Richard, John,
If you are taking the SD card home to do SW updates, why not just buy a second card, update it when necessary, take that to the airfield and swap it over in the PAW? Has the advantage that if the update wasn't successful for any reason, you can always revert to the previous software without having a wasted trip I.e. the PAW will still be useable if you wish to fly. If it goes OK, then the original card becomes the "spare" that you can use for the next update.
NB. In anticipation of receiving the new Bridge, I downloaded 20160511 update and installed it to two SD cards. One worked fine on my Pi B+ PAW. When I received the Bridge, connected it to a Pi 2B and installed the other SD card, it wouldn't boot up for some reason. Fortunately swapping it for the one in the Pi B+ worked and I was then able to enter the licence key etc.
If you have another PAW at home (Pi B+) use this to do the "auto update" using an Ethernet connection to your router. If not, reformat the card and update "manually" via your PC.
Regards,
Chris