Definition
In a turbojet or turbofan engine, the ratio of turbine discharge (exhaust) total pressure to compressor inlet total pressure. It is displayed on a cockpit gauge and used as the primary indication of thrust being produced by the engine.
Plain English
EPR compares the pressure of the air leaving the back of the engine to the pressure of the air entering the front. The bigger that gap, the more thrust the engine is producing. Pilots read it directly off a gauge to know how hard the engine is working.
Context Anchor
Seen on turbine-engine instrument panels and in power-setting procedures for some jet aircraft, especially during takeoff, climb, and cruise.
Derivation
Pressure ratio' simply means one pressure divided by another. The term is descriptive: it is literally the ratio of two pressures measured at two points in the engine — inlet and exhaust.
Why Pilots Care
Pilots use EPR to set thrust accurately and spot engine issues before they become serious.
Grounding Statement
On takeoff in a jet, the target EPR helps confirm that the engine is making the expected push before the takeoff continues.
Intuition Check
EPR is not a single pressure reading, and it is not how fast the engine is turning. It is a comparison of two pressures that helps show how much push the engine is making.
Example Sentence 1
During the takeoff roll, the captain advanced the throttles until the EPR gauges showed the target value from the performance chart.
Example Sentence 2
A sudden drop in EPR during climb prompted the crew to check for compressor stall.