1) An APU bleed valve which opens slowly can cause a low duct
pressure during main engine starts. The slow operation of
the APU bleed valve can be caused by these items:
a) A dirty air supply filter in the valve
b) A dirty rate needle or orifice
c) Dirt below poppet valve in the rate chamber
d) Diaphragm leakage
e) Leakage from the switcher ball valve
f) The air pressure regulator is set too low
g) A stuck regulator
h) Leakage from the pneumatic control lines or the
thermostat
i) The rate valve is set incorrectly.
2) If the APU bleed valve opens quickly, the APU speed will decrease too much. Also, the position of the APU bleed valve will not be controlled. These items can cause the APU bleed valve to open quickly: a) Diaphragm leakage or damaged
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b) The rate valve is incorrectly set c) The air pressure regulator is set too high.
(n) Item 14. WRITE DOWN THE MAXIMUM EGT WHEN IT HAS BECOME STABLE (The EGT operation range is 620°C-649°C (Airplanes without the ETC) and 566°C-649°C (Airplanes with the ETC)):
CAUTION: MAKE SURE YOU MONITOR THE APU OPERATION LIMITS (AMM
_______ 49-11-00/201). DAMAGE TO THE APU CAN OCCUR IF THE EGT IS NOT IN THE OPERATION LIMITS.
1) The power is available for a main engine start and/or the
air conditioning system. 2) A low EGT can cause a slow main engine start. 3) The idle EGT with the bleed valve open (pack valves closed)
subtracted from the maximum stable EGT is the actual APU power available. This power includes the duct leakage.
4) The maximum stable EGT is the temperature control point. This is a function of the load control valve, the thermostat (Airplanes without the ETC) or the ETC (Airplanes with the ETC).
(o) Item 15. WRITE DOWN THE GENERATOR FREQUENCY (396-404 CPS)
1) A frequency that is out of range is related to the fuel control. This frequency can be caused by these items: a) Leakage from the pneumatic line to the fuel control
unit b) Leakage from the acceleration limiter valve c) The metering slide valve on the fuel control unit leaks
or sticks d) The governor is set too low e) A bad fuel pump f) A clogged fuel filter g) The fuel nozzle screen has contamination h) A bad governor spring for the fuel control unit.
NOTE: If the maximum load frequency is subtracted from
____ the idle frequency (item 6) and the difference is more than 10, the fuel control unit (FCU) can be bad. If the difference from the maximum load CPS to the idle CPS increases, the pump can be bad. If the difference is too much but stable, it can be necessary to change the governor spring.
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////////////////////////A / GTCP85-129 SERIES /737-300/400/500/ ENGINES/MAINTENANCE MANUAL ////////////////////////
(p) Item 16. WRITE DOWN THE DUCT PRESSURE (5-20 PSIG)
1) This is used to find the full load operation of the APU.
If a higher than usual pressure is found, the duct blockage
cannot permit the APU to be fully loaded.
(q) Item 17. SET THE ELECTRICAL LOAD TO 100 AMPS
1) The increase of the electrical load to the pneumatic load
will cause an automatic temperature control correction by
the thermostat. This correction will operate the APU bleed
valve and decrease the bleed air flow. During this
correction, the APU EGT must stay stable.
2) If the EGT continues to increase with the electrical load
applied, the automatic temperature control system is bad.
These items can cause the control system to operate
incorrectly:
a) The pneumatic control pressure is bad
b) A low spring tension in the return spring of the APU
bleed valve
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