Aquaplaning

Hydroplaning or aquaplaning by a road vehicle occurs when a layer of water builds between the rubber tires of the vehicle and the road surface, leading to the loss of traction and thus preventing the vehicle from responding to control inputs such as steering, braking or accelerating. It becomes, in effect, an unpowered and unsteered sled.

Hydroplaning also affects aircraft tires in contact with a wet runway and rollercoasters on a wet track.

Causes

Every vehicle function that changes direction or speed, from turning, to accelerating, to braking, places an increased load on the tires. Control of this load relies on the friction between the tire contact points and the road surface. More friction makes for a greater resistance to slipping; if water comes between the tires and the road, friction may be reduced to the extent that the driver may lose control.

A diagram of a hydroplaning tire.

The tread or grooves, sometimes referred to as sipes, of a rubber tire are designed to remove water from beneath the tire, providing high friction with the road surface even in wet conditions. Hydroplaning occurs when a tire encounters more water than it can dissipate. Water pressure in front of the wheel forces a wedge of water under the leading edge of the tire, causing it to lift from the road. The tire then skates on a sheet of water with little, if any, direct road contact, resulting in loss of control.

If multiple tires hydroplane, the vehicle may lose directional control and slide until it either collides with an obstacle, or slows enough that one or more tires contact the road again and friction is regained.

The likelihood of hydroplaning increases with the speed of the vehicle and the depth of the water. Tread wear and underinflation also increase the risk for hydroplaning, as do wider tires. Narrower tires are less vulnerable to hydroplaning because the vehicle weight is distributed over a smaller rubber contact patch, resulting in a greater ability for the tires to press water to the sides, allowing tire contact with pavement.

Bicycles, motorcycles, and similar vehicles with a round-shaped surface toward the pavement are far less likely to hydroplane in normal road use. The contact area with the road is a canoe-shaped patch that effectively squeezes water out of the way. However, because road friction is reduced in wet conditions, the lateral force that the tires can accommodate before sliding is greatly diminished. While a slide in a four-wheeled vehicle is correctable with practice, the same slide on a motorcycle will generally cause the rider to fall, with severe consequences. Thus, despite the relative lack of hydroplaning danger, motorcycle riders must be even more cautious because overall traction is reduced by wet roadways.

See also traction for effects similar to hydroplaning.

In motor vehicles

Response

What the driver experiences when a vehicle hydroplanes depends on which wheels have lost traction and the direction of travel.

If the vehicle is traveling straight, it may begin to feel slightly loose. If there was a high level of road feel in normal conditions, it may suddenly diminish. Small correctional control inputs will be ignored by the vehicle.

If the drive wheels hydroplane, there may be a sudden audible rise in engine RPM and indicated speed as they begin to spin. In a broad highway turn, if the front wheels lose traction, the car will suddenly begin to drift towards the outside of the bend. If the rear wheels lose traction, the back of the car will begin to slew out sideways into a skid. If all four wheels hydroplane at once, the car will slide in a straight line, again towards the outside of the bend if in a turn. When any or all of the wheels regain traction, there may be a sudden jerk in whatever direction that wheel is pointed.

Recovery

To recover while traveling in a straight line, the driver should not turn the steering wheel of the car or apply the brakes. Either action could put the car into a skid from which recovery would be difficult or impossible. Instead, with no change in steering input, the driver should gently ease pressure off the accelerator. Control should then return. If braking is unavoidable, the driver should lightly pump the brakes until hydroplaning has stopped.

If the rear wheels hydroplane and cause oversteer, the driver should steer in the direction of the skid until the rear ties gain traction, and then rapidly steer in the other direction to straighten the car.

Prevention by the driver

The best strategy is to avoid as many contributors to hydroplaning as is possible. Proper tire pressure, narrow and unworn tires, and moderate speeds at or below the speed limit will mitigate the risk of hydroplaning. Avoidance of standing water is another effective prevention strategy.

Stability control systems like ESC, ESP, and DSC cannot replace these defensive driving techniques and proper tire selection. They rely on the same braking mechanism at the driver's disposal, which in turn depends on road contact. While stability control may help recovery from a skid when the vehicle slows enough to regain traction, it cannot prevent hydroplaning.

In aircraft

Hydroplaning may reduce the effectiveness of wheel braking in aircraft on landing or aborting a take-off, when it can cause the aircraft to run off the end of the runway. Hydroplaning was a factor in an accident to Qantas Flight 1 when it ran off the end of the runway in Bangkok in 1999 during heavy rain. Aircraft which can employ reverse thrust braking have the advantage over road vehicles in such situations, as this type of braking is not affected by hydroplaning, but it requires a considerable distance to operate as it is not as effective as wheel braking on a dry runway.

Hydroplaning is a condition that can exist when an aircraft is landed on a runway surface contaminated with standing water, slush, and/or wet snow. Hydroplaning can have serious adverse effects on ground controllability and braking efficiency. The three basic types of hydroplaning are dynamic hydroplaning, reverted rubber hydroplaning, and viscous hydroplaning. Any one of the three can render an aircraft partially or totally uncontrollable anytime during the landing roll.

However this can be prevented by grooves on runways. This was initially developed by NASA for space shuttles landing in heavy rain. It has since been adopted by most major airports around the world. Thin grooves are cut in the concrete which allows for water to be dissipated and further reduces the potential to hydroplane.

Types

Dynamic

Dynamic hydroplaning is a relatively high-speed phenomenon that occurs when there is a film of water on the runway that is at least one-tenth inch deep. As the speed of the aircraft and the depth of the water increase, the water layer builds up an increasing resistance to displacement, resulting in the formation of a wedge of water beneath the tire. At some speed, termed the hydroplaning speed (VP), the upward force generated by water pressure equals the weight of the aircraft and the tire is lifted off the runway surface. In this condition, the tires no longer contribute to directional control, and braking action is nil. Dynamic hydroplaning is related to tire inflation pressure. Data obtained during hydroplaning tests have shown the minimum dynamic hydroplaning speed (VP)of a tire to be 8.6 times the square root of the tire pressure in pounds per square inch (PSI). For an aircraft with a main tire pressure of 24 PSI, the calculated hydroplaning speed would be approximately 42 knots. It is important to note that the calculated speed referred to above is for the start of dynamic hydroplaning. Once hydroplaning has started, it may persist to a significantly slower speed depending on the type being experienced.

Reverted rubber

Reverted rubber (steam) hydroplaning occurs during heavy braking that results in a prolonged locked-wheel skid. Only a thin film of water on the runway is required to facilitate this type of hydroplaning. The tire skidding generates enough heat to cause the rubber in contact with the runway to revert to its original uncured state. The reverted rubber acts as a seal between the tire and the runway, and delays water exit from the tire footprint area. The water heats and is converted to steam which supports the tire off the runway.

Reverted rubber hydroplaning frequently follows an encounter with dynamic hydroplaning, during which time the pilot may have the brakes locked in an attempt to slow the aircraft. Eventually the aircraft slows enough to where the tires make contact with the runway surface and the aircraft begins to skid. The remedy for this type of hydroplane is for the pilot to release the brakes and allow the wheels to spin up and apply moderate braking. Reverted rubber hydroplaning is insidious in that the pilot may not know when it begins, and it can persist to very slow groundspeeds (20 knots or less).

Viscous

Viscous hydroplaning is due to the viscous properties of water. A thin film of fluid no more than one thousandth of an inch in depth is all that is needed. The tire cannot penetrate the fluid and the tire rolls on top of the film. This can occur at a much lower speed than dynamic hydroplane, but requires a smooth or smooth acting surface such as asphalt or a touchdown area coated with the accumulated rubber of past landings. Such a surface can have the same friction coefficient as wet ice.

When confronted with the possibility of hydroplaning, it is best to land on a grooved runway (if available). Touchdown speed should be as slow as possible consistent with safety. After the nosewheel is lowered to the runway, moderate braking should be applied. If deceleration is not detected and hydroplaning is suspected, the nose should be raised and aerodynamic drag utilized to decelerate to a point where the brakes do become effective.

Proper braking technique is essential. The brakes should be applied firmly until reaching a point just short of a skid. At the first sign of a skid, the pilot should release brake pressure and allow the wheels to spin up. Directional control should be maintained as far as possible with the rudder. Remember that in a crosswind, if hydroplaning should occur, the crosswind will cause the aircraft to simultaneously weathervane into the wind as well as slide downwind.

References

• B. N. J. Persson, U. Tartaglino, O. Albohr And E. Tosatti (2004). "Sealing is at the origin of rubber slipping on wet roads". Nature Materials. 3 (7 November): 882–885.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• Smart Motorist - Driving in the Rain
• Airplane Flying Handbook, FAA Publication FAA-H-8083-3A, available for download from the Flight Standards Service Web site at http://av-info.faa.gov.