N1 is measured in mils (thousandth of an inch) and N2 is measured in IPS (inches per second). Why are different methods used?

  • $\begingroup$ Related: What does VIB stand for in EICAS? $\endgroup$
    – Bianfable
    Sep 18, 2021 at 12:44
  • 5
    $\begingroup$ Can you provide a source for your statement? $\endgroup$
    – Afe
    Sep 18, 2021 at 16:23
  • $\begingroup$ Sorry I should've made myself clear. Yes, the vibration sensor is located in No. 1 bearing and turbine rear frame. From CFM56-3 Engine Shop Manual in Engine Operating Limits section, the vibration level limit for N1 is in mils and for N2 is in ips. $\endgroup$ Sep 19, 2021 at 2:26
  • 1
    $\begingroup$ Both seem very odd units for vibration, as it is oscillation that by definition has frequency and amplitude. Neither alone tells enough about the nature and more importantly, the energy of the vibration. $\endgroup$
    – Jpe61
    Sep 20, 2021 at 15:04
  • 2
    $\begingroup$ @Jpe61 both are measured synchronously. i.e. at whatever frequency the corresponding rotor is spinning. $\endgroup$
    – Daniel K
    Sep 20, 2021 at 16:00

2 Answers 2


So first, both measurements are made by accelerometers, and accelerometers are by definition sensitive to acceleration. So the original units are either G or m/s^2. They are converted to different units for analysis and display.

So, what units are most convenient for analysis? It depends on the frequency range. If we have a sinusoidal vibration with displacement amplitude A and frequency $\omega$, then the corresponding velocity will be $A \omega$, and the corresponding acceleration will be $A \omega^2$. So for very low frequencies, i.e. $\omega$ very small, so that acceleration and velocity will both be small. Thus of the three, displacement will be the largest, and most convenient to work with. For very high frequencies, i.e. $\omega$ very large, then acceleration i.e. $A \omega^2$ will be the largest, thus more convenient to work with. For intermediate frequencies, velocity is the largest and thus most convenient.

N1 happens to be a fairly low frequency, due to the slow speed of the fan rotation, so displacement is good. N2 is a medium frequency due to the faster (not but ultra fast, relatively speaking) core rotation.


Because the first sensor is measuring vibration levels on a bearing inside the engine (the #1 bearing) and the second sensor is measuring vibration at the turbine rear frame. The tolerances for vibration on those two are very different. A bearing's tolerance for vibration is extremely small, which is why it is measured in mils. The rear frame's tolerance for vibration is much larger in comparison, which is why it is measured in inches per second.

  • 2
    $\begingroup$ This is incorrect. Although the sensor is mounted on the bearing, it is not primarily measuring vibration generated by the bearing itself, it is measuring vibration generated by the fan rotor. The best place to measure rotor vibration is as close to the rotor as you can get, namely the bearing. Similarly, the sensor on the rear frame is measuring vibration generated by the core rotor (through a stiff load path down to the bearing). It has nothing to do with the "tolerance" of the part for vibration. $\endgroup$
    – Daniel K
    Sep 20, 2021 at 15:58
  • $\begingroup$ You are comparing what it is measuring vs the source of the force it is measuring. Apples and oranges. I also think you are well aware that bearings can fail. The way to tell if it is failing is to check for deviation from expected performance. If the bearing is failing, one of the primary symptoms is excessive vibration. The same for the rear frame. If you think the answer can be better worded, have at it and edit it. But the answer is correct. $\endgroup$ Sep 21, 2021 at 16:06
  • $\begingroup$ Yes, excessive vibration is a symptom of bearing failure. However, based on the comment "From CFM56-3 Engine Shop Manual in Engine Operating Limits section, the vibration level limit for N1 is in mils and for N2 is in ips" the OP is clearly asking about the synchronous fan 1/rev and core 1/rev vibration, which is very specifically related to rotor unbalance. The choice of vibration units for these quantities has nothing to do with the tolerance of the mounting location for vibration or bearing health monitoring. $\endgroup$
    – Daniel K
    Sep 21, 2021 at 22:41
  • $\begingroup$ Again, I said nothing about the mounting location. The OP did. $\endgroup$ Sep 22, 2021 at 23:04

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .