Definition
On a turbojet or turbofan engine, the ratio of the turbine discharge (exhaust) total pressure to the compressor inlet total pressure. EPR is used as the primary indication of thrust being produced by the engine.
Plain English
A number on the engine instruments that compares the air pressure leaving the engine with the air pressure entering it. The bigger the difference, the more thrust the engine is making. Pilots use this number to set and check engine power.
Context Anchor
Seen in jet airplane engine instruments, performance settings, and procedures that call for a specific engine power setting.
Derivation
From the Latin 'pressura' (a pressing) and 'ratio' (a reckoning or proportion). A 'pressure ratio' is simply one pressure divided by another. Here it compares pressure at the back of the engine to pressure at the front, giving a single number that reflects how hard the engine is working.
Why Pilots Care
Pilots use EPR to set precise engine power levels that deliver consistent thrust regardless of altitude or temperature.
Analogy
It is like comparing the pressure before and after a pump to see how hard the pump is working. The important number is the comparison, not just one pressure by itself.
Intuition Check
EPR is not engine oil pressure, fuel pressure, or a direct thrust reading. It is a pressure comparison that the airplane uses as an engine power indication.
Example Sentence 1
During the takeoff roll, the captain confirmed both engines had stabilized at the target EPR before continuing the takeoff.
Example Sentence 2
Maintaining the recommended EPR during climb kept the engines operating at the most efficient power setting.