**Effects of Power Extraction on Turbofan Compressor and Turbine Maps**

The two diagrams below show the effects of shaft power extraction (*shaft power offtake* / *shaft power off-take* for the propulsion of accessories such as fuel-/hydraulic-/oil-pumps, generators etc.) on a turbofan engine at a flight altitude of $H = 11km$ and a speed of $Ma = 0.8$, calculated with [GasTurb 12][1]:

[![Effects of Power Extraction on Turbofan Compressor and Turbine Maps][2]][2]

**Effects of Power Extraction on the Operating Line (Blue)**

- Why are the operating lines (blue) of both the compressor and turbine shifted to the left towards the surge line?
- In particular, why does the non-dimensional corrected mass flow decrease, while the pressure ratio stays approximately constant?  
- The same question from a different angle: Why does the pressure ratio not decrease at a constant mass flow?  
*It seems logical that the non-dimensional corrected relative spool speed decreases, since power is extracted. However, I would assume that the decrease in spool speed would entail a decreasing pressure ratio for the same reason, while the mass flow would stay constant - which is apparently false.*


  [1]: http://www.gasturb.de/
  [2]: https://i.sstatic.net/FMnAg.png