Aquaplaning

Description

Aquaplaning also known as hydroplaning, is a condition in which standing water causes the moving wheel of an aircraft to lose contact with the surface on which it is load bearing with the result that braking action on the wheel is not effective in reducing the ground speed of the aircraft.
The continued incidence of aquaplaning reduces the braking co-efficient to that of an icy or "slippery" runway - less than 20% of that on a equivalent dry runway.



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Causal Factors

A layer of water builds up beneath the tyre in increasing resistance to displacement by the pressure of the wheel. Eventually, this results in the formation of a wedge between the runway and the tyre. This resistance to water displacement has a vertical component which progressively lifts the tyre and reduces the area in contact with the runway until the aircraft is completely water-borne. In this condition, the tyre is no longer capable of providing directional control or effective braking because the drag forces are so low.

Types of Aquaplaning

Dynamic aquaplaning is that which does not begin unless the tyre-pressure moderated ground speed as determined above is exceeded. It leaves no physical evidence on tyre or runway surface.

Viscous aquaplaning arises in the same way as dynamic aquaplaning, but only on abnormally smooth surfaces such as touchdown zones contaminated with excessive rubber deposits where it may begin and continue at any ground speed. Typically, a small amount of water may mix with a surface contaminant. It too leaves no physical evidence on tyre or runway surface.

Reverted rubber aquaplaning occurs when the heat of friction from a locked wheel in contact with the surface causes the reversion of the rubber to its un-cured state and 'boils' the surface moisture into steam. The pressure of the steam raises the centre of the tyre off the surface whilst the edges remain in contact, forming a seal which temporarily traps the steam. The tyre will show clear evidence of rubber reversion and the runway surface will be clearly marked with the path of the wheels as a result of ‘steam pressure cleaning’ beneath the tyre. This is the only type of aquaplaning which leaves physical evidence on the runway surface. It was much more common before anti skid units became widespread and usually only occurs to aircraft so fitted if an emergency brake, which is applied directly rather than through the anti skid units, is used.