What is hydroplaning (aquaplaning) and how should a driver react if it occurs?

1. Tires lose contact with the wet road surface and ride on a film of water; ease off the accelerator gently without braking hard.
2. The engine has lost power after water entered the air intake; brake firmly and stop.
3. The brakes have overheated and faded; pump the brake pedal rapidly to restore them.
4. The power steering has failed; pull the handbrake immediately.