A320 ATC Transponder 1/2 with change of control
 

Hey guys,
I remember on my initial type rating the training captains telling us to change the autopilot over when changing control. Fair enough.
They also said to change the ATC transponder to the associated side of the PF I.e 1 for CPT or 2 for FO. I can't remember for the life of me why this was, something to do with the altimeter being the one sent to ATC or TCAS or something. Any ideas?

If the aircraft has two encoding altimeters, operating the changeover switch causes the second altimeter to transmit its altitude readout. It's only really relevant if the two altimeters show a difference.

It’s an RVSM requirement.

Could you expand on that, Safelife? A requirement for what? Two altimeters? For the PF to have his side transponder selected? An Airbus RVSM requirement?

ATC 1 gets its altitude data from ADIRS 1 while ATC 2 gets its altitude data from ADIRS 2. Also when you put AP1 on, the aircraft maintains selected altitude based on ADIRS 1 data and vice versa for AP2.

The problem is the following:
Flying at FL370 with PFD 1 showing exactly 37000ft and PFD 2 showing 300ft higher than PFD1 (obviously out of tolerance, not realistic)
AP1 is engaged and ATC2 is selected. On ATC radar scope your altitude will show as 37,300ft, and ATC will then have to ask you to stop altitude squawk and cancel RVSM and move traffic away or move you away.

If in this scenario, the pilot selected ATC 1 the altitude indicated on the ATC radar scope will be 37,000ft and the whole cancelling RVSM etc. will not happen. However, RVSM tolerance is still being exceeded in this extreme scenario.

I don't believe that's the case.

First of all, if the difference between your altimeters is more than 200ft in-flight, you should declare yourself as being unable to fly in RVSM airspace.

Second, the ATC should not have problem with your transponder indicating let's say FL372. They could ask you to switch to the other transponder/alt source for their convenience, but you would still be RVSM capable regardless.

To the OP: I wouldn't change AP for short hand-over of controls (e.g. when you do the briefing), but if you do switch it for a prolonged period of time, switching the transponder/alt source to the AP side is appropriate.

FlyingStone, I agree with the three points you raised. My example was a hypothetical case just to illustrate the importance of matching the ADIRS used by the FMGS and the ATC. If not matched, the altitude you are holding and the altitude that is shown the radar scope won't match.

C. Bloggs: Generic RVSM requirement to have XPDR coupled to whichever altimeter is feeding the active AP.

By 2005 the factory Airbus SOP did not have the requirement to set XPDR 1 vs. 2 according to PM/PF (on-side autopilot to be used accordingly). They favoured ATC1 always, electrical emerg conf being the reason I was given.

Later, around 2008, the following note was added:

It's in the fcom nor sop

Thank you all for your replies. That has helped me out a lot