In the course of a recent meeting of the Performance Data Sheets Committee of the Royal Aeronautical Society to discuss thrust measurement in flight, it was pointed out that it is easy to apply the equations of motion incorrectly to the case of an aircraft which is losing mass due to fuel consumption. The rate of change of aircraft momentum is obviously affected by the loss of mass and so, at first sight, it may appear that the margin of thrust over drag required to achieve a given acceleration and /or rate of climb at any particular instant would be affected also. In fact this is not the case. To clarify this point, this account of the momentum and energy balances of an aircraft with an air-breathing engine has been prepared at the suggestion of the Performance Committee.