(a)
Adds fuel pressure to the FCU supply. No recording required. Item 8. RECORD INCREASE IN GENERATOR FREQUENCY (2 HZ MAXIMUM)
(a) If a 3 HZ or greater increase is observed, remove the FCU for bench check.
Item 9. OPEN APU BLEED VALVE AND ISOLATION VALVE WITH DOWNSTREAM PACK VALVES CLOSED
(a)
No recording required. Item 10. RECORD DUCT PRESSURE (38-50 PSIG)
(a)
Indicates condition of APU and used as a reference for the ECS check. Item 11. RECORD APU EGT (370°C MAXIMUM)
(a) Used to check duct leakage. The difference between EGT with the APU bleed valve closed (item 5) and EGT with the APU bleed and isolation valves open determines duct leakage (20°C Maximum).
Item 12. CLOSE APU BLEED VALVE
(a)
To establish a condition for the APU bleed valve opening rate check. No recording required.
(a)
To establish a condition of minimum resistance to observe load control valve,
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regulator and thermostat performance. No recording required. Item 14. OPEN APU BLEED VALVE
(a) No recording required. With both pack valves open or opening when the APU bleed valve is opened, a minimum flow resistance will exist in the pneumatic system allowing the APU bleed valve to seek its maximum rate of opening.
Item 15. RECORD "TIME" FROM APU BLEED VALVE SWITCH ACTUATION TO MINIMUM GENERATOR FREQUENCY (12-14 SECONDS) OR RECORD "TIME" FOR A 200°C EGT RISE (30 SECONDS MAXIMUM) FOR TREND ANALYSIS
(a)
A slow opening APU bleed valve causes low duct pressure during main engine starts. A slow opening APU bleed valve may be due to: 1) Dirty air supply filter in valve (if installed). 2) Dirty or contaminated rate needle or orifice. 3) Dirt under the rate chamber poppet valve. 4) Leaking diaphragm. 5) Leaking switcher ball valve. 6) Air pressure regulator set low. 7) Sticking regulator. 8) Regulator filter dirty. 9) Leaking pneumatic control lines or thermostat. 10) Incorrect rate valve setting.
(b)
A fast opening APU bleed valve will cause excessive speed droop and load cycling. A fast opening APU bleed valve may be due to: 1) Faulty fuel control. 2) Incorrect rate valve setting. 3) Air pressure regulator set too high.
CAUTION: TO AVOID DAMAGING APU, OBSERVE OPERATING LIMITS (REF 49-11-0, MAINTENANCE PRACTICES).
(a)
Established power available for main engine start and/or air conditioning system. 1) Low EGT will cause slow APU starts. 2) Low EGT will cause a slow main engine start.
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NOTE: Idle EGT (Item 11) subtracted from the maximum stabilized EGT is the available APU power (250°C Minimum).
NOTE: Idle EGT with bleed valve open (pack valves closed) subtracted from the maximum stabilized EGT is the actual APU power available for main engine start and/or air conditioning. This takes into consideration duct leakage.
NOTE: The maximum stabilized EGT is the APU temperature control point for both air and electrical load. This is a function of the air pressure regulator, load control valve, thermostat, and fuel control.
Item 17. RECORD GENERATOR FREQUENCY (395-405 HZ) The generator frequency meter is the APU RPM indicator above the APU 95% RPM range.
(a)
Low frequency below 395 HZ is fuel control related.
1) Leaking pneumatic line to the FCU.
2) Acceleration limiter valve leaking.
3) FCU metering slide valve sticking or leaking.
4) Low governor setting.
5) Fuel pump deficiency.
6) Clogged fuel filter.
7) Fuel nozzle screen contaminated.
(b)
High frequency above 405 HZ.
Incorrect governor adjustment.
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NOTE: If maximum load frequency is subtracted from idle frequency (Item 6) and the difference exceeds 10, the fuel control is suspect. If the difference or delta from maximum load HZ to idle HZ is increasing, the pump may be suspect. If the difference or delta is excessive but stable, a spring change in the governor is necessary.
Item 18. RECORD DUCT PRESSURE (6-12 PSIG)
(a) Used to determine full load capability of the APU. If a higher than normal pressure is
recorded, the duct restriction may not allow the APU to be fully loaded. Item 19. SELECT 60 AMP ELECTRICAL LOAD
(a)
The addition of the 60 amp electrical load to the pneumatic load, should cause automatic temperature control correction by the thermostat to modulate the APU bleed valve and reduce bleed air flow maintaining the EGT is recorded.
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