In the case of an engine failure, a helicopter trades its potential energy (altitude) to kinetic energy by allowing the passing air (as it 'falls') to spin the

In the case of an engine failure, a helicopter trades its potential energy (altitude) to kinetic energy by allowing the passing air (as it 'falls') to spin the rotors. This is the helicopter's equivalent of gliding, more or less. The condition is called auto-rotation. Gyros are always in a state of auto-rotation, so if the engine quits they simply begin trading potential energy for kinetic energy, which they're really good at.