text: When the fuel grades are altered during oper- 
  ation and the engine must be’operated on a 
  next lower fuel grade, proper account must be 
  made for the change in the operating limita- 
  tions. This accounting must be made for the 
  maximum power for takeoff and the maximum 
  cruise power since both of these operating con- 
  ditions are near the detonation envelope. 
  In 
  addition, when the higher grade of fuel again 
  becomes available, the higher operating,limits 
  cannot be used until it is sure chat no contamina- 
  tion exists from the lower grade fuel remaining 
  in the tanks. 
  
  Spark plug fouling can provide certain high 
  as well as low limits of operating temperatures. 
  When excessively low operating temperatures 
  are encountered, rapid carbon fouling of the 
  plugs will 
  take place. On the other hand, 
  excessively high operating temperatures will 
  produce plug fouling 
  posits from the fuel additives. 
  
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  Generally, the limited periods of time at 
  various high power settings are set to mini- 
  mize the accumulation of high rates of wear 