What is the first indication in the cockpit when starting an APU?
Refer to figure.
Before engine start, electrical power can be provided either by a Ground Power Unit (GPU) or, when necessary, by an onboard Auxiliary Power Unit (APU) capable of meeting substantial power demands.
The APU, typically a compact axial flow gas turbine, drives an AC generator identical to those fitted to the main engines, supplying 115 V, 400 Hz power via an accessory gearbox.
During the start sequence, the fuel supply and ignition controls are automatically activated as the APU reaches the appropriate speeds. These automated controls safeguard against high Turbine Outlet Temperature (TOT), overspeed, loss of pressure, and excessive oil temperature.
Located in a remote section of the aircraft behind a firewall, the APU not only powers the generator but also drives essential accessories such as fuel pumps and oil pumps. For safety, the APU is designed to automatically shut down if a malfunction occurs.
The APU's air supply is managed through ducted inlets that open and close automatically during startup and shutdown, with their position indicated on the cockpit control panel. Additionally, the APU has its own Fuel Control Unit to manage fuel flow efficiently.
When the APU start sequence is initiated, the APU fuel valve opens, and the indication of fuel flow to the APU will be the first sign that the pilot will notice.
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