They knew exactly how much fuel they had.
They should do, its easy to count to zero. But are you suggesting that knowing you've run out of fuel makes it all OK? The aim must still be to land on Mum wherever she may be, not monitor your fuel burn rate, right up until there is none left.
Lets face it, most Pussers grey funnel line steamers are not exactly 100% reliable when it comes to land on position
And there is more to this than the "T42 doesn't know where it is" argument.
Totalwar you have a very hyd3failure way of putting things don't you? Doh, another rebrief in bound on Monday
