Pilots may use the _____ foot-per-mile rule to help schedule glideslope maintenance.

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A practical descent planning rule of thumb uses about 400 feet of altitude change for every mile of distance on the approach. This simple 400 ft per mile estimate helps pilots quickly check whether the aircraft’s altitude is tracking the expected glide path and, in a maintenance context, whether the glideslope equipment is staying within tolerance. While a true glide slope is about 3 degrees (roughly 318 feet per nautical mile), using 400 feet per mile gives an easy, margin-filled number that’s convenient for planning and spotting anomalies during maintenance checks. For example, at 5 miles from the runway, you’d expect about 2,000 feet of altitude to have been lost; a significantly different reading would flag a potential issue with the glideslope. The other numbers don’t align with this common planning rule and wouldn’t provide the same quick, practical cross-check.