GENERAL ENGINE BLEED Fokker 50
Fokker 50 - Bleed Air System
GENERAL
Bleed-air is used for airconditioning, pressurization, airframe and engine air intake de-icing, and for pressurizing the hydraulic tank. Bleed-air is supplied by the engines.
For aircraft equipped with APU
On the ground bleed-air can be supplied by the APU for air conditioning.
ENGINE BLEED
Tappings Bleed-air is obtained via tappings on each engine, referred to as Low-Pressure (LP) bleed and High-Pressure (HP) bleed. LP-bleed is normally in use during flight. HP-bleed will be used at idling and low engine speeds, when LP-bleed is insufficient. The HP-bleed valve will open automatically provided the BLEED push button, located at the AIR CONDITIONING panel, is blank. Bleed-air from the HP-Bleed is prevented from flowing into the LP-bleed by a check valve.
Duct leak detection A duct leak between the engine tappings and the firewall, resulting in a significant leakage of bleed-air, is monitored by the engine fire detection and warning system.
Distribution Bleed-air from both engines is used to supply the following services: • Airframe, and engine air intake de-icing, see ICE AND RAIN PROTECTION • Hydraulic tank pressurization, see HYDRAULIC SYSTEM. • Airconditioning and pressurization, see below. • If mentioned in aircraft system deviation table: Watertank pressurization, see AIRCRAFT GENERAL.
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BLEED AIR FOR AIRCONDITIONING
Supply Controls and indicators are located at the AIRCONDITIONING panel. Bleed-air is available when the engines are running and the BLEED push buttons are blank. When a BLEED push button is depressed to OFF, the Pressure Regulating/Shut-Off valve (PR/SO) and the HP- bleed valve close. At flight altitudes up to 10.000 feet the supply of bleed-air ceases completely when the propeller of an engine autofeathers. The above also applies when an engine has been shut down manually, provided that: • The POWER lever of the operative engine is at take-off position, and • Either - TO or GA is selected at the Engine Rating Panel (ERP), or the landing gear is extended.
NOTE: • When a PR/SO valve is closed, a VALVE CLOSED light illuminates independent of BLEED push button indications. • FLEX (FLX) is either not used or not installed.
Pressure control There are three modes of pressure control: • Normal • Economy (reduced pressure) • Augmented (increased pressure)
Normal and ECONOMY can be selected manually. However, for reasons of performance the economy mode is activated automatically when take-off power is selected and both airconditioning packs are operative. The augmented mode is activated automatically when one of the airconditioning packs is switched off manually.
Bleed faults • Overpressure If either PR/SO valve should fail in the open position, the rise in pressure may be sufficient to activate a pressure switch. Consequently the relevant BLEED FAULT light illuminates. • Overheat The ducts downstream of the PR/SO valves incorporate temperature switches. An activated switch will result in the closure of the relevant HP-bleed and PR/SO valves. In addition, the relevant BLEED FAULT light illuminates. • Bleed-air leakage A duct leak between the PR/SO valve and the airconditioning packs is detected by temperature switches. An activated switch will result in the closure of relevant HP-bleed and PR/SO valves. In addition, the relevant BLEED FAULT light illuminates. If leakage of bleed-air is detected around the common duct to the airconditioning packs, the HP-bleed and PR/SO valves in both bleed-air systems close.
NOTE: When either of the PR/SO valves is closed, the supply of bleed-air is adequate for continued normal operation.
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FOR AIRCRAFT EQUIPPED WITH APU
APU Bleed
APU bleed-air will be available approximately two minutes after stabilized APU operation; the AVAIL light at the APU BLEED push button will illuminate.
Depressing the push button from AVAIL to ON will open the APU PR/SO valve to provide bleed-air to the airconditioning system.
The APU bleed-air airflow can be reduced by depressing the ECONOMY push button at the AIRCONDITIONING panel to ON.
At lift-off, the APU shuts down (if running) and the APU PR/SO valve will close automatically. If bleed-air leakage is detected the APU PR/SO valve closes and the AVAIL light goes out. If an APU PR/SO valve failure is detected an APU FAULT is presented and the APU shuts down.
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Functional diagram
Functional diagram - For aircraft not equipped with APU
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For aircraft equipped with apu
Functional diagram - For aircraft equipped with APU
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Controls and indicators
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For aircraft equipped with APU
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ALERTS
CONDITION(S) / LEVEL AURAL MWL/MCL CAP LOCAL
ENGINE BLEED FAULT FAULT 1
BLEED
ENGINE BLEED FAULT FAULT + OTHER ENGINE 2
PR/SO VALVE CLOSED CAUTION
For aircraft equipped with APU
APU PR/SO VALVE
FAILURE 2 FAULT CAUTION
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