I still dont understand this tacho charging? Does it mean that if i am taxiying with very little engine power, and when i am 'warming' up the engine, i am actually not paying for aircraft hire?
No it doesn't mean that. However a VDO will tick over 0.1hr every 6 mins all the time the engine is running. The Tacho on the other hand, when idling and warming the engine, may take 10mins to tick over 0.1hr. At cruise power it should take 6mins to tick over 0.1hr. At full power it might take 5mins to tick over 0.1hrs. I plucked the figures out of the air (minutes wise) but hopefully you get the idea.