Recently, I read manual about engine, PW JT8D-217A/C, and there is a chart making me so confused.
Manual show the reference table tells you what number, EPR, is normal in specific temperature and atmospheric pressure.
We all know EPR can be related to engine thrust.
I thought higher Atmospheric Pressure meaning higher air mass density, so EPR should be higher.
At lower temperature making higher efficiency (Higher EPR) is quite easy to understand.
But according to the manual, lower Atmospheric Pressure can produce higher thrust when temperature is constant.
Also when temperature is higher than 102.2F, the EPR doesn't change anymore in different atmospheric pressure.
Why is that?