Special in AIRBUS A320 APU

The APU flight envelope has a wide range of altitude and temperature for ground and flight operations. The Hamilton Sundstrand APS 3200 APU model can be started from pressure altitude of-1000 ft to 41,000 ft with an ambident temperature ranging from -70 degree Celsius to 55 degree Celsius. This APU can supply electrical and pneumatic power independently or simultaneously up to a pressure altitude of 20,000 feet. Above 20,000ft, only electrical power can be supplied, as it takes priority to protect the APU from an overload. On the ground because of the lack of ram air, the APU can be started up to an altitude of 9,200ft.

The operation of the APU is controlled by the MASTER SW, START and BLEED pushbuttons on the overload panel. When the MASTER SW pushbuttons is pressed, the ON light illuminates blue and the APU page appears on the ECAM system display. The ECB initiates a power-up test and the air intake flap opens. A FLAP OPEN message appears when the air intake flap is fully open. The APU fuel valve opens and the APU fuel pump starts running if fuel pressure delivered is low.

When the START pushbutton is pressed, the start sequence is initiated and the ON light illuminates in blue. The starter turns the rotor and after approximately 1.5 seconds, the fuel solenoid valve opens and the ignition system is energized. At 55% rotor speed, the ECB de-energizes the starter and the engines continue to accelerate to 100%. The AVAIL light turns green on the START pushbutton two seconds after the APU rotor speed has reached 95% +2 seconds or 99.5%. On the ECAM MEMO display, the APU AVAIL memo is displayed. If the APU bleed air is used, APU AVAIL MEMO is replaced by APU BLEED. The FCU schedules the fuel flow to maintain a constant speed of 100%. The APU rotor speed is displayed in green on the ECAM APU page. It becomes amber when the rotor speed is 102% and become red when it is 107%.

APU Shutdown

When the MASTER SW pushbutton is released out, the shutdown sequence is initiated and the ON light turns off. The FCU closes the fuel supply to the fuel injectors and the APU decelerates. If APU bleed air was not used, the APU shuts down immediately. If bleed air was used, the APU will run for a maximum cooling period of 120 seconds. It is important to note that batteries must be left in the ON position for the time of cooling period to ensure that the intake flap can be fully closed and the APU maintain dire protection. Below 7% of rotor speed, the fuel valves closes and the APU fuel pumps stops followed shortly by the closure of the intake flap.