Hello. Currently descending thru FL300 with throttles at idle in MACH hold. Surprised to see that EPR is still 1.1.

In a turbofan engine shouldn't the value be around 0.8-0.9 at idle since the engine are causing more drag than thrust ?

Hi,

No even at IDLE when stationary, a running engine can't give an EPR below 1.0 - which indicates that the total exhaust pressure precisely matches the ambient intake pressure. A flight idle of ~1.100 is pretty typical.

On 6/3/2026 at 8:51 AM, richboy2307 said:

Hi,

No even at IDLE when stationary, a running engine can't give an EPR below 1.0 - which indicates that the total exhaust pressure precisely matches the ambient intake pressure. A flight idle of ~1.100 is pretty typical.

Hello,

Unless the Tristar has a different design, the EPR normally compares exhaust and intake total pressure, not ambiant pressure, just like a pitot probe. That means that this intake total pressure will be higher, at speed.

You will observe values around 0.90 EPR during idle descent on modern turbofans such as the A320.

Edit: That does look like a typical total pressure probe on the Tristar's intake:

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Edited 19 hours ago19 hr by Genista

Yes the physical probe equipment on the engine may be the similar, and what they can physically measure will be equivalent but the computers that process this data before being shown to the flight crew are nowhere near the same.

As far as the Tristar is concerned, all the available resources show that it cannot and will not show any indications lower than 1.0. Even if the actual pressure maybe lower than that, it will remain at 1.0 indicated at the very lowest. That being said, descent FLT IDLE readings are typically around 1.05-1.15 range depending on atmospheric conditions so I don't see any cause for alarm.