Hydroplaning

Runway friction values are currently not provided during the summer and when it is raining. Consequently, some discussion of wet runways is in order to assist pilots in developing handling procedures when these conditions are encountered.

A packed-snow or ice condition at a fixed temperature presents a relatively constant coefficient of friction with speed, but this is not the case for liquid water or slush. This is because water cannot be completely squeezed out from between the tire and the runway and, as a result, there is only partial tire-to-runway contact. As the aircraft speed is increased, the time in contact is reduced further, thus braking friction coefficients on wet surfaces fall as the speed increases, i.e. the conditions in effect become relatively more slippery, but will improve again as the aircraft slows down. The situation is further complicated by the susceptibility of aircraft tires to hydroplane on wet runways.

Hydroplaning is a function of the water depth, tire pressure and speed. Moreover, the minimum speed at which a non-rotating tire will begin to hydroplane is lower than the speed at which a rotating tire will begin to hydroplane because a build up of water under the non-rotating tire increases the hydroplaning effect. Pilots should therefore be aware of this since it will result in a substantial difference between the take-off and landing roll aircraft performance under the same runway conditions. The minimum speed, in knots, at which hydroplaning will commence can be calculated by multiplying the square root of the tire pressure (PSI) by 7.7 for a non-rotating tire, or by 9 for a rotating tire.

Hydroplaning is a function of the water depth, tire pressure and speed.

This equation gives an approximation of the minimum speed necessary to hydroplane on a smooth, wet surface with tires that are bald or have no tread. For example, the minimum hydroplaning speeds for an aircraft with tires inflated to 49 PSI are calculated as:

  • Non-rotating tire: 7.7 X √49 = 54 kt
  • Rotating tire: 9 X √49 = 63 kt

When hydroplaning occurs, the aircraft’s tires are completely separated from the actual runway surface by a thin water film and they will continue to hydroplane until a reduction in speed permits the tires to regain contact with the runway. This speed will be considerably lower than the speed at which hydroplaning commences. Under these conditions, the tire traction drops to almost negligible values, and in some cases, the wheel will stop rotating entirely. The tires will provide no braking capability and will not contribute to the directional control of the aircraft. The resultant increase in stopping distance is impossible to predict accurately, but it has been estimated to increase as much as 700 percent. Further, it is known that a 10 knot crosswind will drift an aircraft off the side of a 200 feet wide runway in approximately 7 seconds under hydroplaning conditions.

Notwithstanding the fact that friction values cannot be given for a wet runway and that hydroplaning can cause pilots serious difficulties, it has been found that, under light or moderate rain conditions, well-drained runways seldom accumulate sufficient standing water for hydroplaning to occur.

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