Definition
A visual or instrument-based reference that shows the pilot whether the aircraft is on, above, or below the correct vertical descent path to the runway. The term covers both ground-based visual systems (such as VASI and PAPI lights beside the runway) and the cockpit instrument that displays the electronic glide slope signal from an Instrument Landing System.
Plain English
Something that tells the pilot if their descent toward the runway is at the right angle — too high, too low, or just right.
Context Anchor
Seen during instrument approach flying and on cockpit displays that provide vertical guidance to a runway.
Derivation
Glide refers to a steady descent with engine power reduced. Slope refers to the angle of that descent. Together, glide slope describes the angled path an aircraft follows down to the runway. Indicator simply means the device that shows it.
Why Pilots Care
Staying on the glide slope keeps the airplane at a safe height and touchdown point, especially when visibility is low.
Intuition Check
Do not read “glide” as meaning the engine is off. Here, the glide slope is simply the desired downward path to the runway, and the indicator shows whether the aircraft is on that path.
Example Sentence 1
On final approach, the glide slope indicator showed two red and two white lights, confirming the aircraft was on the correct descent path.
Example Sentence 2
When the GS indicator showed full deflection below, the pilot added power to regain the path.