text: If the fixed geometry engine is operated at a 
  constant V (TAS) in subsonic flight and con- 
  stant N (RPM) the inlet velocity, 
  inlet ram, 
  and compressor pressure ratio are essentially 
  constant with altitude. An increase in alti- 
  tude then causes the engine air mass flow to 
  decrease in a manner very nearly identical to 
  the altitude density ratio. Of coutsc, this de- 
  crease in mass flow will produce a significant 
  thrust of the engine. 
  e&ct on the output 
  Actually, the variation of thrust with altitude 
  is not quite as severe as the density variation 
  temperature 
  because favorable decreases in 
  occut. The decrease in inlet air temperature 
  will provide a relatively greater combustion 
  gas &ergy and allow a greater jet velocity. 
  The increase in jet velocity somewhat offsets 
  the decrease in mass flow. Of course, an in- 
  crease in altitude provides lower temperatures 
  below the tropopause. Above the tropopause, 
  no further favorable decrease in temperature 
  takes place so a more rapid variation of thrust 
  will 
  take place. The approximate variation 
  of thrust with altitude is represented by figure 
  2.11 and some typical values at specific alti- 
  tudes ate as follows : 