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APU Door Positioning
The APU door is positioned automatically as a function of Weight-On-Wheels (WOW), Mach number and APU speed. When RUN is selected on the APU control panel and the airplane is on ground, the APU door is commanded to the full open position. When the airplane is in flight, the FADEC schedules the door position.
Volume 2 Flight Crew Operating Manual Rev 3, Apr 25, 2005
APU SYSTEM OPERATING SEQUENCE (CONT'D)
APU Loading
The FADEC controls the ready to load signals, one for electrical loading and one for pneumatic loading. Electrical loading is activated 2 seconds after the APU speed reaches 99%. If the APU drops below 95%, electrical loading is not available (on ground and in flight).
On ground, pneumatic loading is available 60 seconds after the APU reaches 99% if the APU pre-start EGT was < 149°C. If the APU pre-start EGT was ≥ 149°C, pneumatic loading is available on the ground 2 seconds after the APU reaches 99%.
In flight, pneumatic loading is available 2 seconds after the APU reaches 99%, regardless of the pre-start EGT. Bleed air extraction is limited to 30,000 feet.
The APU LCV is opened upon receiving a bleed air request from the airplane bleed management control system or the APU BLEED switch on the BLEED/AIR COND/ANTI-ICE panel is selected. Open and closed positions of the control valve are determined by the status of weight-on-wheels, main engine starting and environmental control system signals. The bleed and load management systems are responsible to ensure that the bleed air request is activated only when another source of bleed air is not commanded. Example: L engine bleed valve open and/or the R engine bleed valve open and crossbleed valve opened (control valve remains closed).
The normal position for the APU BLEED switch is AUTO. The AUTO position automatically integrates control of the APU LCV. The manual ON position will command the LCV to open as selected, but only if alloed by the FEDEC (to garantee APU self-preservation). The manual OFF position will command the LCV to close as selected.
APU Shutdown
When the “OFF”position on the APU control panel is selected, the FADEC controls the shut down sequence. The ready-to-load signals are cancelled, commanding the APU load valve closed and reducing the APU speed to 70% (below 20,000 feet) or 100% (above 20,000 feet) for a 60 second cooldown period.
NOTE
If the APU switch is selected to RUN prior to completion of the 60 second cooldown period, the APU returns to 100% and continues normal operation.
After cooldown is completed, the APU FADEC shuts down the APU and all loads and fuel demands are cancelled. When the speed drops below 25%, the inlet door is closed, APU operating hours and fault information are stored in the FADEC and the APU indications are removed from EICAS.
If the BATT MASTER switch is selected OFF, fire handle is pulled, or either APU shutoff switches (External Services Panel or APU compartment) is selected, the APU FADEC will cancel the cooldown period and initiate an immediate shut down.
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