It’s so simple you say. Start the clock and you should know what your endurance is?
WRONG
I have got airborne in a helicopter with faulty fuel gauges. They SHOWED 1:30 fuel but were over-reading by 15 minutes worth. In other words the fuel was not there at the start. It got very tense when the other warnings came on.
The only time you can be confident of your contents is when the tank is brimmed full or empty.
If this aircraft had faulty gauges, it could have lifted with god only knows what in the tanks. The stopwatch call would be irrelevant
As I said above, it happened to me