When a percentage thrust has been determined for take off the auto thrust system (Airbus) or the auto throttle (Boeing) will send the data to the engines FADECs to provide the required thrust.

When the levers are set at FLEX (Airbus) or when the levers push button is pressed (Boeing) the engines will develop the takeoff thrust which is displayed as an N1 or as an EPR.

It seems the FADECs will command the same N1 (EPR) for all engines since the displayed N1 (EPR) are the same for both (all) engines. Am I right?

Considering the engines may have different age and therefore producing different thrusts for the same indicated N1 (EPR), a thrust assymetry will result. Is it true? How this is acceptable at take off?

Considering the engines total running hours (aging of the engine) are known by the operator, is there a way to enter such a data to the auto thrust or to the auto throttle or to the FMS in order to get at take off really equal thrusts and not just equal N1(EPR)?

  • 2
    $\begingroup$ N1 is not the same as EPR. N1 is RPM whereas EPR is Engine Pressure Ratio, calculated as Turbine Outlet Pressure / Compressor Inlet Pressure. EPR can be used as an accurate indicator of thrust regardless of the age of the engine (assuming the sensors work correctly), whereas N1 cannot. $\endgroup$ Jun 7, 2019 at 12:03
  • $\begingroup$ Thank you, so let us concentrate on N1 with respect to the question. Further if I understand well your answer it means both engines will deliver very similar thrust all the time, but from one day to another the power will depend on atmospheric conditions. Thank you. $\endgroup$
    – user40476
    Jun 7, 2019 at 12:59
  • $\begingroup$ N1 doesn't really say anything about thrust, just how fast the low pressure compressor wheel is turning. If two engines of the same model but different years of manufacture, cycles and hours are producing the same EPR under the same atmospheric conditions, the thrust produced is the same. What I don't know is if and how FADEC takes that into consideration. I would assume it is one of the sensors it is looking at, but don't know for sure at the moment. That is why I did not make my comment an answer. $\endgroup$ Jun 7, 2019 at 13:02
  • $\begingroup$ Considering a GE engine (N1) It would sound very strange to get difference of thrust at take off with engines of different age, the trouble is that the indicated N1s are similar at take off, as far as I know. $\endgroup$
    – user40476
    Jun 7, 2019 at 13:35

1 Answer 1


@JuanJimenez's comments are correct. N1 (and the accuracy of EPR) are not indicators of a precise thrust force.

So to answer the title question, generally no, moreover the acceleration of each engine may be different, and that is why the pilots go through a stabilization step, where they apply some thrust, make sure they're responding in a sufficiently similar manner, then they proceed with takeoff thrust. The pilots will correct any yaw with the rudder, and any remaining effect could be similar to a very light crosswind.

Also see: How is thrust of engines on the two sides of an airplane kept exactly equal to prevent yawing? The same phenomenon due to wear and tear is discussed. It is trivial for the most part since there is a rudder.

Of course this affects fuel consumption in-flight as there will be a tiny yaw as well that needs to be trimmed out.

But don't take it from me, check the Airbus article, Engine Thrust Management - Thrust Setting at Takeoff.

Every engine has its own performance level due to manufacturing tolerances. In addition, engine performance evolves with time due to wear and ageing. As a consequence, the acceleration profiles may slightly differ from one engine to another on an aircraft (fig.3), even if fitted with new engines.

Similarly, the idle thrust can slightly differ from one engine to the other, moving the acceleration profile to the left on the graph (fig.4).

Earlier I said generally no, because there are always exceptions (never say never on the internet). The articles notes that the Trent engines of the A330 and A380 automatically manage the stabilization step.

  • RE notice: On old aircraft before FADEC pilots were manually adjusting the thrusts to be equal (...)

It's because before FADEC the link between each lever and engine was a cable, and each cable depending on its age and environmental factors, acted differently – different stretching, friction, set torque, etc.

With FADEC, this is no longer a problem. The same electronic command is sent. And depending on the target, e.g., x N1, this is what the engines aim for, regardless of each engine's age and the fact that exact N1 values don't mean the same thrust force. Pilots pre-FADEC, like pilots with FADEC, don't know the thrust that is being applied in lbf/kN.

  • $\begingroup$ Please consider the basic meaning of the question as done by @JuanJimenez. Of course the question is not about the acceleration profile, but about the FADEC final commanded values, taking into account aging or not. $\endgroup$
    – user40476
    Jun 7, 2019 at 15:00
  • $\begingroup$ When I flew the CAE Cat 3 ERJ sims at the American Eagle flight academy in the late 90's the instructor told me Rolls-Royce engines on those birds require no stabilization pause when throttling up for takeoff. In fact, I did pause out of habit, and he specifically told me "Oh, that's not necessary on these, just go full throttle." $\endgroup$ Jun 7, 2019 at 15:16
  • $\begingroup$ @user40476: The acceleration profile was additional yet very relevant information. For the title question, as I wrote, the answer is no. They don't take the age into account. That's why I also wrote about a slight yaw after proceeding with the takeoff thrust (and in-flight). $\endgroup$
    – user14897
    Jun 7, 2019 at 15:29
  • $\begingroup$ Any reference about this answer. Ar far I am concerned I hope to get more answers, I find it very strange to get non symmetrical thrusts at take off with these modern aircrafts such as the 777 or the Airbus series having GE engines. Even though I have no explanations, if it was like that with N1 engines and not with EPR engines why would any operator choose a GE engine? Really I hope to get more precise answers? $\endgroup$
    – user40476
    Jun 7, 2019 at 16:16
  • $\begingroup$ @user40476: I have added a link to a related post, and I don't have a reference, it is there, it is trivial, and it also affects EPR engines. (Not as huge as you think.) And yes, hopefully more answers are posted, the more the merrier. $\endgroup$
    – user14897
    Jun 7, 2019 at 16:43

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