Taken from Tekkit Wikia:
Pressing "jump" twice: Causes the player to fly in the same manner as Creative Mode flight (double-click "jump" to fall, hold "jump" to ascend, hold "sneak" to descend). Right-click abilities are still available. If you open your inventory and remove the Swiftwolf's Rending Gale while flying, you will fall. Consumes 1 Redstone Dust every 6 seconds, 1 Coal every 12 seconds or 64 EMC from a Klein Star every 6 seconds. If you press "G" while flying you will shut off this ability.
Rendering Gale runs on fuel. So you have to keep a constant supply. Such as Redstone/Dust, Coal,Alchemical Coal, Mobius Fuel.