Definition
The act of lowering the landing gear from its retracted (stowed) position into the down-and-locked position in preparation for landing. In retractable-gear airplanes, gear extension is normally accomplished by selecting gear down on the gear handle, which commands the hydraulic, electric, or mechanical system to drive the wheels out of their wells, swing them into position, and engage the down-locks that hold each leg rigid for landing.
Plain English
Putting the wheels down. The pilot moves the gear handle to the down position and the airplane lowers its wheels and locks them in place so it can land on them.
Context Anchor
Seen during before-landing checks, approach, and landing gear malfunction procedures.
Derivation
"Extend" comes from the Latin extendere, meaning to stretch out. The gear is being stretched out away from the airplane's belly into the airflow, the opposite of retraction (drawn back in).
Why Pilots Care
Failure to achieve proper gear extension can force an emergency gear-up landing, risking aircraft damage and injury.
Intuition Check
Do not read gear extension as simply “moving the gear handle.” In this context, it means the landing gear has actually moved to the landing position and is confirmed ready for landing.
Example Sentence 1
On downwind, abeam the touchdown point, the pilot called for gear extension and confirmed three green lights before turning base.
Example Sentence 2
When the normal system failed, the pilot performed manual gear extension using the emergency hand pump.