ELECTRICAL-SOURCE PRIORITY AND TRANSFER
Normally operating engine generators have priority over APU generation and external power. With the engines stopped, external power selected ON takes priority over the APU generator. An available APU can therefore be running normally without supplying the main buses. Read the connected source on ECAM ELEC before interpreting an unchanged diagram as a failed transfer. [22, PDF p. 6]
For a ground-power-to-APU transfer before start, establish APU availability and generator readiness, select external power OFF at the appropriate stage and confirm APU supply. Coordinate the physical cable disconnection after that confirmation. This is an electrical transfer followed by a ground-service action; the sound of the APU establishes neither one by itself. [22, PDF pp. 6–8] [14, PDF pp. 1034–1038]
At the stand with engines running, prepare the replacement source before shutdown and monitor the transfer as the engine generators go offline. Their normal priority explains why the replacement may not take over earlier. The checks at Frankfurt and Heathrow follow the same source logic even though the order of physical events differs. [22, PDF p. 6] [14, PDF p. 1161]
IN PRACTICE: Transfer between ground electrical sources
Before you begin: Revisit this exercise after the APU-start workshop. Both engines are off, external power is supplying, the APU is AVAIL, and APU GEN is in its normal enabled selection.
1. Identify the source currently carrying the load
Action. Call ELEC and read the live supply path. With the engines off and external power selected, confirm that external power is supplying even though the running APU is available.
Check. The starting condition is understood: an available APU does not by itself mean its generator has replaced external power. [22, PDF p. 6] [22, PDF pp. 7-9]
2. Confirm the replacement is ready
Action. Check APU AVAIL and the normal APU GEN selection before removing the external supply. Confirm there is no generator-related fault that would defeat the intended transfer.
Check. The replacement source is available and permitted to supply the network. [24, PDF pp. 8-9] [22, PDF pp. 7-9]
3. Release the higher-priority external source
Action. Select EXT PWR OFF while monitoring ELEC. With this normal engine-off configuration and the APU generator available, observe the APU supply path taking over.
Check. The page shows the new supplying source and the buses remain powered. The transfer is established before cable removal. [22, PDF p. 6] [14, PDF p. 1161]
4. Remove the service after the transfer
Action. Confirm the new supply is stable, then disconnect the ground-power service through the chosen ground-service workflow. Recheck ELEC after disconnection.
Check. The aircraft remains supplied by the APU and the removed ground service is no longer part of the power plan. [14, PDF pp. 1034-1038] [22, PDF pp. 7-9]
5. Practise the reverse direction when required
Action. To return to external power with engines off, connect the GPU, wait for AVAIL and select EXT PWR ON. Confirm the external supply path before treating the APU generator as unloaded.
Check. The transfer back agrees with source priority. Later engine shutdown uses the same read-the-page discipline, with engine generators retaining priority until they go offline. [22, PDF p. 6] [14, PDF p. 1161]
Complete when: The selected source has taken the load before the previous service is removed. The actual source and the next intended change are clear.