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Aquaplaning

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A diagram of a hydroplaning tire

Hydroplaning or aquaplaning by the tires of 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. If it occurs along all four wheels, the vehicle becomes, in effect, an uncontrolled 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 relies on the friction between the tires and the road surface. If water comes between the tires and the road, friction may be reduced to the extent that the tires may slip, and the driver may lose control.

The grooves of a rubber tire are designed to disperse water from beneath the tire, providing high friction 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, and loss of control results. 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 risk of hydroplaning increases with the depth of standing water and the sensitivity of a vehicle to that water depth.[1][2]

Water depth factors

Ruts in a road.
  • Depth of compacted wheel tracks and longitudinal depressions
    Heavy vehicles can cause ruts in the pavement over time that allow water to pool, negatively impacting draining.
  • Pavement micro and macrotexture[3]
    Concrete can be preferable to hotmix asphalt because it offers better resistance to rut formation, though this depends on the age of the surface and the construction techniques employed while paving. Concrete also requires special attention to ensure that it has sufficient texture.
  • Pavement cross slope and grade[4]
    Cross slope is the extent to which the cross-section of a road resembles an upturned U. Higher cross slopes allow water to drain more easily. Grade is the steepness of the road at a particular point, which affects both drainage and the weight of the vehicle. Vehicles are less likely to hydroplane while traveling uphill, and far more likely to do so at the trough of two connected hills where water tends to pool. The resultant of cross slope and grade is called Drainage Gradient or "Resulting Grade". Most road design manuals world wide require that the Drainage Gradient in all road sections must exceed 0.5 %, in order to avoid a thick water film during and after rainfall. Areas where the Drainage Gradient may fall below the minimum limit 0.5 % are found at the entrance and exit of banked outer curves. These hot spots are typically less than 1 % of the road length, but a large share of all skid crashes occur there. One method for the road designer to reduce the crash risk is to move the cross slope transition from the outer curve and to a straight road section, where lateral forces are lower. If possible, the cross slope transition should be placed in a slight up- or downgrade , thereby avoiding that the Drainage Gradient drops to zero. The UK road design manual actually calls for placing the cross slope transition in an artificially created slope, if needed. In some cases, permeable asphalt can be used to improve drainage in the cross slope transitions.
  • Width of pavement
    Wider roads require a higher cross slope to achieve the same degree of drainage.
  • Roadway curvature
  • Rainfall intensity and duration

Vehicle sensitivity factors

  • The driver's speed, acceleration, braking, and steering
  • Tire tread wear and contact patch shape
    The longer and thinner the contact patch, the less likely a tire will hydroplane. Tires that present the greatest risk are wide, lightly loaded, and small in diameter. Deeper tread dissipates water more easily.
  • Ratio of tire load to inflation pressure
    Underinflated tires are more prone to hydroplaning, especially as vehicle weight increases.
  • Vehicle type
    Combination vehicles like semi-trailers are more likely to experience uneven hydroplaning caused by uneven weight distribution. An unloaded trailer will hydroplane sooner than the cab pulling it. Pickups towing RVs present similar problems.

There is no precise equation to determine the speed at which a vehicle will hydroplane. Existing efforts have derived rules of thumb from empirical testing.[5] In general, cars hydroplane at speeds above 45 MPH (72 km/h), where water ponds to a depth of at least 1/10 of an inch (2,5 mm) over a distance of 30 feet (9 meters) or more.

Motorcycles

Motorcycles benefit from narrow tires with round, canoe-shaped contact patches. Narrow tires are less vulnerable to hydroplaning because vehicle weight is distributed over a smaller area, and rounded tires more easily push water aside. These advantages diminish on lighter motorcycles with naturally wide tires, like those in the supersport class. Further, wet conditions reduce the lateral force that any tire can accommodate before sliding. While a slide in a four-wheeled vehicle may be corrected, the same slide on a motorcycle will generally cause the rider to fall. Thus, despite the relative lack of hydroplaning danger in wet conditions, motorcycle riders must be even more cautious because overall traction is reduced by wet roadways.

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 have no effect.

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 tires 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 reduced speeds from those judged suitably moderate in the dry will mitigate the risk of hydroplaning. Avoidance of standing water is another effective prevention strategy.

Electronic stability control systems 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.

Cruise Control - Must Dos In cruise control, the intelligent intervention normally done by an expert driver is not normally done; here, the feed (to the engine) is largely controlled by speed of the vehicle and hence the machine tries to compensate on the speed requirement with more feed to the engine(as the signalling system of cruise control is unaware of the possibilities of hydroplaning). The higher acceleration hence results in the increased slippage at this situation and can result in a precarious output.

Suggested is to turn off the cruise control in such wet / icy roads. Technologies are available in market and more are in development to ensure that an intelligent intervention is assured for a vehicle running on cruise control. These are mostly pre-fitted from the company of production.

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

Viscous

Viscous hydroplaning is due to the viscous properties of water. A thin film of fluid no more than 0.025 millimetres[6] 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.

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 0.25 millimetres deep.[6] 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 generally related to tire inflation pressure. Tests have shown that for tires with significant loads and enough water depth for the amount of tread so that the dynamic head pressure from the speed is applied to the whole contact patch, the minimum speed for dynamic hydroplaning (Vp)in knots is about 9 times the square root of the tire pressure in pounds per square inch (PSI).[6] For an aircraft tire pressure of 64 PSI, the calculated hydroplaning speed would be approximately 72 knots. This speed is for a rolling, non-slipping wheel; a locked wheel reduces the Vp to 7.7 times the square root of the pressure. Therefore, once a locked tire starts hydroplaning it will continue until the speed reduces by other means (air drag or reverse thrust).[6]

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 change the water film into a cushion of steam which keeps the tire off the runway. A side-effect of the heat is it causes the rubber in contact with the runway to revert to its original uncured state. Indications of an aircraft having experienced reverted rubber hydroplaning, are distinctive ‘steam-cleaned’ marks on the runway surface and a patch of reverted rubber on the tire.[6]

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).

Reducing risk

Any hydroplaning tire reduces both braking effectiveness and directional control.[6]

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.[clarification needed]

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[6] as well as slide downwind.[clarification needed]

See also

  • traction for effects similar to hydroplaning.

References

Inline
  1. ^ http://www.crashforensics.com/papers.cfm?PaperID=8
  2. ^ Glennon, John C. (2004). Roadway Safety and Tort Liability. Lawyers & Judges Publishing Company. p. 180. ISBN 193005694X. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  3. ^ http://www.atlantaclaims.org/files/Newsletters/2006/10October/10-06%20Claimscene.pdf
  4. ^ http://www.roadex.org/Publications/docs-RIII-EN/Health%20Issues%20-%20RIII.pdf
  5. ^ http://www.sago114.com/Inc/down_new.asp?TB_NAME=TB_MENU&idx=130&f_field=filename1&f_dir=TB_MENU&f_gubun=1
  6. ^ a b c d e f g "1/2009 G-XLAC G-BWDA G-EMBO Section 1" (pdf). Air Accidents Investigation Branch. 2009: 58, 59. 0.25 mm for worn tyres and 0.76 mm for new tyres {{cite journal}}: Cite journal requires |journal= (help)
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