• Will an engine pressure ratio indicator (EPR) show the same at full mil thrust as with full reversed thrust?

  • What does the EPR indicate at afterburner active? Will it just show the same as for max mil thrust?


1 Answer 1


What is EPR?

[It] is the total pressure ratio across a jet engine, measured as the ratio of the total pressure at the exit of the propelling nozzle divided by the total pressure at the entry to the compressor.

Placing an EPR probe in the A/B (afterburner) tube would not be a wise choice, and therefore in an A/B aircraft you wouldn't find an EPR reading. What is more useful is the NP (nozzle position). As that is a good indication of A/B level. N1 is still used of course.

In the civil world, GE and CFM don't have EPR sensors/readouts, unlike P&W and RR.

Not having an EPR avoids issues like EPR probe icing. But the downside is that it's harder to compute engine FADEC parameters.

More info here: What is the difference between EPR and rotor speed as thrust setting parameter?

Below are the F-14 engine instruments (analog tapes and dials are also present in the cockpit).

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TBT is turbine blade temperature. EGT is also not used as it too would not survive the A/B stream.

EPR and Reverse

On the Boeing 747-400 with RR or P&W engine options, the EPR window is blanked when reverse is activated.

What the engine is redirecting is the bypass flow. So the EPR will still read as normal, which can be quite confusing to the crew especially in max reverse, hence the blanking.


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