The plane has a glide-slope of about 4:1, which means with 1km of height it has a flight radius of about 4km. This rule only works to an altitude of about 10km, beyond this the atmosphere gets too thin. It should also be noted that it takes approximately 2 hours to ascend to 30km (100,000 ft), in this time the balloon may easily drift for more than 200 km (130 miles) so alternative landing site should be placed down wind at regular spacings (normally around 60km apart) to prevent early release.