Definition
An autopilot mode that captures and tracks the glideslope signal of an ILS, automatically commanding pitch changes to keep the aircraft on the correct vertical descent path to the runway.
Plain English
A setting on the autopilot that flies the aircraft down the correct descent angle toward the runway during an instrument approach, so the pilot does not have to control pitch by hand.
Context Anchor
Seen during instrument approaches when the autopilot or flight director is set up to follow vertical guidance to the runway.
Derivation
G/S is shorthand for 'glideslope' -- the slanted radio beam that defines the proper descent path to a runway. 'Mode' here means a selectable behavior of the autopilot. So 'G/S mode' literally means 'the autopilot setting that follows the glideslope.'
Why Pilots Care
It supplies precise vertical guidance that enables safe landings in low visibility while lowering pilot workload.
Intuition Check
Do not assume G/S mode controls the whole approach. It controls vertical tracking of the glide slope; the pilot must still confirm the airplane is properly set up and following the correct course.
Example Sentence 1
After intercepting the localizer, the pilot verified that G/S mode armed and then captured as the aircraft reached the glideslope.
Example Sentence 2
The autopilot held the aircraft on the glide path once G/S mode was armed and captured.