Seeing as the iPhone has an incredibly short battery life when in GPS mode and the battery can't be swapped for a spare how were you chaps thinking of charging the phone up whilst in flight?
Well I have a
Juicepack from Mophie and also one of their smaller
Juicepack Air.
A bit bigger than a replacement battery, but easier to change - and without powering down. Just slid it out of one and into the other.
Engadget review.