Water on the runways reduces the friction between the tires and the ground, and can reduce braking effectiveness. The ability to brake can be completely lost when the tires are hydroplaning because a layer of water separates the tires from the runway surface.
This is also true of braking effectiveness when runways are covered in ice. When the runway is wet, the pilot may be confronted with dynamic hydroplaning. Dynamic hydroplaning is a condition in which the airplane tires ride on a thin sheet of water rather than on the runway's surface.
Because hydroplaning wheels are not touching the runway, braking and directional control are almost nil.
To help minimize dynamic hydroplaning, some runways are grooved to help drain off water; but most runways are not.
Tire pressure is a factor in dynamic hydroplaning. By the simple formula in figure 9-19, the pilot can calculate the minimum speed, in knots, at which hydroplaning will begin. In plain language, the minimum hydroplaning speed is determined by multiplying the square root of the main gear tire pressure in pounds per square inch (p.s.i.), by nine. For example, if the main gear tire pressure were at 36 pounds per square inch, the airplane would begin hydroplaning at 54 knots.
Landing at higher than recommended touchdown speeds will expose the airplane to a greater potential for hydroplaning. And once hydroplane starts, it can continue well below the minimum, initial hydroplaning speed.
On wet runways, landing into the wind can maximize directional control. Abrupt control inputs should be avoided. When the runway is wet, anticipate braking problems well before landing and be prepared for hydroplaning. Opt for a suitable runway most aligned with the wind. Mechanical braking may be ineffective, so aerodynamic braking should be used to its fullest advantage.
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