A fully loaded 747 has a glide ratio of 15:1 meaning that it travels 15 feet horizontal for every foot of vertical drop. That means at 35,000 feet it could travel about 100 miles. The new 787 Dreamliner is around 20:1.
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Just found on a techie forum that in a 747 in clean configuration (ie flaps and gear up) you should aim for V2+80kts as your best glide speed.
The A380 has a glide ratio of about 15:1, which means that for every 15 kilometers (9 miles) it travels horizontally, it loses 1 kilometer (0.6 miles) of altitude. Therefore, if an A380 is cruising at 12 kilometers (7 miles) above sea level, it can glide for about 180 kilometers (110 miles) before reaching the ground.
Without engine thrust, a 747-200 has a glide ratio of roughly 15:1, meaning it can glide forward 15 kilometres for every kilometre it drops. The flight crew quickly determined that the aircraft was capable of gliding for 23 minutes and covering 91 nautical miles (169 km) from its flight level of 37,000 feet (11,000 m).
The lift-drag ratio for commercial airliners is about 16:1. The distance that a 747 could glide is a function of altitude. Gliding from cruise altitude of 30,000 ft, or about five miles would have a range of 80 miles.
With it's efficient wings the glide ratio of the B 777 should be around 15:1. Extrapolating, from a Flight Level of 390 gives us a theoretical gliding distance of about 110 Miles.