A new or overhauled engine's thrust on a manufacturer's test bed is measured against a load cell or thrust meter at given RPM, TGT (Turbine Gas Temperature),fuel flow and EPR (Engine Pressure Ratio).
When the certified engine is installed in the airframe and ground run, after taking into account local atmospheric conditions,intake and jet pipe losses, the maximum EPR, or thrust, and TGT and fuel flows can be cross checked.
In a test flight, given the correction factors for altitude and speed etc, against the known parameters of EPR, TGT, RPM and fuel flow, the engine can be seen to be giving required performance. If the the engine parameters are correct but aircraft performance (i.e. speed and rate of climb) are low then suspect the airplane's weight and/or drag (flap, doors, panels etc poor fit) as possible culprits. Its not fair to expect an engine to push a dirty heavy airplane up that big hill on a hot day is it?
