DOCTOR A300,
You are of course correct in saying that if the EPR system does not include the by-pass flow in its calculations, the resulting accuracy will deteriorate as the engine gets older, in much the same manner as N1 (as an indication of thrust) does. But more modern engines also measure the fan outlet pressures. In effect they take separate measurments of the the pressure rise in the hot stream and that in the by-pass stream. They then integrate the results to take account of the different mass flows. This sytem should (in theory at least) provide a much more accurate indication of thrust than can be provided by N1 alone.
The real problem with EPR systems is that blockage of the low pressure tapping at the compressor inlet can cause the system to over indicate the EPR. If the autothrottle or pilot are using EPR to set take-off thrust, this over-indication can have disasterous consequences, with insufficient thrust being avaialble to complete the take-off.
It is rather more difficult to imagine a situation where an N1 measuring system would give such an over-indication, but even the most inconceivable situations tend to occur eventually!
If however the problem is one of a sudden loss of fan/compressor efficiency, due to FOD for example, then an EPR system will be far more effective than an N1 system as an indication of thrust.
PITCAIRN,
I think that you are being a bit unfair in comparing a duplicated N1 system with a non-duplicated EPR system. What would be the effect if both the primary and secondary N1 systems went down?
A duplicated EPR system would case no more problems than an equivalent N1 system.