Definition
The specific airspeed a helicopter pilot establishes and maintains during an autorotation to achieve the optimum balance between rate of descent and forward glide distance after an engine failure or loss of engine power. This speed is published in the rotorcraft flight manual for each helicopter type and is the speed at which the descending air through the rotor system keeps the rotor blades turning at a useful RPM, allowing a controlled descent and a successful landing flare.
Plain English
The airspeed the pilot flies at after the engine quits, so the rotor keeps spinning on its own and the helicopter glides down safely instead of dropping straight out of the sky.
Context Anchor
Seen in helicopter emergency procedures and autorotation training, especially when setting up the descent after reducing or losing engine power.
Derivation
Autorotation comes from the Greek 'auto' (self) and Latin 'rotare' (to turn). It literally means 'self-turning' -- referring to the rotor turning by itself from the airflow passing up through it, with no engine power driving it.
Why Pilots Care
Maintaining this speed ensures adequate rotor RPM for flare and touchdown; too fast or too slow can result in a hard landing or loss of rotor control.
Intuition Check
Autorotation speed does not mean how fast the rotor blades are spinning. It means the helicopter’s airspeed during autorotation; rotor speed is a separate thing the pilot also monitors.
Example Sentence 1
After the engine failed, the pilot lowered the collective and pitched for the published autorotation speed of 65 knots.
Example Sentence 2
The student practiced entering autorotation speed from a hover to simulate a low-altitude power loss.