The thrust, noise and emission elements of the performance of a jet engine are of vital importance in the takeoff phase of operation of the aircraft. The thrust and fuel consumption elements, and their variation with altitude and airspeed, are of vital importance in the climb and cruise phases of operation of the aircraft.
The thrust from a jet engine is determined by the velocity of exhaust gases measured relative to the engine.
The velocity of the exhaust gas as it leaves a jet engine is <math>V_j;</math>:G. Van Wylen and R. Sonntag, Fundamentals of Classical Thermodynamics, Section 9.18
Alternatively, <math>V_j;</math> is related to the temperatures of the exhaust gas:
<math>V_j^2=2 C_ (T_4-T_5)</math><br />
Kerrebrock, Jack L. (1992), Aircraft Engines and Gas Turbines, The MIT Press, Cambridge, Massachusetts USA. ISBN 0 262 11162 4
G. Van Wylen and R. Sonntag (1985), Fundamentals of Classical Thermodynamics, 3rd edition, John Wiley & Sons, New York NY ISBN 0 471 829331 1