APU REGULATION, LOADING AND SHUTDOWN LOGIC
The APU is a self-contained turbine installation with a power section, a load compressor and an electrical generator. Its starting and control functions are not the same as the aircraft’s selection of the outputs. The electronic control box supervises the unit and its sequence. Air intake, fuel, ignition, acceleration and load availability have to develop in a coherent order. The APU page provides evidence of that process; the sound of a running unit cannot establish generator connection or the downstream bleed configuration. [64, PDF pp. 2–4]
MASTER SW prepares the control sequence, and START requests starting after the normal prerequisites. The controller manages acceleration and reports availability through the appropriate indications. APU speed, exhaust-gas temperature, intake status and fault information answer different questions. A successful AVAIL indication means the unit has reached the relevant availability condition, not that the buses and packs are necessarily using it. The latter require their own electrical and pneumatic paths to be selected and confirmed. [64, PDF pp. 5–9]
Pneumatic and electrical loads share the APU’s available capability, but their transfer paths remain separate. The supplied architecture permits the APU to support starting, conditioning and electrical backup within the applicable envelope. Takeoff air conditioning supplied by the APU can change the demand placed on the main engines; that belongs in the performance configuration. APU starting, bleed supply and generator operation can have different envelope restrictions. Do not turn the fact that an APU start is permitted in a particular condition into permission for every simultaneous output. [64, PDF pp. 2–4] [67, PDF pp. 2, 7–8]
Normal shutdown includes unloading and controller-managed completion; a fire/emergency shutdown has a different purpose and isolation path. In the supplied fire architecture, an actual APU fire on the ground can trigger automatic shutdown and extinguishing, whereas the test has a separate inhibit context. Study the required result and receiving supplies rather than making the MASTER selection the whole completion criterion. After removal of APU use, confirm which source carries electrical loads and which source supports conditioning or start air. This distinguishes a normal handover from an unexplained loss of both services. [64, PDF pp. 5–6] [78, PDF pp. 3–4, 7–8]
APU compressor, turbine, generator and air path in the original source. [64, PDF p. 2]