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Old 2nd Apr 2011, 08:45   #1 (permalink)

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

Thanks PPRuNe for having this spotters corner.
I see this question posed often: "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. In other words, when these % RPM numbers are being compared between each other and between engines what would prompt some action by the pilot? What causes these values to differ more or less across conditions? Or, what does it mean if the values differ between engines?

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Old 10th Apr 2011, 19:43   #2 (permalink)
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A brief explanation...

First the turbofan engine in the pic:

In this ex. we have a double spool front fan turbo-jet engine. As you probably allready know, the air is compressed backwards from the compressor stages into the compustion chamber where it is mixed with fuel and ignited, then forced backwards to drive the turbine stages which, because they are on the same shaft/spool, will drive the front compressor stages . Each spool has its own rotation speed. The one in green is the Low Pressure compressor/turbine (N1) and the one in purple is the High Pressure compressor/turbine (N2).

Engines are designed to work safe and proper for a number of cycles. If they are to do that, they mustn't get overstressed and pushed beyond the limits (N usually over 100% or EGT above a certain limit, at least not for a long period of time). The significant, limiting factors are:

Temperature inside the engine (which is proportional to EGT, what we are able to measure)
The rotation speed, RPM (N) of the spools.

Since airplanes operate in different atmosferic conditions at all times, engines are not able to produce the same amount of THRUST for a constant RPM (N) value. Static Air Temperature is the most significant factor for this. In a hot day, having to take off from a short runway, pilots may have to increase N speed in order to get the thrust needed for a safe take off. The opposite applies for a cold day on the same runway, pilots may then choose a lower N value in order to get the thrust required. Even when runway, atmosferic conditions and obstacle clearence are not a problem, pilots will still set the lowest N possible in order to spare the engines.

I hope this sorted out things a little bit!


Last edited by Jetpipe.; 10th Apr 2011 at 22:23.
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Old 11th Apr 2011, 02:02   #3 (permalink)
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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.
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Old 12th Apr 2011, 22:02   #4 (permalink)
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Just to muddy the waters further, you can also measure the power output (thrust is what we're after) by using EPR, the Engine Pressure Ratio. This is measured by comparing inlet and outlet pressure values over the whole engine. The higher the EPR value, the more oomph (technical term). On older types (eg Fokker70/10 with RR Tays) at take off we calculate, or lookup, an EPR value to get the required thrust, and type it into the thrust rating panel. On newer types (eg Airbus A320 family) we type in the required assumed outside air temperature, and this will calculate the EPR for us.

On a daily basis we only look at N1 and N2 during engine start. The starter motor is connected to the high pressure spool, and therefore we get a positive reading and increase in N2 first, before the fuel is introduced and light up occurs. After we have a positive reading of N2, N1 will come to life. If it doesn't, then we have a problem (fan frozen to the engine casing? Who did the walkround inspection properley). However N2 is the main rotation parameter we check against other happenings in the start sequence (fuel on, flame ingition, starter motor disengage etc etc).

If however there is a problem with the EPR sensing system we are able to change the engine operation to N1/N2 mode instead and set power requirements accordingly. The 737 crash into the bridge over the Potomac in Washington DC was a prime example of setting a correct EPR, but having incorrect power set due to pressure sensors being iced over. The N1 and N2 values were way to low for the thrust they thought they were getting.

Hope this helps.
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Old 12th Apr 2011, 23:13   #5 (permalink)
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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.

Last edited by Northbeach; 12th Apr 2011 at 23:46.
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engine, thrust

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