Airbus AP engagement and XPDR
 

Howdy, quick question that I havent been able to find an answer for and its wrecking my head.

Why does the TCAS/ATC selector, ie 1 or 2 have to correspond to the on side AP engagement. ie Capt is flying, AP1 will be used. On the Xpdr/TCAS box, its selected to ATC 1 (selector under stby/auto/on) when squawk code is inserted at the start of a leg.

It came up during line training when I was on the 320 but Ive totally forgotten. Maybe its just our company SOP but we are pretty well aligned with Airbus SOPs. Same procedure on our 330 fleet.

Cheers

AP1, Xpdr 1, Radar 1 etc.
 

I do not think it matters. It is only a way of ensuring that everything is alternated/cycled in sync with the AP.

It is in the Airbus SOP FCOM-PRO-NOR-SOP-Cockpit Preparation


The way i read it, if not rvsm select sys 1 regardless of which AP in use because only sys 1 is available if Emer elec config.

If RVSM, select the ATC corresponding to the AP in use. This is to ensure that Air traffic will get the "correct" altitude readout corresponding to which AP is in use ( assumming there is actually a difference between ADIRU 1 and 2 ). Eg, if AP 1 is selected and it levels off at FL 330 but ATC no 2 is selected and ADIRU 2 is reading FL 329. So in this case, air traffic will see you as levelling at FL 329 and not 330.

That's what i think. Was explained to me by my captain not too long ago.
Hope he's right on this :)