This runway in San Antonio is laid in such a way that one way is 40 degrees south of east – therefore the number 13 and the reverse way is 40 degrees north of west – thus the number 31. So, the reverse of one direction has the exact reverse number. The only other possibility is if you write a runway direction as “02” instead of “2” (20 degrees east of north). The reverse direction will be actually 20 (20 degrees west of south).

But what is unique about the 13-31 combination is that the square of 13 is 169. And the square of 31 is 961. And they are reverse of each other!!

So one way and the reverse way of this runway has not only reverse numbers, their squares are also reverse numbers!!!

I agree, I really need to get a life 🙂