Better 'reception' while mid-air? PLOT-HOLE!

So it's explained that Samwise Gamgee's "time machine" device can't send anyone back more than 10 minutes... except when they're in an airplane, high above the earth - in clean view of hundreds of cell towers... right?

Then can someone please explain how our intrepid heroes are able to time travel back an entire day while CRASHING INTO A MOUNTAIN? ... I thought it only worked at high-altitude, in clear range of towers... and yet, Stuart activated it when they were literally 20-feet above the side of a mountain.

There were many, many things wrong with this otherwise interesting concept of a film, but this plot hole really stood out to me. Anyone else agree?

- Jon