During an engine trim check, what pressure should be used for calculating thrust?

The calculation of thrust during an engine trim check is most accurately conducted using turbine discharge pressure, referred to as P7. This pressure reflects the conditions immediately downstream of the turbine, where the energy conversion is most relevant to the thrust produced by the engine.

At this stage, the pressure is fully representative of the thermal and kinetic energy converting into thrust. It takes into account the modifications in airflow and the specific engine design characteristics. Using turbine discharge pressure enables a clearer understanding of engine performance metrics under actual operating conditions.

While the other options like compressor discharge pressure, fuel pressure, and ambient pressure can offer some insights into engine operation, they do not provide the direct correlation with thrust generation necessary for a trim check. For instance, the compressor discharge pressure pertains to the state of air compressed before combustion, while fuel pressure relates to the fuel system and not directly to thrust output. Ambient pressure, on the other hand, is influential at different altitudes but does not reflect the immediate operational condition of the engine itself.

Thus, focusing on turbine discharge pressure ensures that the thrust calculations are grounded in the conditions that most accurately reflect engine output performance.