The   type   numerals   and   type   letter   symbol   are assigned  consecutively  by  each  of  the  services.  The numerals begin as follows: !    100—Air Force !    400—Navy !    700—Army Model Indicator The third part is the model indicator. It is a dash and a model number, or a dash and a model number with a suffix letter. Each  configuration  of  the  engine  has  an  assigned model number. Each of the services assigns a block of numbers that are used consecutively. !    100—Air Force !    400—Navy !    700—Army NOTE: If   one   service   uses   another   services' designated engines, the designation remains the same unless a model change is required. Only in this case will the model indicator change to indicate the engine has been modified. F401-PW-400 is an example of a MIL-STD-1812 engine designation. !    F Turbofan !    401 Second Navy turbofan in designation system !    PW Pratt and Whitney Aircraft Division, United Aircraft Corporation !    400 First  Navy  model  of  this  particular engine Q6-18. What are the two engine designation systems used to identify aircraft power plants? Q6-19. What  does  the  letter  X  or  Y  preceding  the basic designation signify? Q6-20. What are the three parts of the MIL-STD-1812 designation system? Q6-21. F401-PW-400 is an example of what engine designation system? POWER PLANT SAFETY PRECAUTIONS LEARNING OBJECTIVE: Recognize power plant safety precautions that apply to the intake ducts, exhaust area, and engine noise. Operational  readiness  of  a  maximum  number  of aircraft power plants is necessary if naval aviation is to successfully perform its mission. Keeping aircraft and power plants in top operating condition is the principal function of naval aviation maintenance personnel. This maintenance work must be performed without injury to personnel. Every person connected with power plant maintenance is responsible for discovering and eliminating   unsafe   work   practices.   In   the   following section,    we    will    discuss    a    few    standard    safety precautions.   You   must   follow   these   precautions   to prevent injury to yourself or others working on or near aircraft jet engines. INTAKE DUCTS The air intake ducts of operating jet engines are an extreme hazard to personnel working near the aircraft. Ducts are also a hazard to the engine itself if the area around  the  front  of  the  aircraft  is  not  kept  clear  of debris. The air intake duct develops enough suction to pull  an  individual,  or  hats,  eye  glasses,  etc.,  into  the intake.    The    hazard    is    obviously    greatest    during maximum  power  settings.  Protective  screens  for  the ducts     are     part     of     the     aircraft's     ground-handling equipment. These screens must be installed prior to all maintenance turnups. EXHAUST AREA Jet    engine    exhausts    create    many    hazards    to personnel. The two most serious hazards are the high temperature and the high velocity of the exhaust gases from the tailpipe. High temperatures are present several hundred feet from the tailpipe. The closer you get to the aircraft,  the  higher  the  exhaust  temperatures  and  the greater the danger. When  a  jet  engine  is  starting,  sometimes  excess fuel  will  accumulate  in  the  tailpipe.  When  this  fuel ignites,  long  flames  shoot  out  of  the  tailpipe  at  very high velocity. You will want to stay clear of this danger at all times. 6-19