"Auto" ignition switch has four selections.
1,
NORM,
2,
BOTH.
FAFC controls ignition selection at all times.
If switch is set to
NORM during a ground start the FAFC will use either number 1 OR number 2 ignition. Assuming a successful start (EGT rise and N3 > 50%), if the engine is then shut down, during a subsequent start, the FAFC will use the other ignition system. So if you sit at the gate and successfully start/shutdown the engine 100 times in a row, then 50 starts will be on sys 1 and 50 starts will be on sys 2. If on the 101st start attempt the engine fails to light, during the next attempt, the FAFC will activate BOTH igniters.
How does the FAFC determine "ground" ?
My notes tell me that the ignitors alternate after each "ground start" and that the ADC provides the air-ground signal
You might be in the incorrect part of your notes. FAFC assumes "ground" if MACH is less than 0.2. Nil PSEU inputs at all.
Below from the Manual,
"Igniter Selection Logic is part of the Autoignition Logic. If the
airplane is in flight (FAFC calculated Mach number greater than 0.2)
or a second ground Autostart attempt from a failure of the first is
being made, both Igniters are always selected."
As for Flap and NAI inputs, if the switch is left in NORM, then "neither" igniter is activated regardless of flap/NAI. If however, the switch is in 1, 2 or BOTH then the selected ignition will be activated with flap/NAI selection.
"If the Auto Ignition Switch is in NORM, external automatic control
of the Engine Ignition Systems is inhibited because there is no
ground for R7738 through the Switch.
The Switch must be in 1, 2, or BOTH, to enable external automatic
control of the Ignition system and to select which Ignition Unit(s)
will be powered."
Even though ignition doesn't come on with flap/NAI with switch in NORM, the FAFC is always monitoring the engine for flameout/stall and will command BOTH igniters ON if decelleration rate is too low and/or N3 RPM drops below idle range.
"Regardless of the position of the Auto Ignition Switch, if the FAFC
detects an Engine flameout, it immediately arms the Auto Ignition
Logic.
The FAFC always uses both Ignition Systems whenever a flameout is
detected, regardless of the position of the Auto Ignition Switch."
Clear as mud!!
.