From today's The Australian:
The risky business of landing on wet runways
By BYRON BAILEY
Last week in Perth Airport, as I waited to board a Qantas flight, I heard a passenger address announcement that Newman Airport was closed and that passengers should contact their company to make arrangements to travel the next day. Cyclone Blake had passed through the Newman area previously and I assumed that was the cause.
Later, I saw on television the picture of a QantasLink aircraft, due to a slight overrun on landing, sitting off the end of the sealed Newman runway with its nose gear apparently bogged in the non-sealed surface. It was raining heavily.
Heavy rain presents a visibility problem when landing at around 200km/h and it also presents a braking action problem when landing, due to surface water on the runway.
A wet runway is defined as water up to a depth of 8mm. Over that it is defined as contaminated and manufacturers cannot guarantee adequate stopping performance.
The problem is hydroplaning (or aquaplaning) where — above a critical speed — the wheel is lifted off the surface and rides on the film of water, causing the plane to skid. The general formula for working out hydroplaning speed is nine times the square root of the tyre pressure. However, the Federal Aviation Administration recently reworked the formula due to some overruns. The dynamic (aircraft taking off) wheel-rotating hydroplaning speed was lowered slightly, but in the static hydroplaning landing case of stationary wheels the FAA reduced the formula for hydroplaning onset speed to about 7.5 times the square root of the tyre pressure.
This means for a typical jet with a tyre pressure of around 200psi and a landing speed of around 130 knots the pilots should, after a firm landing to break through the surface water to ensure tyre contact and wheel spin up, use lift dump and thrust reversers as soon as possible and then, when below the hydroplaning onset speed of around 100 knots, apply one firm brake application and let the aircraft’s brake antiskid system do its job.
Car drivers as well need to be aware that in heavy rain with tyre pressures of around 35psi, in the interests of safety, speed should be reduced, especially with under-inflated tyres.
Also, once hydroplaning starts and the wheels lock up, the skidding due to hydroplaning can continue down to a much lower speed.
Byron Bailey is a former RAAF fighter jet pilot and flew B777s as an airline captain.
My bolding.