Which markers are used in ILS for navigation and what are their typical positions?

• A. DME, VOR, TACAN markers.
• B. Outer marker, middle marker, inner marker along the approach path.
• C. Beacon, compass locator, inner marker.
• D. Glideslope, localizer, marker beacon.

Answer: (B)

Marker beacons are the navigational cues used specifically for ILS approaches, and the three that are normally installed are the outer marker, middle marker, and inner marker. These beacons lie along the final approach path and provide distance cues as you fly the approach.

The outer marker is placed several miles from the runway threshold—typically about 4 to 7 miles. It helps you confirm you are on the correct final approach path well before you get near the runway. The middle marker is located much closer to the runway, roughly half a mile to about a mile from the threshold (around 0.5–0.8 NM). It continues to confirm you’re on the glide path as you descend. The inner marker sits very close to the threshold, about 0.1 NM (approximately 600 feet) from touchdown, signaling you are near the decision point and final alignment with the runway.

These markers also provide distinct visual and audio cues (marker lights and tones) to help you identify exactly where you are on the approach. The other options mix in nav aids that aren’t the ILS marker beacons themselves (or omit the trio of markers), so they don’t accurately describe the standard ILS marker system.