RB211 FAFC Ignitor Selection (744)

[NSEU]

I've been trying to figure this out for a few days now.

My notes tell me that the ignitors alternate after each "ground start" and that the ADC provides the air-ground signal. However, I can find no links between the PSEU and the ADC which would tell the ADC that it was on the ground.

The only air/ground signal that the ADC's seem to get is during a CMC test (and that requires the CMC Ground Test Switch to be on)... shown as "FT" on the schematics.

Anyone have a better knowledge of this selection process?

Thanks.
Cheers.
NSEU

[mono]

At a guess airspeed rising then falling above a certain value. Typically 80 kts.

[gas path]

I think the (switching) control is entirely in the EPROM (logic) chips in the FAFC. The other external inputs being (T/E) flap position and NAI switch selection.

[aveng]

Could it be that you usually only start the engines on the ground - and each subsequant switch selection cycles the ignitors?

[Spanner Turner]

"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!!

.