at throttle positions below idle. A separate fuel shutoff lever  is usually provided on engines equipped with thrust reversers. Prior to start, particular attention should be paid to the  engine air inlet, the visual condition and free movement of  the compressor and turbine assembly, and the parking ramp  area fore and aft of the aircraft. The engine is started by using  an external air power source, APU, or an already operating  engine. Starter types and the engine starting cycle have been  discussed previously. On multi-engine aircraft, the engines  are usually started by an onboard APU that supplies the air  pressure for a pneumatic starter on each engine. Air bled from  the APU is used as a source of power for starting the engines. During the start, it is necessary to monitor the tachometer,  the oil pressure, and the exhaust gas temperature. The normal  starting sequence is: Adherence to the procedure prescribed for a particular engine  is necessary as a safety measure and to avoid a hot or hung  start. A successful start is noted first by a rise in exhaust gas  temperature. If the engine does not light up, meaning that fuel  starts to burn inside of the engine within a prescribed period  of time, or if the exhaust gas starting temperature limit is  exceeded, a hot start, the starting procedure should be aborted.  Hot starts are not common, but when they do occur, they can  usually be stopped in time to avoid excessive temperature  by observing the exhaust gas temperature constantly during  the start. When necessary, the engine is cleared of trapped  fuel or gases by continuing to rotate the compressor with  the starter, but with the ignition and fuel turned off. If the  engine did not light off during start after the allotted time,  about 10 seconds although this time varies from engine to  engine, the fuel must be shut off as the engine is being filled  with unburned fuel. A hung start is when the engine lights  off, but the engine will not accelerate to idle rpm. Move the fuel shutoff lever to the off position if an engine  fire occurs, or if the fire warning light is illuminated during  the starting cycle. Continue cranking or motoring the engine  until the fire has been expelled from the engine. If the fire  persists, CO$_{2}$ can be discharged into the inlet duct while it is  being cranked. Do not discharge CO$_{2}$ directly into the engine  exhaust, because it may damage the engine. If the fire cannot  be extinguished, secure all switches and leave the aircraft. If  the fire is on the ground under the engine overboard drain,  discharge the CO$_{2}$ on the ground rather than on the engine.  This also is true if the fire is at the tailpipe and the fuel is  dripping to the ground and burning. Checking turbofan engines for proper operation consists  primarily of simply reading the engine instruments and then  comparing the observed values with those known to be correct  for any given engine operating condition. After the engine  has started, idle rpm has been attained, and the instrument  readings have stabilized, the engine should be checked  for satisfactory operation at idling speed. The oil pressure  indicator, tachometer, and the exhaust gas temperature  readings should be compared with the allowable ranges. Takeoff thrust is checked by adjusting the throttle to obtain  a single, predicted reading on the engine pressure ratio  indicator in the aircraft. The value for engine pressure ratio,  which represents takeoff thrust for the prevailing ambient  atmospheric conditions, is calculated from a takeoff thrust  setting curve or, on newer aircraft, is a function of the  onboard computer. This curve has been computed for static  conditions.  [Figure 10-74]  Therefore, for all precise thrust  checking, the aircraft should be stationary, and stable engine  operation should be established. If it is needed for calculating  thrust during an engine trim check, turbine discharge pressure  (Pt7) is also shown on these curves. Appropriate manuals  should be consulted for the charts for a specific make and  model engine. Engine trimming procedure is also covered  in Chapter 3, Engine Fuel and Fuel Metering Systems.  The engine pressure ratio computed from the thrust setting  curve represents thrust or a lower thrust call part power  thrust used for testing. The aircraft throttle is advanced to  obtain this predicted reading on the engine pressure ratio  indicator, or the part power stop is engaged in the aircraft.  If an engine develops the predicted thrust and if all the other  engine instruments are reading within their proper ranges,  engine operation is considered satisfactory. Full authority  digital engine controls (FADEC) engine controls (computer controls) also have means of checking the engine with the  results displayed on the flight deck. The sensitivity of gas turbine engines to compressor inlet air  temperature and pressure necessitates that considerable care  be taken to obtain correct values for the prevailing ambient  air conditions when computing takeoff thrust. Some things  to remember are: 1.  Rotate the compressor with the starter; 2.  Turn the ignition on; and 3.  Open the engine fuel valve, either by moving the  throttle to idle or by moving a fuel shutoff lever or  turning a switch.  1.  The engine senses the air temperature and pressure at  the compressor inlet. This is the actual air temperature  just above the runway surface. When the aircraft is  stationary, the pressure at the compressor inlet is  Ground Operation Engine Fire Engine Checks Checking Takeoff Thrust Ambient Conditions 10-58