If the front engine EPR probes are blocked and the thrust lever is advanced to take-off thrust, what will the EPR gauge do and how will thrust be affected?

• A. Read Correctly; Thrust Normal
• B. Under-read; Thrust Higher
• C. Over-read; Thrust Lower
• D. Fluctuate; Thrust Unpredictable

Answer: (C)

The key idea here is that EPR is a pressure ratio based on two sensor readings: the pressure at the compressor inlet and the pressure at the engine exhaust. If the front EPR probes are blocked, the inlet pressure reading is distorted (typically underestimated because the sensor isn’t sampling the true inlet pressure). With the exhaust pressure being read normally, the calculated ratio ends up higher than the engine’s actual pressure ratio. In other words, the gauge over-reads.

Because the reading is falsely high, the crew (or automation) may set or expect more thrust than the engine can actually deliver. The real thrust produced will be lower than what the over-read EPR would imply, so thrust ends up being lower than the commanded takeoff thrust.