N1, N2 - Why would a pilot want to know?

[Northbeach]

Good questions all............

when these % RPM numbers are being compared between each other and between engines what would prompt some action by the pilot?

The % RPM indicates to the pilots the amount of power the engines are developing under the given conditions. Our aircraft are required to be able to successfully complete the takeoff, after a given point, should one of the engines fail. The way I know that the takeoff will be successful is to be assured that the operating engine is producing the required thrust designed for this contingency. Thrust is measured in N1. Once the engines develop required N1, calculated prior to takeoff and known to the flight crew, and I achieve a speed on the runway, then if an engine fails I will have the performance required to safely takeoff, climb to a determined altitude and return for a safe landing.

I have the ability to select varying amounts of thrust for takeoff. It is advantageous, from a maintenance, reliability and longevity perspective, to minimize the temperatures, speeds and pressures developed in the turbojet engines. Mechanical things last longer if they are not subjected to the maximum stress they were designed to endure all the time. If the runway length, temperature, altitude of the airport and total amount of weight combination I am trying to get airborne allow I may select a lower amount of thrust while still meeting the safety requirements to be able to fly (under certain circumstances) should one engine fail during the takeoff phase. The way I know how much thrust is being developed is to check N1.

What causes these values to differ more or less across conditions?

Different temperatures and local atmospheric pressure, whether or not engine anti-ice protection has been selected, and the pressurization and temperature demands required by the crew the will cause these RPM values to differ more or less across conditions. To lift a certain fixed weight off a runway will take a slightly different amount of power depending on whether or not anti-ice protection has been selected on and what the outside air temperature is. Each one of these variables will have an effect on the amount of power the engine is required to produce to achieve the thrust necessary to meet the performance parameters.

If force is mass multiplied by acceleration, and the mass (density) of the air changes (air temperature gets much hotter and the air density decreases) then to get the same amount of force (thrust) I must accelerate that less dense air at a greater speed. N1 rotation is going to be higher to move more of the less dense air to achieve the same amount of thrust.

but I'm not clear on what these values mean when taken together by a pilot

There are normal relationships between N1 & N2 that exist across all the operating spectrums: engine start, idling, break away thrust, takeoff, cruise, descent. A pilot will look at the indications to see whether the engine is healthy. Being vigilant of RPM is a means of monitoring engine integrity. Observing changes in N1 is also a means of verifying certain other events have taken place. For example turning on engine and airfoil anti-ice will place additional demands on the engine that will be indicated by a change in N1. Although this is not the primary means of verifying the action has taken place it is a valid secondary cross check.