PRESSURE RATIO 34 12 15 25 30 20 Figure 10-71.  Measuring turbine wheel to exhaust cone clearance.  Figure 10-70.  Measuring the turbine blades to shroud (tip)  clearances.  Figure 10-72.  Engine pressure ratio indications.  Engine Ratings Turbine Engine Instruments Engine Pressure Ratio Indicator The flat rating of a turbine engine is the thrust performance  that is guaranteed by the manufacturer for a new engine under  specific operating conditions, such as takeoff, maximum  continuous climb, and cruise power settings. The turbine inlet  temperature is proportional to the energy available to turn  the turbine. This means that the hotter the gases are that are  entering the turbine section of the engine, the more power is  available to turn the turbine wheel. The exhaust temperature  is proportional to the turbine inlet temperature. Regardless of  how or where the exhaust temperature is taken on the engine  for the flight deck reading, this temperature is proportional to  the temperature of the exhaust gases entering the first stage  of inlet guide vanes. A higher EGT corresponds to a larger  amount of energy to the turbine so it can turn the compressor  faster. This works fine until the temperature reaches a point  when the turbine inlet guide vanes start to be damaged. EGT  must be held constant or lowered as the result of a prolonged  hot section life and, at the same time, provide the thrust to  meet the certification requirements. Before high bypass turbofan engines, some older types of  engines used water injection to increase thrust for takeoff  (wet). This is the maximum allowable thrust for takeoff. The  rating is obtained by actuating the water-injection system and  setting the computed wet thrust with the throttle, in terms of a  predetermined turbine discharge pressure or engine pressure  ratio for the prevailing ambient conditions. The rating is  restricted to takeoff, is time-limited, and has an altitude  limitation. Water injection is not used very much on turbine  engines any more. Engine pressure ratio (EPR) is an indication of the thrust  being developed by a turbofan engine and is used to set power  for takeoff on many types of aircraft. It is instrumented by  total pressure pickups in the engine inlet (Pt2) and in the  turbine exhaust (Pt7). The reading is displayed in the flight  deck by the EPR gauge, which is used in making engine  power settings.  [Figure 10-72] 10-55