regarding EPR
 

why does EPR rise with altitude? just wondering

thanks

Because....errrr....outside pressure decreases with altitude? And does so faster than the pressure inside the engine, since at higher levels jet engines can run at higher power settings.

Air density is decreased at altitude, therefore the mass flow of air per sec through the engine is decreased. So, in order to produce X lbs of thrust at altitude, the air passing thru the engine has to be accelerated more compared to lower FL's. EPR measures the differences in intake and exhaust pressure, which is a direct indication of the change in speed of the airflow through the engine, and as mass flow is constant for a given altitude, temperature and power setting, the EPR gives an indication of thrust. Therefore to maintain a particular thrust as an aircraft climbs the EPR must be increased.