Pilots out there, what would this mean?

Question

Hello,
I have been flying now with my stepfather in his Cirrus for some time therefore I have a relatively good understanding of flight, however I am not entirely sure about something on a jet aircraft.
If on a B737 or a B757 there was something on the EICAS that said it had a High n1 yet a low n2 and the EPR was increased, then what would this mean or would those circumstances be somewhat impossible.

RickH · Accepted Answer

Unless you start to assume really contorted and contrived instrument failures, then there is something wrong with the N2 sensor.  Why?

EPR is the real measure of the actual output power of the engine.  It stands for Engine Pressure Ratio, and is the ratio of the inlet air pressure to the outlet air pressure.

N1 is the measure of how fast the fan section of the engine is turning and is directly proportional to EPR as is N2, and on those airplanes with a third engine section, N3.

So if two sensors are showing an increase in power, and one is showing a decrease, then under normal circumstances, we would tend to believe the two that are in agreement.  We would normally continue to our destination, and make a logbook entry detailing the problem with the N2 sensor.  I'm assuming no other abnormal indications from the engine.  

If it was surging, vibrating, having compressor stalls, or any other abnormal indication, we would try to operate it at reduced power to keep it available.  As a last resort, we would shut it down.

BTW, EICAS in the simplest sense, is just a TV tube that shows all of the engine gauges, it won't say things like "High N1"; it simply shows round dial gauges for N1, N2, EPR, EGT, etc.  If a limit has been exceeded, the gauge will turn red until reset by maintenance.

Freedom Wins IV · Answer

Not really possible since his N1 is higher and EPR is increasing, meaning power is being applied at lower altitudes UNLESS at high altitude.

Pilots out there, what would this mean?

2 Answers