NavList:

I wrote:  E.g. if an airline or air force flew regularly between two places neither N/S or E/W of each other, e.g. UK to Cyprus, it might be worthwhile producing an Oblique Mercator between the two points, because this would retain as many of the six properties of the ideal chart as possible for this particular route.

I thought it wise to check on this before someone else did, and here it is.  http://webarchive.nationalarchives.gov.uk/+/http://www.aidu.mod.uk/enroute.html This document states that most RAF en-route charts were, at the time of the archive, Oblique Mercator projections.  They probably still are.  If you follow the link, you’ll see the meridians and parallels don’t go straight up and down and across the cut of the chart, but they do cross at right angles, so the chart is still conformal.  DaveP