in most cars.
When a runway is covered with snow, the airport authorities would plough the runway and clear the snow. They would determine the friction index of the runway surface and pass the information to the pilots. A pilot would only land when he thinks the condition is safe for him to do so and would not touchdown if the runway condition is reported to be poor or slippery!
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