I have read the paper "Damage Propagation Modeling for Aircraft Engine Run-to-Failure Simulation" (Reference). I am doing analysis to find which sensors to monitor for the faults injected in the simulated datasets.

One such fault is high pressure compressor which caused the engines to be marked as failure.

My analysis gave me the following sensors:

  • Corrected fan speed (NRf in rpm),
  • Physical fan speed (Nf rpm), and
  • Engine pressure ratio (P50/P2).

I could not find the proper answers online. Could anyone be kind enough to ascertain how related are these sensors for the occurrence of HPC fault in a turbofan engine.

I'm not from the aviation background. I greatly appreciate any valid suggestions/references or answers.

  • $\begingroup$ Hi, what type of fault do you consider ? They can be of multiple types: stall, overspeed, underspeed, vibration levels ... $\endgroup$
    – BambOo
    Apr 15, 2018 at 18:09
  • $\begingroup$ @BambOo, In the document, it says that to simulate HPC degradation HPC flow and efficiency modifiers were used. The data outputs Response surface of HPC stall margin as a function of efficiency and flow losses simulating degradation in HPC module.Health index is determined by increasing EGT (decreasing EGT margin) as compared to HPC stall margin for all three degradation trajectories. Each degradation trajectory was simulated until the health index reached zero. Sensed margins (fan, HPC, LPC and EGT) were used to compute health index to determine simulation stopping criteria. $\endgroup$
    – Joe
    Apr 15, 2018 at 20:21

2 Answers 2


It sounds like you are looking for an understanding of how the HPC efficiency affects the overall engine performance. My suggestion here would be use NASA's excellent engine simulator: https://www.grc.nasa.gov/www/k-12/airplane/ngnsim.html One of the many variables that you can play around with is HPC efficiency. Note that as you decrease HPC efficiency, overall EPR is going to drop. In a real engine, the controller will notice this and dump in more fuel to compensate and get back to the original EPR. In this simulator, you will have to do that by adjusting the throttle controller.

Here's a screenshot of the engine simulator running. The one on the left has a high compressor efficiency and the one on the right has a low compressor efficiency (note how i've changed the throttles and the different fuel flow). Note how the temperatures on the right are all higher. This a symptom of a less efficient component. The engine has to work harder to generate the same thrust. There are a number of other outputs. You can also play around with other components, e.g. see how a degraded turbine efficiency looks versus a degraded compressor efficiency.

The only thing that this simulator does not have is rotor speed. For that, although I've not run out the calculations, I think that fan speed won't change too much with HPC efficiency, at least for a high bypass configuration. Like I said, the control is going to change fuel flow to get back to the same thrust (via EPR), and since the majority of the thrust comes from the fan, the fan speed will be about the same. The core speed, on the other hand, should go up. Because the core has to work harder to produce the same amount of energy output.

enter image description here


I read your reference a bit, and it seems that the goal was to model degradation without looking to deep in the underlying phenomena. So basically you are just interested in the efficieny of the HPC module.

The loss of performances and therefore the wear of the module can be assessed through different probes, but the main value is the pressure ratio between the inlet and the outlet of the HPC module.

This ratio will highly depend on multiple wear phenomena occuring inside the module, especially the wear of the structures due to rotor to stator rubs. Also, small damages due to fatigue failures may lead to pressure ratio losses.

  • $\begingroup$ Thank you for having a glance at the paper. I am trying to understand if there is any way the corrected fan speed, physical fan speed and energy pressure ratio effects HPC ? $\endgroup$
    – Joe
    Apr 15, 2018 at 22:40
  • $\begingroup$ @BambOo. Your answer is not wrong - but there's a bit more too it. First, the pressure ratio you mention, isn't always measured in the standard instrumentation set of an engine (you need the total pressures, not static, which is what is normally measured). Second, the ratio changes with corrected rotor speed. So you also have to measure N2, and the engine inlet total pressure and temperature to give corrected N2. Thirdly, efficiency is defined for a compressor as the actual temperature rise, compared to the isentropic temperature rise, that should occur for the given pressure rise. $\endgroup$
    – Penguin
    Apr 16, 2018 at 10:45
  • $\begingroup$ ...So, you also have to measure the total temperature at the inlet and outlet of the compressor (which I've never seen in a production engine). But, if you do have all these measurements, you should be able to detect a change in compressor efficiency, if it's big enough above the noise level. $\endgroup$
    – Penguin
    Apr 16, 2018 at 10:47
  • $\begingroup$ @Joe. It's the other way around. HPC efficiency affect these parameters (and others ) not vice-a-verse. Daniel's answer gives a good overview. The details depend on what the engine control is trying to do. (See aviation.stackexchange.com/questions/32301/…). In simple summary, a GE engine will adjust fuel flow to keep corrected N1 constant, a PW or RR engine will adjust fuel flow to keep EPR constant. In both cases, the fuel flow will go up, as the HPC degrades, but not necessarily by the same amount. $\endgroup$
    – Penguin
    Apr 16, 2018 at 11:03
  • $\begingroup$ ...All other parameters will except EPR, or corrected N1, will shift, because every thing is thermodynamically linked. $\endgroup$
    – Penguin
    Apr 16, 2018 at 11:04

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