Other Applications of the 1 in 60 Rule — Page 214, Lesson 196

Let’s pick up with the 1 in 60 rule and push it into two new applications: adjusting your rate of descent on approach, and using VOR/DME to find how far off track you are and even your range from a VOR. First, the descent problem. Example 3: you’re approaching London Heathrow on a 3° glide slope, and you decrease speed from 140 knots to 120 knots. The question is: what change in ROD — rate of descent — must you make to stay on the glide slope? The rule of thumb: decrease speed means decrease ROD. For a 3° glide slope only, we can use the 5 × rule. Change in ROD equals 5 × change in ground speed, and this applies only to 3° glide slopes. So 5 × 20 = 100 ft/min. You decrease ROD by 100 ft/min. That links back to Example 1 — if your original ROD was 600 ft/min, the new ROD becomes 600 minus 100 = 500 ft/min. Now Example 4: approaching London City airport, glide slope 5.5°, you reduce ground speed from 120 to 110 knots. Again, decrease speed means decrease ROD. First, for a 3° glide slope, change in ROD = 5 × ground speed change = 5 × 10 = 50 ft/min. But this is a 5.5° glide slope, so we scale it: change in ROD = 50 × 5.5/3 = 92 ft/min. So you decrease ROD by 92 ft/min. The key point: the 5 × rule is only valid for 3°; for any other angle, you scale proportionally. Now the VOR/DME problems. Here’s a typical exam example. You’re flying along an airway to VOR/DME ‘Q’. The airway QDM is 271°(M). Your Radio Magnetic Indicator shows your QDM to Q as 266°(M), range 48 NM. How far are you off the airway centre line, and to which side? We use the Track Error formula: Track Error = (Distance off × 60) / Distance gone. Modifying it: Angle off (TE) = (Distance off (DO) × 60) / DME range (DG). So 5° = (DO × 60) / 48. Rearranging: DO = (5 × 48) / 60 = 4 NM. So you’re 4 NM to the right of the centre line. Note: you’re off track but still within the airway, because most airways are 10 NM wide — 5 NM either side of centre line. Finally, finding range from change of VOR bearing. Suppose you’re tracking 090°(M) at 180 knots ground speed. At 1100 hrs, the QDM to the VOR is 002° (or QDR from it is 182°). Five minutes later, QDM has changed to 357° (or QDR 177°). What’s the range R from the VOR to the aircraft at the closest point of approach? The closest point of approach occurs when the aircraft is perpendicular to track — at a relative bearing of 270°, or when QDM is 000°. You can treat this as two separate right-angled triangles of 2° and 3° at the top, but it’s easier to do both sums at once and treat it as a near-right-angled triangle with a 5° angle at the top. The aircraft has a ground speed of 180 knots. Five minutes of flight gives a ground distance of 15 NM between the first and second bearing. So an angle of 5° subtends a distance of 15 NM along track. For each 60 NM of range R, the angle will subtend 5 NM of range. Therefore the range must be 3 × 60 = 180 NM. Alternatively, use the formula: Z = (opposite × 60) / Range R, where opposite is 15 NM along track. So 5 = (15 / R) × 60. Rearranging: R = (15 / 5) × 60 = 180 NM. So there you have it — the 1 in 60 rule applied to descent ROD changes and to VOR/DME crosstrack and range problems.