CHANGING THE POWER SOURCE WITHOUT GETTING LOST
The electrical system has a preferred order of sources. Normally operating engine generators take priority over the APU generator and external power. The APU is the Auxiliary Power Unit, the small gas-turbine unit in the tail that we will start next. With the engines stopped, external power selected ON takes priority over the APU generator. This is why an apparently successful APU start may leave the main electrical diagram looking unchanged: the selected ground source is still doing its job. Check the connected source on ECAM ELEC before deciding that the APU has failed to take over. [22, PDF p. 6]
Before departure, the transfer from ground power to the APU begins with a ready replacement. Establish APU availability and generator readiness. At the appropriate stage, select external power OFF, then check the ELEC page to confirm that the APU now supplies the aircraft. Arrange physical disconnection of the ground cable after that confirmation and the required coordination. There are two linked actions here: changing the aircraft's electrical source and disconnecting a ground service. The sound of the APU running outside cannot confirm either the bus connection or removal of the cable. [22, PDF pp. 6–8] [14, PDF pp. 1034–1038]
After landing at Heathrow, the engine generators may still be supplying while you prepare a replacement source at the stand. Their normal priority explains why that prepared replacement does not need to take over immediately. Establish its readiness before shutting down the engines, then observe the supply change as the engine generators go offline. The scene is different from the departure transfer, but the source logic is familiar. Read the aircraft's actual arrangement instead of trying to make a lower-priority source supply while the normally preferred generators are still operating. [22, PDF p. 6] [14, PDF p. 1161]
Think through the handover aloud if that helps: this source is supplying now; that source is ready to replace it; the change has been commanded; the new supply has been confirmed. The order gives each check a reason. It also helps you understand why AVAIL is encouraging without proving that a source has already taken the load. We will use the same distinction between readiness and actual supply when looking at APU air. [22, PDF p. 6]
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.