A320 FADEC Question
 

I have a question.
How does FADEC alternate igniters and FADEC Channel after successive engine starts sequence?
FADEC A Ignition A →FADEC B Ignition A→ FADEC A Ignition B→FADEC B Ignition B?

The igniters A and B alternate exactly like you say.

As far as FADEC, the FCOM does not differentiate between FADEC A or B. They say there's FADEC (ECU) that has two channels. No more information, I am afraid.

DSC-70-80

ON THE GROUND
‐ Automatic start:
• During a first automatic start attempt only one igniter is supplied. The FADEC automatically alternates the igniters and/or channels used in successive start sequences.

As noted, the EEC channel alternates every engine start (assuming both channels are healthy - if faults are present there is logic that overrides the normal channel alternation logic).
Each channel alternates the igniter on subsequent starts. So the channels end up going A - B - A - B. But the igniters end up going 1-1-2-2-1-1-2-2....

Clear?

The OP asks how it is done.

I don't know the exact answer, but with pre-computer technology you could use mechanically latching relays in the starting circuits to alternate igniter set A, B, A, B etc for each start in sequence.

With today's FADECS it is a simple matter to register in the software which igniters were used for the last start and therefore use the other set for the next start.

The FADEC is a full blown computer so it's probably just software.

There is non-volatile memory (NVM) in each channel of the FADEC which stores various data including which channel was in control and which igniter was used on the previous start.
It's a simple matter for the FADEC to check NVM when it initializes and decide what to do for this start.
On the early generation FADECs, NVM was very limited (in fact overall memory was limited as well - the original PW2000 only had 24k of RAM memory per channel :eek:). But the current generation of FADECs have lots of RAM and NVM)

All the logic above is perfectly acceptable, however for this particular question there seems to be a different algorithm behind. The igniters as displayed on ENG page alternate A <-> B on each consecutive start.

So perhaps F(i)+A -> F(i)+B -> F(ii)+A -> F(ii)+B. (speculation).

I have no first hand knowledge of the Airbus since I worked for Boeing. So I can't definitively state how the A320 works. But all the Boeing installations work the same way - the FADEC channel alternates every engine start, assuming equal channel health (note one minor exception, some installations won't alternate channels if the previous engine run didn't go above idle), and each channel alternates igniters every engine start. So while the channels alternate every start, the igniters alternate every other start. To do otherwise would mean that each channel would end up using the same igniter every time, and a fault in one channels ability to use an igniter could go undetected indefinitely.
BTW, on Boeing installations, there is no flight deck indication of the igniter being used.

I've seen this. I was under the impression the igniters alternate every start. So FADEC channel should switch as FD says...?

In some cases the FADEC will use both igniters.

From FCOM DSC-70-80:

Airbus does show which igniter circuit(s) are in use.

Looking at the picture in the FCOM it could just be that both FADEC channels can use either igniters if required but channel A sticks to ignitor A and channel B sticks to ignitor B.

Channel A - Igniter A
Channel B - Igniter B
Channel A - Igniter A
Channel B - Igniter B ....... and so on

But both channels have access to both ignitors if required. This would cause equal use of both channels and igniters?