If the brakes are on and the plane doesn't move it's like having thrusters bolted to the earth: you push some air in one direction and what the thrusters bolted to (the earth) must go in the other direction - there's the law of conservation of momentum and you don't get any momentum out of the fuel, just energy. If you look at the situation from a frame of reference where you spin with the earth (so the earth seems to be at rest at first) it's simple to see where the energy's going to: part of it makes the air go faster and the other part makes the earth start to spin. From a point of reference where the earth is already spinning at the start it's be a bit less obvious: when your plane points west you actually slow down the rotation of the earth, so it afterwards has less energy, thus that bit and the energy from the fuel is all gone into speeding up the air while, when you point the plane east, the rotational speed of the earth will increase while the rotation of the atmosphere will be slowed, i.e. part of the energy of the air plus the energy of the fuel will all go into the earth. And, finally, from a frame of reference where the rotation of the earth doesn't change (i.e. one that's gets accelerated with the earth, which is the one you're probably thinking in when you sit in the cockpit) it looks as if the energy from the fuel has all gone into just accelerating air. So the answer to where the energy goes depends on the frame of reference you use to look at it - but there are the proper transformations in physics for going from on frame to the others. But because of the conservation of momentum thing take good care to make as many starts going east as you do going west or you may stop the earth from turning - or speed it up that much that centrifugal forces outweight gravity and you will lose your job because everyone then can fly without a plane;-)