The 1 in 60 rule is based on similar right-angled triangles, but don't let that put you off!
Basically, imagine a long skinny right-angled triangle with the two sides at right angles being 60 units and 1 unit long respectively.
It so happens that the angle formed between the long side and the hypotenuse is 1 degree.
Now make the short side 2 units long; the angle becomes 2 degrees, and so on. This works for smaller angles, say less than about 15 degrees or so.
The similar triangles part goes like this: Using a triangle with long side 60 units and short side 4 units, the angle as described above would be 4 degrees. Say your long side was then 30 units and the short side 2 units, that would still be a 4 degree angle, and so on for all the other similar triangles – long 120, short 8 etc.
In flying terms, it means that if you have drifted 1 mile across track for 60 along, your track angle error is 1 degree. You would use it like this:
Say you departed on track overhead an airfield and flew your planned heading (let’s say 090). If the wind was exactly as forecast, you should stay right on track.
You take a pinpoint after 40 miles along track and find you’re 2 miles left of track.
2 in 40 is the same as 3 in 60 (just using the same proportion) or 3 degrees, so you have actually made good a track 3 degrees to the left of your planned track.
To parallel your planned track, you would turn right 3 degrees onto 093.
However, you’re still 2 miles left of track. To get back to track, you use the 1 in 60 rule again.
From the ‘parallel track heading’, you want to change heading further right to go back to track, and it depends on how quickly you want to get back as to how big a change you make.
To get back on in 60 miles, 2 in 60 is 2 degrees, so turn right a further 2 deg onto 095.
To get back on in 30 miles, 2 in 30 is 4 in 60, so turn 4 degrees further, onto 097.
You can pick any distance you like – if there was a good feature on track in 37 miles, I’d say something like this to myself – 37 is very close to 40. 2 in 40 equals 3 in 60, so I’ll turn right 3 degrees more from the parallel track heading, onto 096.
That’s basically it – do a bunch of practice ones of these on paper using different distances flown, miles off track, and distance to go to get back on track. Very useful once you get the hang of it; happy mental DR-ing.
PS Gaunty, 90 kts is about my speed these days, not that I ever went that quick previously – only ever had a joyride in those fast pointy things...