Supposing this was an AC fitted with the 4th aux tank, can you tell me how the 4th indicator was displayed? I have never flown a 4 tank 73 but remember reading somewhere that the 3 indicator triangle became a square - If this is the case then neither intuitive or ergnomic in design, and real potential for error.
However if this was the 3 tank varient then down to 100kg IS a bit "running on fumes"
