What happens to EPR during the initial lag phase when thrust is increased?

During the initial lag phase when thrust is increased, engine pressure ratio (EPR) initially decreases before it starts to increase. This occurs because, at the onset of thrust application, there may be a delay in the response of the engine parameters to changes in throttle settings. Specifically, when thrust demand is increased, the engine may not immediately produce the expected increase in exhaust gas temperature and pressure due to the inertia of the engine components and the air-fuel mixture adjustments.

As the engine accelerates and begins to produce more thrust, the EPR metric will start to reflect the increased performance as it measures the ratio of the pressure in the exhaust compared to the inlet pressure. Understanding this behavior is crucial for the correct assessment of engine performance during the critical phases of operation, particularly in commercial aviation where precise thrust management is essential for safety and efficiency.