Chaps ...
I was taught Mental DR ... the point I'm making is that to work through a full Nav Flight Planing exercise, when beginning ones flying traing, provides skills that are "foundational" for the future ... giving you the confidence to challenge instrumentation ... clearly if the ground/on-board kit goes AWOL you have at least the ability to do it from scratch ?
I'm not suggesting one whips out the Dalton while doing 480Kts at 500' ... not even those dinky little Daltons issued the the Harrier boys
