Definition
A small opening in the underside of the pitot tube that allows moisture, condensation, and small debris to escape, preventing water from collecting inside the tube and disturbing the ram-air pressure used by the airspeed indicator.
Plain English
A tiny hole on the bottom of the pitot tube that lets water drip out so it does not block the airflow the airspeed indicator depends on.
Context Anchor
Seen in pitot-system blockage discussions and during preflight inspection of the pitot tube.
Why Pilots Care
If the drain hole becomes blocked along with the pitot inlet, trapped pressure can produce constant or erroneous airspeed indications during climb or descent.
Intuition Check
Do not think of a drain hole as just any random opening. In this context, it is a designed opening in the airspeed pressure system that lets moisture escape without being the main air inlet.
Example Sentence 1
During preflight, the pilot checked that both the pitot inlet and the drain hole were clear of obstructions.
Example Sentence 2
Ice blocking both the inlet and the drain hole caused the airspeed indicator to remain fixed during the climb.