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Each box of lights is equipped with an optical apparatus that horizontally splits light output into two phases: red and white. Depending on the viewing (and thus the descent) angle, a pilot will see either the red or the white portion.

A normal glidepath is indicated by an even split in red and white lights. If an airplane is beneath the glidepath, more red lights visible to the pilots; if an airplane is above the glidepath, more white lights are visible.

During aircraft descent, this system may be activated by the pilot by keying the airplane microphone with the airplane's communication radio tuned to the CTAF .

Sources

• An FAA Aircraft Accident Report of Federal Express Flight 1478