1. This was a simple slide rule enabling a pilot to calculate speed, distance fuel. I consisted of a fixed outer disk for miles or gallons and a rotating inner disk for minutes, hours or kilometres.

2. APR stands for Aero Products Research.

3. On the back of the card on which the key ring was mounted there were advertisements for other APR flight computers. On the back of the rule itself there is a summary of the items to be included in a formal flight plan.

4.Interestingly there is a gauge point for pi. Whilst this does not often enter in pilot's calculations it suggests that the designers also considered the rule might have other applications.
 

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