The Pilots Guide To Aircraft Hydroplaning

Hydroplaning is defined as a condition that exists when an aircraft's tires rise off the runway surface and ride on the surface of a contaminant, like a water ski.

For an aircraft to hydroplane, an airport surface, such as a runway or taxiway, must have a contaminant present.

The FAA states that a runway is considered contaminated whenever standing water, ice, snow, slush, frost in any form, heavy rubber, or other substances are present.

When an aircraft hydroplanes, a layer of fluid forms between the tires and airport surface.

The fluid layer between the aircraft's tires and airport surface lifts the tires off the surface and results in a partial or complete loss of contact—this loss of contact results in a loss of ground controllability and braking efficiency.

As a result, when an aircraft hydroplanes on landing, the expected landing distance will increase. If this increase in distance is not accounted for, an aircraft may run the risk of overrunning a runway.

A contaminated surface causes all types of hydroplaning. However, the type and degree of hydroplaning is dependent on the following factors:  

• The runways surface type (grooved, grass, asphalt, etc.)
• The depth and type of contaminant
• Tire pressure
• Groundspeed
• Anti-skid effectiveness or locked wheels

When an aircraft hydroplanes, it will experience one of the following types of hydroplaning:

• Dynamic Hydroplaning
• Reverted Rubber Hydroplaning; or
• Viscous Hydroplaning

Dynamic hydroplaning can be expected when there is a film of water on a runway that is at least one-tenth of an inch deep.

Dynamic hydroplaning occurs when an aircraft is traveling at relatively high speeds. As an aircraft's speed increases, water on the runway surface can not be displaced by the tire and forms a wedge of water under the aircraft's tires. This wedge resists the tire and builds up pressure between the tire and runway surface. At a certain speed, termed the dynamic hydroplaning speed (Vp), the wedge's pressure equals the aircraft's weight, resulting in the aircraft's tire to rise off the runway surface.

The dynamic hydroplaning speed is directly proportional to the aircraft's tire inflation pressure. It is equal to 8.6 times the square root of the tire pressure in pounds per square inch.

While the dynamic hydroplaning speed is the minimum speed required for dynamic hydroplaning to occur, once an aircraft has started to dynamically hydroplane, it may continue to hydroplane significantly slower than the dynamic hydroplaning speed.

Reverted rubber hydroplaning occurs when a thin film of water is on a runway, and a pilot allows for the aircraft's tires to enter a prolonged locked-wheel skid.

A pilot is most likely to cause an aircraft's tires to enter a prolonged locked-wheel skid while performing heavy braking. Frequently, reverted rubber hydroplaning and a locked-wheel skid occurs after an encounter with dynamic hydroplaning as the pilot may have had the brakes locked while attempting to slow down the aircraft.
When an aircraft tire experiences reverted rubber hydroplaning, the skidding tire generates a lot of friction and heat. This heat causes the rubber on the tire to revert to its uncured state. Hence the name reverted rubber hydroplaning.

In its uncured state, the rubber forms a seal between the tire and runway and prevents water from exiting the tire's footprint. This trapped water is then heated up by the tire's friction and forms steam, which lifts and supports the tire off the runway surface.

Viscous hydroplaning requires a thin film of fluid no more than one-thousandth of an inch deep and a smooth or smooth acting surface.
Viscous hydroplaning is due to the viscous properties of water. An aircraft tire experiencing viscous hydroplaning cannot penetrate the thin film of fluid and instead rolls on top of it.

Viscous hydroplaning can occur at speeds much slower than dynamic hydroplaning but does require a smooth acting surface, unlike dynamic hydroplaning.

Smooth surfaces can include asphalt runways or touchdown zones coated with rubber from previous landings. These surfaces can cause a tire to behave as if it was traveling across wet ice.

Regardless of the type, hydroplaning will always occur when operating on a contaminated runway. 

However, The degree of hydroplaning depends on:

• The type of runway surface
• The type of contaminant and its depth
• The PSI of each individual tire
• The aircraft's ground speed
• Whether the aircraft tires are locked or not

To prevent or compensate for hydroplaning, a pilot should keep the following in mind:

1. Choose a runway that has a grooved surface.
2. Touchdown as slow as safely possible
3. Use moderate braking, and if hydroplaning is suspected, raise the nose of the aircraft and use aerodynamic braking until brakes become effective
4. Apply braking force to a point just short of a skid; if a skid occurs, release brake pressure and allow the wheels to spin back up to speed
5. Attempt to maintain directional control with rudder, and avoid nose wheel steering if possible
6. Conducting a firm touchdown on a contaminated runway can reduce the degree of hydroplaning at touchdown. 
7. If hydroplaning occurs with a crosswind, the crosswind may cause an aircraft to weather vane into the wind and travel down the runway simultaneously.