To provide circling offset when approaching a runway at 90°, overfly the runway and time for 20 seconds (Category D) or 15 seconds (Category C) before turning downwind.

Eddie Sez:

If you plan to circle by approaching the landing runway with a 90° offset, finding the time to delay your turn is a simple matter of computing turn radius and speed. You can find the turn radius by taking the speed in nautical miles per minute, squaring it, and dividing the answer by nine or ten. More about that: Rules of Thumb / Turn Radius.

I first learned about this rule of thumb at the Air Force Instrument Instructor's Course (AFIIC) where we were told it is derived from the 60-to-1 concept. But is that true? Well let's find out. But first, we'll define the rule, provide an example of its application, and then we'll provide a mathematical proof of the rule.


Definition

To provide circling offset when approaching a runway at 90°, overfly the runway and time for 20 seconds (Category D) or 15 seconds (Category C) before turning downwind.


Example

Figure: Teterboro VOR/DME Circle to Runway 19, from Eddie's notes.

In the old days, when the weather was good and Teterboro was landing on Runway 19, the only approach choice was to circle from a course almost 90° to the landing runway.

In our GV we would fly this circling approach at 120 knots for a turn radius of 0.44 nm. (Explained below.) Running the math you end up with an offset time of 13 seconds. Our rule of thumb is for 15 seconds which ends up being perfect, given the turn to downwind is slightly more than 90°.


Proof

Many pilots use a 15 second offset when approaching a circling runway from a perpendicular heading. They start timing when overhead the runway, 15 seconds later they roll onto downwind. Sometimes it works, sometimes it doesn't. As is turns out, 15 seconds is a compromised number. The real number is slightly lower for an airplane circling at 120 knots, slightly higher for one at 150 knots. Does "slightly higher" or "slightly lower" matter? Perhaps not. But if your ground speed is affected by winds or pressure altitude, it would be nice to know how to adjust, eh?

Figure: Circling 90° to runway, from Eddie's notes.

You will need to time overhead the runway and then you need to turn to the correct downwind heading, adjusted for the winds.

r= ( nm min ) 2 /9

r= ( 150 60 ) 2 /9

r = 0.69 nm

t= d v = ( 0.69 150 )x3600 (sec/hr)

Time to delay downwind turn after crossing runway at typical Category D speeds:

t=17seconds

For typical Category C speeds, 120 knots, r = 0.44 nm, t = 13 seconds

Of course these times do not include the roll in and roll out of the turn, both of which increase the turn radius. Despite this, it may be a good technique to round up to the nearest 5 second increment.

To provide circling offset when approaching a runway at 90°, overfly the runway and time for 20 seconds (Category D) or 15 seconds (Category C) before turning downwind.


More About This:

For more about this rule of thumb, see: Instrument Procedures / Circling Approach.


Bottom Line

So what about the claims this rule of thumb is based on 60-to-1? My conclusion: No, it has more to do with trigonometry.

Rule of Thumb 60-to-1? Trigonometry π
Circling Approach 90° Offset    

60-to-1 — Rule of thumb is based on the mathematical relationship of a 360° circle and/or 6076' to 1 nm.

Trigonometry — Rule of thumb is based on the relationship to a right angle and the derived trigonometric functions.

π — Rule of thumb is based on the relationship of a 360° circle, the number π, and/or 6076' to 1 nm.