Definition
In a helicopter equipped with an automatic flight control system or autopilot, the collective servo is the powered actuator that moves the collective pitch control on behalf of the pilot. It receives commands from the flight control computer and physically drives the collective up or down to adjust main rotor blade pitch on all blades simultaneously, thereby controlling vertical movement, climb, descent, and engine power demand.
Plain English
It is the small motor-like device the autopilot uses to raise or lower the collective lever for the pilot. When the autopilot wants the helicopter to climb or descend, this is the piece that actually moves the control.
Context Anchor
Seen in helicopter instrument flying and emergency procedures when discussing hydraulic or flight control servo failures.
Derivation
Servo comes from the Latin servus, meaning servant. A servo is a device that 'serves' the pilot by moving a control on command. Collective refers to the collective pitch lever, named because moving it changes the pitch of all rotor blades collectively (together at the same time).
Why Pilots Care
A failed collective servo can cause the autopilot to disconnect without warning or produce uncommanded pitch changes, requiring immediate manual control in instrument conditions.
Grounding Statement
In normal use, the pilot moves the collective lever and the servo supplies much of the force needed to change the rotor blade angle.
Intuition Check
Collective does not mean a group of pilots or people here. It means the control changes all main rotor blades together, rather than changing one blade differently from another.
Example Sentence 1
When the collective servo failed in cruise, the helicopter began a slow uncommanded descent until the pilot disengaged the autopilot and took manual control.
Example Sentence 2
After the collective servo failed, the pilot took the controls and continued the ILS by hand.