However if you were ever questioned about logging your flight time and responded I just logged the VDO time, CASA would not give a damn. However in a commercial operation if you used Tacho you could find yourself having exceeded flight time limitations, I'm only stateing where the logging of flight time becomes a strict process that CASA will care about. As I stated earlier for private ops, who cares. The only issue being if you are logging it for experience purposes you will shaft yourself out of hours needlessly. Standard factors we'd apply in the old days is .2-.3 for tachos and .3-.4 for airswitch for say a session of circuits, reduced number for say navex. that's if you couldn't be arsed looking at your watch at blocks on/off. CASA seemed happy with that. Engineers had factors for converting Tacho use for airframe calcs and for reverse flight switch back to engine time with no tacho. The reason I went further to explain commercial reasoning is that inevitably someone will ask that question if you just focus on private ops. In any case the same rule applies for both as to how flight time is calculated, just one is more critical legally.
(And I still have no idea how logging tacho time as pilot flight time results in “wasting TBO hours” as suggested by India Four Two.)
Yeah that one I have no idea, in a private operation anyway, it would only waste TBO if you owned the aircraft and wanted to minimise cost per logged hour for experience, maybe that's the angle. For a hired aircraft you would just waste unnecessary hire time if you logged 20% less each flight for experience gaining.