[ajuc]

Most helicopters can autogyro - that means they can land without working engine, using air flow to speed up the rotor, and then using the rotor to slow down the fall to relatively gentle landing. More or less like the seeds of some trees fly.

Same goes for gyrocopters, which are sth in between regular plane and helicopter (they have powered propeler to gain horizontal speed, and unpowered rotor that is speed up by autogyro effect that replaces the need for wings).

Of course in dense 3d traffic you may collide with sth before you can slow down the fall, so there will be a need for some system that sygnalizes everybody below you to quickly get out of way.