Originally Posted by
alf5071h
The 'problem' is how to detect blocked sensors, incorrect thrust, aircraft acceleration.
On newer FADEC equipped engines, the FADEC does cross checks of inlet pressure (P2) with aircraft total pressure to detect blocked inlet probes, and models the expected turbine exit pressure (P5) against the various engine parameters (mainly rotor speeds) to detect errors in the P5 measurement. If either of those cross-checks fail it will invalidate EPR - this should happen prior to 80 knots and result in a low speed abort.
That technology simply wasn't available on the JT8D which resulted in a few takeoff accidents (the most famous being the 737-200 that ended up in the Potomac.) Which is why it's been implemented on FADEC.
Several years ago, I got a couple of 'oh-god-thirty' calls from Boeing when a Singapore 747-400 experienced multiple RTO's due to EPR on one engine repeatedly being invalidated prior to 80 knots. They eventually found a partially plugged P5 probe (turbine exit pressure). It wasn't completely plugged, so during a normal engine ground run-up (which are done with a gradual accel) it read OK, but during TO power set, the partially plugged probe would add enough of a lag to the P5 measurement would fail the synthetic cross check and invalidate EPR.