Auxiliary power units typically comprise three main sections–the power, load compressor, and gearbox, and they are designed to operate at 115V and 400 Hz to reduce systemic inefficiency. These voltage and frequency values help make aircraft lighting and air conditioning possible for passenger comfort. Since APUs are fundamental to an aircraft’s well-being and standard operations, learning more about their functionality becomes paramount.
The sequence of starting up APUs is similar to turning on a car’s ignition. First, a pilot tests the APU’s fire suppression system as a safety measure and then proceeds to open the flap covering the inlet for air. This step is crucial because all engines, even APUs, need air for operations, or else they will not start at all. Next, the pilot presses a button to initiate the APU’s internal electronic starting sequence, which is different from the main engine. Once the APU is running, it starts to spin a turbine for generating electricity through its connection with a generator. This electricity is then used by the aircraft’s electrical system to power up the components connected to it and the air compressor to create bleed air. This bleed air is used in the aircraft’s air conditioning and HVAC system.
Applications of Auxiliary Power Units in Aircraft Systems
The electricity generated by APUs is used for lighting a grounded aircraft’s cabin and cockpit, alongside any other areas that may need to be lit for everyone’s convenience on board. In the APU’s absence, passengers would experience complete darkness on board before the flight takes off, as it is the primary source of power while engines are shut off. Meanwhile, the APU also powers the plane’s air conditioning system, and passengers may experience a brief disruption of the AC supply as the plane prepares to take off. This is because there is a transfer of the HVAC system’s source of power from the APU to the engines’ bleed air.
Apart from powering up the air conditioning and electrical system, the APU also acts as a source of power for the main engines’ electronic starting sequence. With the combined effect of the main engines’ electricity and bleed air being generated by the APU, the first engine starts up and provides bleed air to other engines. Overall, using the APU is far more efficient for the aircraft during initial operations than forcing its main engines to start from scratch.
When an aircraft is airborne, there is no need for it to rely on its secondary power source which is the APU. Aircraft fuel capacity is limited, and APUs consume a large amount of fuel for operations, which can be more than 100 gallons per hour. This is especially true for large commercial planes, and APU usage is kept to a minimum to ensure overall fuel efficiency. This is why APUs are not always running while flying as engine power and other resources are then relied on for the remainder of the flight.
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