Glidepath Rate of Descent
The rate of descent that holds a glidepath, from the ground speed: ROD = ground speed x 101.27 x tan(angle) ft/min, and the path itself is 6076.12 x tan(angle) ft/nm -- 318 ft/nm at a standard 3.00 degree ILS, the exact FAA TERPS figure that fixes the tangent form. At 120 kt on a 3-degree path that is 637 ft/min, which the 'ground speed x 5' rule rounds to 600. Because it scales with GROUND speed, a tailwind or a faster approach demands a higher descent rate to stay on the same path. A planning aid, not a clearance; the approach chart and the pilot in command govern.
Formula and source
rod_fpm = ground_speed_kt x 101.269 x tan(glidepath_angle); ft_per_nm = 6076.12 x tan(glidepath_angle). The 101.269 converts knots (nm/hr) to ft/min (x 6076.12 / 60).
Required rate of descent on a glidepath (FAA Instrument Flying Handbook; TERPS Order 8260.3), by name; the approach chart and the pilot in command govern.
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