Which substitution is permitted when an ILS component is inoperative?

Which substitution is permitted when an ILS component is inoperative?

• A. ADF or VOR bearings which cross either the outer or middle marker sites may be substituted for these markers.
• B. DME, when located at the localizer antenna site, should be substituted for the outer or middle marker.
• C. A compass locator or precision radar may be substituted for the ILS outer or middle marker.
• D. LOC minimums should be substituted for ILS minimums whenever the glide slope becomes inoperative.

Answer: (C)

The key idea is what you can substitute for a failed ILS marker to preserve guidance on the approach. If the outer or middle marker is inoperative, you are allowed to substitute a compass locator or, when available, precision radar to provide the necessary positional cue along the final approach segment. The compass locator delivers a marker-like fix at the same general location as the marker beacon and helps you identify where you are relative to the runway threshold, keeping the approach properly sequenced and your decision altitude appropriately informed. A precision radar can also supply a comparable fix or range cue. The other options don’t provide an approved, equivalent cue for the OM or MM: ADF/VOR bearings don’t replicate the precise distance/fix information those markers supply, DME at the localizer site isn’t an approved substitute for OM/MM, and switching to localizer-only minimums is a different procedure, not a substitution for the marker beacons themselves.

The key idea is what you can substitute for a failed ILS marker to preserve guidance on the approach. If the outer or middle marker is inoperative, you are allowed to substitute a compass locator or, when available, precision radar to provide the necessary positional cue along the final approach segment. The compass locator delivers a marker-like fix at the same general location as the marker beacon and helps you identify where you are relative to the runway threshold, keeping the approach properly sequenced and your decision altitude appropriately informed. A precision radar can also supply a comparable fix or range cue. The other options don’t provide an approved, equivalent cue for the OM or MM: ADF/VOR bearings don’t replicate the precise distance/fix information those markers supply, DME at the localizer site isn’t an approved substitute for OM/MM, and switching to localizer-only minimums is a different procedure, not a substitution for the marker beacons themselves.