Definition
The angle, measured upward from the horizontal, at which the ILS glideslope signal is projected from the transmitter near the runway touchdown zone, defining the descent path an aircraft follows on a precision approach. The standard projection angle is approximately 3 degrees, though some installations use a steeper angle where terrain or obstacles require it.
Plain English
The slope of the invisible beam the glideslope sends out from beside the runway. It is tilted upward at about 3 degrees, and an aircraft riding that beam down to the runway is descending along that same slope.
Context Anchor
Seen in instrument landing system approach discussions and on approach information when checking the glideslope for a runway.
Derivation
Projection here means 'thrown forward' from Latin proicere -- to cast or throw out. The glideslope transmitter projects (throws out) a radio beam tilted upward into the approach airspace, and the angle of that throw is the projection angle.
Why Pilots Care
The angle determines the height at which the aircraft crosses the runway threshold and must remain within tight tolerances for safe landings.
Grounding Statement
Picture a straight, invisible ramp leading down to the runway; the GS projection angle is the slope of that ramp.
Intuition Check
Do not confuse the GS projection angle with the airplane’s pitch angle. It is the angle of the electronic glide path, not the angle of the aircraft’s nose.
Example Sentence 1
The approach chart showed a GS projection angle of 3.00 degrees, giving us a standard descent profile to the runway.
Example Sentence 2
A change in GS projection angle requires a new obstacle clearance evaluation for the approach.