text: Since the turbojet engine is composed of 
  many different distributed masses and elastic 
  structure, there are certain vibra~tory modes 
  and frequencies for 
  the shaft, blades, etc. 
  While it is necessary to prevent any resonant 
  the normal 
  conditions from existing within 
  operating range, there may be certain vibra- 
  tory modes encountered in the low power range 
  common to ground operation, low altitude 
  If 
  endurance, acceleration or deceleration. 
  certain operating RPM range restrictions are 
  specified due to vibratory conditions, opera- 
  tions must be conducted with a minimum of 
  time 
  increased 
  this area. The greatly 
  stresses common to vibratory conditions are 
  quite likely 
  to cause fatigue failures of the 
  offending components. 
  
  in 
  
  The operating limitations of the engine are 
  usually specified by various combinations of 
  RPM, exhaust gas temperature, and allowable 
  time. The conditions of high power output 
  and acceleration have relatively short times 
  allowable to prevent abuse of the powerplant 
  and obtain good service life. While the al- 
  times at various high power and 
  lowable 
  acceleration condition appear arbitrary, 
  the 
  purpose is to reduce the spectrum of loading 
  which contributes the most rapid accumulation 
  of creep and fatigue damage. 
  In fact, in some 
  instances, the arbitrary time standards can be 
  set to suit the particular requirements of a 