FAA Aircraft Dispatcher (ADX)Navigation and ATCMedium
A dispatcher is briefing a captain on ILS approach lighting/marker beacon indications. What does passage of the middle marker beacon signify during a CAT I ILS approach?
- AThe aircraft has crossed the runway threshold and should begin the flare
- BThe aircraft's GPS has acquired vertical guidance for the missed approach
- CThe aircraft is over the outer fix, roughly 4 to 7 miles from the runway threshold
- DThe aircraft is approximately at the decision height point, about 3,500 feet from the threshold
Show answer & explanationAnswer & explanation
Correct answer: D. The aircraft is approximately at the decision height point, about 3,500 feet from the threshold
The middle marker (amber light, 1300 Hz tone in alternating dot-dash) is positioned approximately 3,500 feet from the runway threshold and normally corresponds to the decision height on a CAT I ILS approach, indicating the point where the pilot must have visual reference to continue.
Why the other options are wrong
- A. Threshold crossing has no dedicated marker beacon indication.
- B. Marker beacons provide no GPS or missed approach guidance function.
- C. That describes the outer marker, not the middle marker.
Middle Marker (MM)
An ILS marker beacon located about 3,500 feet from the runway threshold that approximates the CAT I decision height.
- Amber light, 1300 Hz tone, alternating dot-dash
- Located near DH on CAT I ILS
- Complements outer and inner markers along the approach course
Memory trick: 'Blue is far, Amber is near, White is where you flare'