Hi quarkhopper,
To answer your questions:
"What does N1, N2 mean?" (example
in this thread) Well, I know what these mean but I'm not clear on what these values mean when taken together by a pilot.
A pilot is only interested in N1 as a general rule. The only time they would be interested in N2 is during the start procedure or if they had an abnormal indication of N2 during normal operation. Pilots do not need to compare N1 versus N2 during operation. N2 means nothing to a pilot. Engineers however use the N2 value in many different diagnostic scenarios when troubleshooting or testing an engine.
In other words, when these % RPM numbers are being compared between each other and between engines what would prompt some action by the pilot?
As above pilots dont need to compare N1 versus N2. They are really only interested in N1 which is the direct indicator of engine power output. Unless N2 is reading an abnormally incorrect value the pilot is not concerned with it. The pilots are however interested in differing values of N1 between individual engines as this indicates different power outputs for the same atmospheric conditions. This will normally be picked up by the pilot in the form the autothrottle computer driving the throttles to different positions to achieve the same N1 setting between engines. The normal action by the pilot in this scenario is to report the 'throttle split' he sees visually in the cockpit to the engineer when he lands. Generally speaking a 'throttle split' of 1/2 handle knob or less is acceptable.
What causes these values to differ more or less across conditions?
The atmospheric temperature and pressure of the day is what causes these values to differ across conditions.
Or, what does it mean if the values differ between engines?
If the values differ between engines for the same atmospheric pressure and temperature it could indicate that the engine is out of trim or has some other fault that may or may not be able to be adjusted back to the normal operating parameters, or it could simply be an indicator of the overall health of the basic engine. That being said N1 should not vary between engines. As stated above N1 is the power output of the engine and if a pilot wants 90% N1 delivered (for example) the engine should give it to him but the N2 and EGT may be a lot higher if the engine is not as healthy as the other(s) bolted to the aircraft. This is normal and no cause for concern to the pilot unless the EGT starts to get abnormally high within the operating parameters.
These are very general answers in the hope they will clarify your queries and there will be scenarios that are different to what I have described however this should answer your questions adequately.