LowNSlow -
Ditto to Nishko, except I'd go further and say that my Pilot III chews up batteries 'a lot'! Quoted time per the manual is "up to eight hours" without use of the backlighting. I reckon "up to six hours" using 4 x AA Duracells would be closer to the mark.
Consider buying the cigarette lighter adapter, if your plane has the required fag lighter installed.