ENGINE INDICATING

民航

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ENGINE INDICATING

民航

1. General A. The engine indicating systems described in this chapter include the engine pressure ratio (EPR) indicating system, a tachometer system to measure the speed of the low pressure compressor (N1) and high pressure compressor (N2), an exhaust gas temperature (EGT) indicating system, and an airborne vibration monitoring system. B. Each system provides a reading of engine operating conditions on indicators located on the center instrument panel. This information enables the monitoring of engine output and maintaining a selected flight performance. 1. General A. The engine pressure ratio (EPR) indicating system shows the engine power output and is used for setting engine thrust and for monitoring engine performance. The EPR indicating system consists of one inlet pressure (Pt2) sensing probe, six-exhaust pressure (Pt7) sensing probes, an engine pressure ratio transmitter and a pressure ratio indicator for each engine. B. The engine inlet and exhaust pressures, sensed by the pressure sensing probes, are transmitted to the pressure ratio transmitter. The transmitter converts the exhaust and inlet pressures into a ratio, provides output signals proportional to the EPR and transmits the signals to the EPR indicator located in the flight compartment. The indicator transforms the electrical input signals into the indicator pointer shaft rotation and digital three-wheel counter to show the engine pressure ratio. A test receptacle, used to attach a master indicator, is included in the circuit to provide a means of adjusting and checking the system (Fig. 1). On airplanes incorporating EPR-activated takeoff warning system, refer to Chapter 31, Instruments, for a description. C. On airplanes incorporating a Performance Data Computer System (PDCS), an EPR indicator incorporating a second servo loop and a driven bug is installed on the pilot's instrument panel. 2. Inlet Pressure Sensing Probe A. The engine inlet pressure (Pt2) is sensed by a probe similar to a pitot tube. This probe is mounted through the center of the nose dome with the open end of the tube facing the inlet air stream. The vent hole in the probe functions as the probe ice detector by decreasing engine inlet pressure (increasing EPR) when icing occurs. The probe is anti-iced by the engine anti-ice system. 3. Exhaust Pressure Sensing Probe A. Each engine has six exhaust (discharge, Pt7) pressure-sensing probes projected into the stream of turbine exhaust gases. The probes are connected to a common manifold for obtaining an average pressure of the exhaust gases. Exterior connection to the manifold is made at a single point through the fan discharge outer duct at approximately the seven o'clock position (Fig. 1). 4. Engine Pressure Ratio Transmitter A. The engine pressure ratio transmitter converts the exhaust pressure (Pt7) and the inlet pressure (Pt2) into a ratio, and generates three-phase electrical signals corresponding to pressure changes in the engine. It consists of two bellows (multi-cell diaphragms), a sensing mechanism, an amplifier, a motor-gear train, and a synchro transmitter. The engine pressure ratio transmitters are located at STA 570 on the right and left side of the airplane in the air conditioning bay. B. The engine exhaust and inlet pressures are applied to the bellows assembly of the transmitter. A change in either of these pressures cause differential bellows movement. The bellows movement affects the sensing mechanism, which, with the aid of the amplifier and the motorgear train, causes the (synchro transmitter) rotor to rotate and generate three-phase electrical signals. 5. Engine Pressure Ratio Indicator A. The engine pressure ratio indicator provides pointer and digital readout of the engine exhaust pressure ratio (Pt7/Pt2) and on some airplanes pointer and digital readout of the Command EPR input signal. The indicator consists of a synchro receiver, servomotor, geartrains, coarse indicator pointer and numerical counter, command index and numerical counter, command set knob, graduated dial face, integral lights, and case. The indicator is not hermetically sealed. The dial face is graduated from 1.0 to 2.6 EPR. Each counter consists of three rotating drums with numerals, readable from 0.50 to 2.00 EPR. Downward rotation of drums indicates pressure ratio increase. The indicator incorporates a failure warning flag, which drops in front of coarse counter numerals when power is not received, when voltage is too low, or when sustained mechanical malfunction of indicator occurs. The indicator displays Command EPR over the range of 1.0 to 2.6 EPR by means of a moving index against a fixed scale and a three wheel numerical counter. Increasing EPR will be clockwise rotation of the index and downward rotation of the counter wheels and is set by clockwise rotation of the set knob located on the lower right front face. 6. Operation A. The system operates on ac power (Fig. 2). B. The engine exhaust and inlet pressures are sensed by the pressure sensing probes. These pressures act on the bellows assembly of the pressure ratio transmitter, causing differential bellows movement whenever either of the pressures change. The relative bellows movement effects the sensing mechanism of the EPR transmitter which, with the aid of the amplifier and motor-gear train, cause the sychro transmitter rotor to rotate and generate three-phase electrical signals. The generated electrical signals are transmitted to a respective pressure ratio indicator over a three-wire system. The indicator converts the electrical signals into the pointer shaft rotation or indicator pointer movement corresponding to the pressure change in the engine (Fig. 2). C. On airplanes incorporating PDCS, an EPR-driven command bug is used to indicate the optimum EPR setting for maximum economy of operation in various flight modes. 1. Engine Pressure Ratio Indicating System Test (Preferred Procedure) A. General (1) Testing the engine pressure ratio (EPR) indicating system consists of two phases. The transmitter inlet (Pt2) and exhaust (Pt7) pressure lines are tested for leakage, and an operational test on the entire system is performed. The components included in this systems test are the pressure ratio transmitter, indicator, and the inlet and exhaust pressure lines. NOTE: The order of the two test phases should not be reversed. If systems operational test is performed prior to the leakage test, erroneous results may be obtained because of possible leaking lines or transmitter. (2) Testing the engine pressure ratio indicating system will not include the inlet pressure sensing probe and its connecting line through nose dome because of the difficulties in closing the vent hole in the nose dome for the testing purposes. Also, the exhaust pressure probes and manifold will not be included in the test due to the difficulties involved in obtaining access. Therefore, it should be kept in mind, that if the first phase of the test does not reveal any leakage, and the second phase indicates faulty systems operation, the cause may be assumed to be in the areas mentioned above. (3) Refer to step 2 for an alternate EPR test procedure. B. Equipment and Materials (1) An air pressure source with two individually regulated outlets, the pressure of which can be accurately set at pressures between 25.00 and 100.00 inches of mercury absolute. All pressures are to be monitored by pressure gages with an accuracy of + 0.5%. Shutoff valves must be provided on the source side of all gages (Fig. 501). (2) Gage - Pressure, Model C, 0 - 50 or 0 - 100 inch HgA, 0.1% Full Scale, Hiese (3) Shutoff Valve (4), A311 - 1/8" ID, Dwyer Instr. (4) T-Fitting (4), A343-1 - 1/8" ID, Dwyer Instr. (5) Fitting (2), A339 - 1/8" ID, Dwyer Instr. (6) Tubing 30 feet, A225 - 1/8" ID, Dwyer Instr. (7) Wheatstone Bridge, Shallcross 638-R, Shalltronix C. Prepare for Test (1) Obtain access to inlet pressure line test "TEE" by removing cowl panels on engine(s) to be tested (Ref Chapter 71, Power Plant). (2) Observe the following precautions for maintenance in engine exhaust section. (a) Pull applicable engine START circuit breaker on circuit breaker panel P6-2, and placard circuit breaker with DO NOT CLOSE. (b) Placard thrust reverser controls with DO NOT OPERATE. WARNING: SERIOUS INJURY CAN OCCUR IF ENGINE IS STARTED AND/OR IF THRUST REVERSER IS OPERATED. (c) Open ENGINE-1 and ENGINE-2 EPR circuit breakers on circuit breaker panel P6. (3) Obtain access to exhaust pressure line connection on engine(s) by opening cowl panel on engine(s) (Ref Chapter 71, Power Plant). (4) Disconnect exhaust pressure line from exhaust pressure manifold outlet elbow on engine(s) (Fig. 501). D. Test Engine Pressure Ratio Indicating System CAUTION: DO NOT EXCEED 42 INCHES OF MERCURY ABSOLUTE (20.63 PSIA) ON INLET PRESSURE LINE, OR 80 INCHES OF MERCURY ABSOLUTE ON EXHAUST PRESSURE LINE WHILE LINES ARE CONNECTED TO TRANSMITTER. MAXIMUM PRESSURE DIFFERENTIAL SHALL NOT EXCEED 51 INCHES OF MERCURY (25.05 PSIA) DURING THESE TESTS TO PREVENT PERMANENT DAMAGE TO THE TRANSMITTER. (1) Test Inlet Pressure Line Leakage (a) Connect test hose from test apparatus to inlet pressure line Pt2 test "TEE" on bottom of engine aft of cowl flange (Fig. 501). Do not connect test hose to exhaust pressure line at this time.