Why didn't they use the aircraft earlier?

At the end of Edge of Tomorrow, Cage and Rita convince the J squad to embark on the gunship to fly directly to Paris. Since they're down to their final "life", we know that it provided a fast and safe transportation to the final battle's theater.

So why did they insist on going by foot to the barrage? We see that this is by far the hardest way to do it, and in fact the entire plan seems senseless (they knew the entire beach is a trap to kill them, so why assume there will be cars available farther inland?). It seems like they could just as easily taken the ship (which doesn't even require a crew) and flew to the Alps.