Aquaplaning / Hydroplaning

Aquaplaning is the phenomenon of a tire skating (not rotating) over the runway surface on a thin film water. It is caused by the buildup of a layer of water beneath the tire, which becomes a wedge between the tire and the runway. This wedge of water effectively lifts the tire off the ground, reducing the friction forces to practically zero. Therefore, wheel braking has no effect, and directional control also may be lost. Aquaplaning is extremely significant on jet transport aircraft and can increase the stopping distance required significantly.

What are the three types of aquaplaning/hydroplaning?

The three types of aquaplaning are

1. Dynamic: This is due to standing water on the runway when the tire is lifted off and completely supported. That is, it is a function of fluid density.

2. Viscous: This occurs when the surface is damp and provides a very thin film of fluid that cannot be penetrated by the tire. Viscous aquaplaning can occur at or persists down to much lower speeds than simple dynamic aquaplaning. It is particularly associated with smooth surfaces and is quite likely to occur in the touchdown area, which is often liberally smeared with rubber deposits. That is, it is a function of fluid viscosity.

3. Reverted rubber or rubber reversion: This refers to the tire becoming tacky. It requires a long skid, rubber reversion, and a wet surface. The heat from the friction between the tire and the wet runway surface boils the water and reverts the rubber, which forms a seal that delays water dispersal. The steam then prevents the tire from contacting the runway. That is, melting rubber traps the steam, which causes aquaplaning.

Note: All three types of aquaplaning can occur during one landing run if the conditions are correct.

How do you calculate the aquaplaning/hydroplaning speed?

The approximate minimum true ground speed in knots at which aquaplaning can be initiated, is calculated using the following formula

9 X square root of the tire pressure in pounds per square inch for takeoff or,

7.6 X square root of the tire pressure in pounds per square inch for landing

However, it is generally regarded that 9 times the square root be used also for the landing, although in theory only 7.6 times the square root is required at landing, followed by 9 times the square root during the rollout.

Note: Aquaplaning occurs at any speed above this minimum speed. On takeoff, there is normally a considerable gap, i.e., 20 to 50 knots, between the minimum aquaplane speed and Vi; e.g., minimum aquaplane speed of 115 knots and a Vl speed of 145 knots. On landing or a rejected takeoff, if aquaplaning occurs, it may continue at speeds lower than the calculated aquaplane speed. This is so because the calculation gives us a speed below which aquaplaning will not start. Tire wear is obviously an important factor; the more worn the tire, the more likely it will be to aquaplane. A general awareness of this should be retained.

How do you control an aquaplane?

You control an aquaplane by using the antiskid braking systems, which releases the brakes if it senses a skid. This allows the wheel to rotate and to push through the water layer, making contact with the runway again. If you have no antiskid systems or they are inoperative, then manually brake using the same principle, namely, if you feel a slip, then release the brakes, allow the skid to stop, and then reapply the brakes.

 

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