Aircraft Turbine Engines: N1 and N2 Indicators and Measurements

While many pilots may be well versed with the various power settings and power principles of piston engines, some may not be familiar with turbine engine power. In general, turbine engines express power in terms of N1 and N2, and power settings relate to the speed limits of engine rotation. This is due to the difference in operations between both engine types, turbine engines producing thrust for flight rather than power. To help you better understand the functionality of turbine engines, we will discuss what N1 and N2 measurements mean.

While various turbine engines may differ in their operations from one another to a degree, most follow the basic steps of air intake, air compression, fuel and air ignition within the combustor, turbine powering, and exhaust of spent gases out of the nozzle. With these standard steps, turbine engines can produce the thrust they need to keep an aircraft steady in the sky during flight operations. The compressor and turbine will both feature a high and low-pressure section, and the two turbine sections will rotate independently from one another. In many of these configurations, a two-spool design is present.

When discussing N1 in regards to a turbine engine, N1 refers to the rotational speed of the low-pressure turbine section and the compressor spool. Typically, such values are represented as a percentage of the spool’s maximum standard operating RPM. N2, meanwhile, is the measurement of the high-pressure sections which is similarly recorded as a percentage of the spool’s maximum standard operating RPM. As the two engine spools operate separately from one another, N1 and N2 speeds will regularly differ, their gap widening as power settings are lowered.

N1 and N2 values are quite important for flight, each being referenced during different phases of an operation. For example, the N1 power setting speed is relied on during procedures such as takeoff, cruise, and the landing approach. N2 power setting speed, meanwhile, serves to assist pilots in providing ample power to the bleed air systems, fuel pumps, generators, hydraulic pumps, and other various apparatuses that are engine driven.

In order to monitor N1 and N2 settings and speeds, indicators are often present within the cockpit in an area visible and accessible to the pilot. The N1 indicator may either be a standalone device that is either analogue or digital, or may serve as a part of an Engine Indicating and Crew Alerting System (EICAS) or Electronic Centralized Aircraft Monitor (ECAM) system. N2 indicators are similar in their placement, also featuring the option of being standalone or as a part of a larger system. In the instance that an engine features a three-spool design, the N2 indicator may be used as an indication of the intermediate pressure spool rather than monitoring the high pressure spool. With the use of both gauges, pilots can better determine the functionality of their turbine engine to carry out various flight procedures safely and efficiently.

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