An A321 Captain's AOA heater failure was alerted on the ECAM. After selecting ADR3 the FMGEC and Autopilot continued operating from ADR1 data and leveled of 300 FT above the FCU selected altitude.

Narrative

Climbout passing thru 20;000 FT; Captain flying; Autopilot 1 engaged; Altitude reporting 1 ON. ANTI-ICE CAPT AOA on ECAM. ECAM directs AIR DATA SWTG.....CAPT ON 3 Done. No QRH follow up. ECAM Non-Normal Supplemental says we should see the following status message on enhanced aircraft - INOP SYSTEMS: CAPT PITOT; L STAT; R STAT; CAPT AOA. What we actually saw on the status page after performing the ECAM action - INOP SYSTEMS CAT 3 DUAL CAPT AOA. Not what the book says we should see but more or less normal for the situation. At level off at FL350 the altimeters read as follows: CAPT 35;300; F/O 35;300; STBY 35;300. How can that be? A quick check of the altitude window in the FCU shows 35;000. Switched AIR DATA back to Normal. Altimeters now read: CAPT 35;000; F/O 35;300; STBY 35;300. Return switch to CAPT 3 and all three altimeters again read 35;300. Begin to suspect ADR 1. Switch to Autopilot 2 and descend so as to make all altimeters read 35;000. Get an almost immediate call from Center asking us to confirm altitude as they show us 300 FT low. Of course we suspect we initially leveled off 300 FT too high and are now at the correct altitude. Switched altitude reporting to 2 and problem with Air Traffic Control solved. Contacted dispatch via ACARS and continued to our destination. On descent we got numerous NAV ALTI DISCREPANCY warnings. The ECAM fix is AIR DATA switch - AS REQUIRED. We left it in CAPT ON 3 per previous ECAM because it appeared ADR 1 was 300 FT wrong. After landing ACARS printed maintenance message that read in part: Warnings XA15Z ADR 1; XA16Z F/CNTL; XA22Z NAV ALTI DISCREPANCY; XA23Z ANTI-ICE CAPT; AOA XD42Z NAV ALTI DISCREPANCY. At no time did we ever see an ADR 1 or F/CNTL message nor did any faults illuminate on the overhead panel Parked at gate. Briefed maintenance. The event occurred because the aircraft did not operate as it was supposed to. AIR DATA CAPT 3 should take ADR 1 out of the equation. It didn't. ADR 3 data was displayed on the Captain's PFD and exactly matched the First Officer's PFD and the ISS. But somehow the bad ADR 1 data was still used by the autopilot and altitude reporting systems. There is no provision for switching autopilots in either the ECAM procedure for CAPT AOA or; coincidentally; in the ADR 1 fault procedure. With the aircraft properly set up there should be no way the aircraft should be able to level off at an altitude not selected by the pilot and displayed on his PFD. But it did. Not very hazardous at 35;000 but at low altitude! First you've got to figure out what went wrong with the aircraft. Some ideas: Software within the ECAM decided the AOA problem was the reason for the ADR 1 fault and F/CNTL fault reported after the flight and suppressed them. There would have to be additional software that decided to leave ADR 1 in control of the autopilot despite the fact that CAPT 3 was selected. My understanding of the system is that what you see on your PFD and FCU is what you should get but not in this case. The Captain was looking at ADR 3 information but ADR 1 was driving the aircraft on autopilot 1 and we couldn't see ADR 1 information. And remember that CAT 3 DUAL message we saw on the status page but were not told to expect in the ECAM Supplemental manual? Interestingly; the QRH has a procedure that directs you to change autopilots and recycle FAC 1 and FAC 2. Unfortunately this procedure only applies if you have a CAT 3 DUAL INOP message with no other malfunctions so it would have been of no help to us. Request you get this information out as soon as possible to all pilots. For obvious reasons pilots need to know that under certain conditions; the aircraft may not fly to the altitude they select.