How far out is the glideslope reliable?

• A. 10 nm
• B. 15 nm
• C. 5 nm
• D. 20 nm

Answer: (A)

Glideslope reliability depends on staying inside its designed service volume. The glideslope transmitter provides vertical guidance only within a specific coverage area, where the signal remains stable and accurate enough to trust for descent. If you go beyond that outer boundary, the signal tends to degrade, may drift, or even produce false readings, so it’s no longer reliable for vertical guidance.

So the best answer is the one that identifies that outer limit of usable reception. The other distances describe ranges that are either comfortably inside that coverage (still reliable but not the furthest point you can rely on) or outside it (where you shouldn’t trust the glideslope at all).