Definition
A confirmed condition in which the landing gear has been fully extended into its down position and the mechanical locks (downlocks) have engaged to hold each gear leg rigidly in place, ready to support the airplane's weight on landing. Confirmation is provided to the pilot through position indicators -- typically three green lights, one for each gear leg -- and on some airplanes by a mechanical indicator or audible signal.
Plain English
The wheels are not just lowered -- they are lowered and physically locked into place so they cannot fold up when the airplane touches down. The pilot verifies this by checking the cockpit indicators before landing.
Context Anchor
Seen in retractable-gear airplane checklists, cockpit gear position lights, and landing checks before touchdown.
Why Pilots Care
Confirms the airplane will not suffer a gear collapse on touchdown.
Intuition Check
Do not assume moving the gear handle down means the gear is down and locked. The phrase means the gear itself has fully extended and the lock indication confirms it is secure.
Example Sentence 1
On final approach, the pilot called out 'three green -- gear down and locked' before continuing to landing.
Example Sentence 2
Three green lights told the crew the gear was down and locked before they crossed the threshold.