Definition
Backup procedures specified by the aircraft manufacturer for lowering the landing gear when the primary (normal) extension system has failed. These methods bypass the failed component -- typically the hydraulic, electrical, or pneumatic actuating system -- and use mechanical means such as a hand crank, manual release handle, gravity free-fall, or a separate emergency hydraulic or pneumatic source to extend and lock the gear down.
Plain English
If the normal way of putting the wheels down doesn't work, the airplane has one or more backup ways to get them down. These backup methods are listed in the aircraft's flight manual and usually involve hand-cranking, pulling a release, or letting gravity drop the gear into place.
Context Anchor
Seen in landing gear malfunction procedures, abnormal or emergency checklists, and training for retractable-gear airplanes.
Derivation
Alternate comes from a Latin idea meaning “one after the other” or “another choice.” Extension comes from a Latin word meaning “to stretch out.” In this aviation use, extension means moving the landing gear out and down into its landing position, not stretching it.
Why Pilots Care
Allows safe extension of the landing gear during a system failure, avoiding a wheels-up landing that could damage the aircraft and endanger occupants.
Intuition Check
Alternate does not mean a casual choice between equal options here. It means the backup method used when the normal way of lowering the landing gear is not working or should not be used.
Example Sentence 1
After the gear handle was selected down and no green lights illuminated, the pilot referred to the POH and used the alternate method of gear extension to manually crank the gear down.
Example Sentence 2
Before landing, the instructor demonstrated the alternate methods of gear extension on the training aircraft.