you would want to plant the aircraft firmly on the ground.
This advice is in the Boeing manuals in order to encourage you
not to waste runway. The aircraft slows down much faster on the runway than floating above it - so aquaplaning conditions are not the time to stretch the hold-off attempting a "greaser". Having said that, as stated above, a soft or firm touchdown has no effect on any subsequent aquaplaning.
Speed in MPH = 9 X (the square root of the tyre pressure in PSI).
This formula is only relevant if the depth of the water (above the highest points of the surface) exceeds the depth of the tyre tread. You really only need be concerned for flooded runways - this is why 3mm water depth figures in the charts so much. It is worth noting your tread depth visually on the walkaround, if you are operating in wet conditions. It is also why you nay have landed on many wet runways, but never experienced the sudden loss of friction from aquaplaning.
This is also why grooved runways are so effective - water sitting in the grooves doesn't count, and braking on a wet grooved runway is considered by Boeing as equivalent to that on a dry runway. (As an aside, "chevron cuts" - the V-shaped cuts you see on tyres are caused by overly-heavy braking on a grooved runway.)
In 12,000 hours, I have only aquaplaned twice, both times in jets, both times landing in tropical downpours on flooded runways. The experience is much like landing a taildragger - in that, as above, you must constantly "fly" the aircraft by staying lively on the controls (ailerons into wind, rudder to keep straight) until the speed drops (which happens veeerrrryy slowly
) below the aquaplaning speed and the brakes become effective. The aircraft skitters about just like driving on ice at high speed
.
It is also important that your crosswind landing technique is good enough that you are certain you will touch down with the aircraft tracking down the runway centreline - any drift on touchdown on a flooded runway will probably see you off into the bushes.
