Originally Posted by
JediDude
Correct me If I'm wrong but surely a tug towing an aircraft and an aircraft powering itself are going to use exactly the same amount of energy over the same distance? Surely the emmisions saved from burning aviation fuel is going to be offset by the increased emmisions from the tugs?
No Jedi, but good question.
The energy to move the plane at a given speed over a given distance is, as you suggest, exactly the same whether the plane is pushed by elephants or conventionally propelled by its engines.
The additional, think of it as parasitic, energy required to make the energy available to propel the plane, will vary vastly depending on the power source. The total energy price is Energy to Move the Plane + Energy Wasted (mostly as heat) To Make Available the Energy to Move The Plane.
Airplane engines are designed to be efficient at cruise speeds and levels and are decidedly inefficient on the ground at low speed. I expect that self-powering a jet to the runway burns a lot more total fuel than towing one.
So if the tug burns less fuel, perhaps its emissions are less. Doncha think?