What is standby ignition for?

There are 2 ignition systems for each engine (Ignition Sys 1 and Ignition Sys 2). Normally, 115V AC Bus #1 feeds Ignition Sys #1 and 115V AC Bus #3 feeds Ignition Sys #2.

With the Standby Ignition Switch in NORM:

• If AC Bus #1 fails, then Ignition Sys #1 (on all four engines) will be powered by the Standby Bus.
• If AC Bus #3 fails, then Ignition Sys #2 (on all four engines) will be powered by the Standby Bus.

Note that losing a bus is very rare. All generators feed all buses on the 744, except during Autoland.

When the Standby Ignition Switch is selected to 1 or 2, the Standby Bus will power the selected (1 or 2) Ignition system on all four engines.

If you lose Main AC power (1 & 3), then, irrespective of Standby Ignition Switch position, Standby Power will provide power to both (#1 and #2) Ignition systems.

The Standby Switch shouldn't normally be touched on the 744. The EECs control ignition switching automatically (cycling between sys 1 and 2 on each leg and controlling single/dual ignition as required).