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

I want to walk you through some more applications of the 1 in 60 rule — this time in the context of glide slope management and VOR/DME navigation. Let’s start with Example 3. You’re approaching London Heathrow, which has a 3° glide slope. Your aircraft decreases speed from 140 knots to 120 knots. The question is: what change in rate of descent — ROD — must you make to stay on the glide slope? The principle is simple: decrease speed means decrease ROD. For a 3° glide slope only, we can use what’s called the 5 × rule. That rule says the change in ROD equals 5 times the change in ground speed — and this applies only to 3° glide slopes. So here, the change in ground speed is 20 knots — from 140 down to 120. Multiply that by 5, and you get 100 feet per minute. So you need to decrease your ROD by 100 feet per minute. That links back to an earlier example — it would give you a new ROD of 600 feet per minute. Now Example 4. This time you’re approaching London City airport, which has a 5.5° glide slope. You reduce ground speed from 120 knots to 110 knots — a change of 10 knots. Again, decrease speed means decrease ROD. First, for a 3° glide slope, the change in ROD would be 5 × 10 = 50 feet per minute. But because the glide slope here is 5.5°, not 3°, you need to scale that result. You multiply the 50 feet per minute by 5.5 divided by 3. That gives you approximately 92 feet per minute. So you decrease your ROD by 92 feet per minute to maintain the 5.5° glide slope. Now let’s move to VOR/DME problems. Here’s a typical exam example. You’re flying along an airway to a VOR/DME station called ‘Q’. The airway QDM — that’s the magnetic bearing from the station to you, or the track you should be following — is 271° magnetic. Your Radio Magnetic Indicator shows your actual QDM to Q as 266° magnetic, and the DME range is 48 nautical miles. The question: how far off the airway centre line are you, and to which side? We use the track error formula. Track error equals distance off times 60 divided by distance gone. But we modify it for this situation. The angle off — which is the track error — is the difference between your actual QDM and the airway QDM: 271 minus 266 gives 5°. The DME range is 48 nautical miles. So the formula becomes: 5° equals distance off times 60 divided by 48. Rearranging, distance off equals 5 times 48 divided by 60. That gives 4 nautical miles. And since your actual QDM is 266°, which is less than the airway QDM of 271°, you are to the right of the centre line. So you’re 4 nautical miles to the right. Note that you are off track but still within the airway — most airways are 10 nautical miles wide, meaning 5 nautical miles either side of the centre line. Next, we look at finding range from a change of VOR bearing. This uses the 1 in 60 rule to work out range from a VOR using the change in bearing over time. Suppose an aircraft is tracking 090° magnetic at a ground speed of 180 knots. At 1100 hours, the QDM to the VOR is 002° — or equivalently, the QDR from the VOR is 182°. Five minutes later, the QDM has changed to 357°, or the QDR is 177°. We want to find 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 the track — that is, at a relative bearing of 270°, or when the 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 nautical miles between the first bearing and the second. So an angle of 5° subtends a distance of 15 nautical miles along the track. For each 60 nautical miles of range R, the angle will subtend 5 nautical miles of range. Therefore, the range must be equal to 3 times 60, which is 180 nautical miles. Alternatively, you can use the formula. The opposite side — the 15 nautical miles along track — divided by the range R, multiplied by 60, equals the angle. So 5° equals 15 divided by R times 60. Rearranging, R equals 15 divided by 5 times 60, which gives 180 nautical miles. So in summary: for glide slope changes, use the 5 × rule for 3° slopes and scale for other angles. For VOR/DME off-track problems, use the track error formula with DME range. And for finding range from a changing VOR bearing, treat the bearing change as an angle subtending the distance flown along track, then solve for range using the 1 in 60 relationship.