Definition
A small toothed gear inside an airspeed indicator that is fixed to the shaft carrying the indicator needle. As the instrument's mechanism rotates a larger gear (the sector), its teeth turn the handstaff pinion, which in turn rotates the needle across the dial face.
Plain English
A tiny gear attached to the same shaft as the airspeed needle. When the gear turns, the needle turns with it.
Context Anchor
Seen in diagrams of the inside of a mechanical airspeed indicator, such as Figure 5-11 in the Instrument Flying Handbook.
Derivation
Handstaff' comes from the German Uhrmacher tradition meaning the staff (shaft) that carries the hand (needle) of an instrument. 'Pinion' comes from the French 'pignon,' meaning a small gear that meshes with a larger one. Together it means 'the small gear on the needle's shaft.'
Why Pilots Care
Pilots do not adjust this part in normal flying, but understanding it helps explain how a pressure change inside the airspeed indicator becomes movement of the airspeed needle.
Analogy
Think of the minute hand on a clock. Behind the dial, that hand is fixed to a small gear on a shaft, and a larger gear drives it. The handstaff pinion plays the same role for the airspeed needle.
Intuition Check
Do not read “handstaff” as a handle or tool held in the hand. In this instrument, the “hand” is the pointer, and the handstaff is the shaft that holds it.
Example Sentence 1
As the diaphragm expanded with increasing airspeed, the linkage rotated the sector, which drove the handstaff pinion and moved the needle clockwise around the dial.
Example Sentence 2
Wear on the handstaff pinion can cause the airspeed pointer to stick or jump during flight.