Date: 201205
Local time of day: 1801-2400
Flight conditions: Mixed
Weather elements / visibility: Rain; Thunderstorm; Turbulence
Light: Daylight
Aircraft: A321
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Air Data Computer
Component problem: Design; Malfunctioning
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Human-Machine Interface
Anomaly: Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Weather / Turbulence
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: General Maintenance Action
Contributing factors: Aircraft; Weather
Primary problem: Ambiguous

Narrative: We were flying a heavily loaded A321 at FL350 in an effort to stay on top of the clouds.  The optimum altitude was FL340; and the max altitude was FL365.  This was a newer aircraft with enhanced instrumentation.  There was a line of thunderstorms in our path and we chose the best route to get between the thunderstorms and back on course.  As we started to go between two thunderstorms; the gap between them began to fill in and we found ourselves in IFR conditions in intermittent moderate rain and moderate turbulence.  The SAT was -53 and I didn't write the TAT down until later; but I think it was about -10; which was warm considering the very cold SAT and the fact we were at FL350.  I was flying and I was quite concerned about the plane's ability to maintain its airspeed and altitude in the moderate turbulence; and consequently I was directing most of my concentration towards maintaining control of the aircraft.  I was worried that the flight conditions might worsen and we would be forced to descend; possibly requiring declaration of an emergency should ATC be unable to immediately comply with our request.  While I was preoccupied with basic control; we got an ECAM chime.  The First Officer announced that we had a failed ADR2 [Air Data Reference].  About one second later the situation corrected itself.  In about another five seconds we got a second ECAM chime.  The First Officer announced it was the same problem.  It too corrected itself about a second later.  About five seconds later we got a third ECAM chime which was also failure of ADR2; and it quickly corrected itself.  During all of this my flight instruments continued to work normally; but I was now also preoccupied with how to deal with the possible loss of my flight instruments; should the situation worsen or start involving ADR1.  While I dealt with the turbulence; icing; and marginal operating envelope; the FO consulted the QRH; ECAM indications; and the supplemental ECAM booklet.  He could not find QRH guidance (I later found that he should have looked at immediate actions; loss of airspeed indications) but he did find the ECAM guidebook page for loss of ADR2; which directed him to select First Officer to ADR3; which he did.  I don't know for sure; but I suspect his selecting First Officer to ADR3 stopped us from getting additional ECAM cautions.  About two or three minutes later we exited the precipitation and the ADR2 problems stopped happening.  The First Officer by then he had reached the point in the supplemental ECAM guide where it directed us to turn off ADR2.  I asked him not to do that; as I didn't think there was a problem any longer.  I also asked him to reselect First Officer to ADR2 for the same reason.  We were in VFR clear of the weather by then.  He did as I asked and his instruments worked normally.  The balance of the trip was normal; except that selecting First Officer to ADR3 caused loss of CAT3 dual; and it would not come back. Probably super cooled icing over the ADR2 pitot and static ports.  We debriefed one another after the incident ended; and I feel that my main concern at the time was that I was determined to keep the wings level and the pitch and thrust constant; regardless of what might happen next.  The idea of selecting the autothrust push button on the fwd panel to off crossed my mind; as I didn't want any fluctuation of thrust; although up to that point FADEC had been working normally.  It should be noted that prior to entering the precipitation I selected the ignition on.  I chose not to select engine anti ice on; as the SAT was -53.  Should this happen again; I would choose to select the engine anti ice on as well.  The First Officer stated that he remembered similar scenarios from sim training and based on that training he was quick to select First Officer to ADR3.  In discussing what had happened with the First Officer; I was surprised to learn that he had lost all altitude and airspeed indications when the ADR2 failures occurred; as opposed to them working but providing erroneous information.  His airspeed and altitude sections on his PFD were blank.  I had been too preoccupied with making sure the plane stayed under control to actually look at what his panel displayed; and I relied on the First Officer to tell me what the ECAM's were for.

Callback: The reporter states that an extensive logbook entry was made but he has not been apprised of the maintenance findings.  He believes that current procedures and training are adequate for dealing with this situation but since the failure was intermittent and his instruments were not affected the QRH guidelines were not employed.  The autopilot and auto thrust remained engaged throughout the event.



Synopsis: A321 Captain experiences an ADR2 intermittent failure at FL350 in IMC while flying in the vicinity of thunderstorms.  After three ECAM warnings and self corrections the First Officer selected ADR3; which ended the ECAM warnings and returned his airspeed and altitude indications.  Once clear of the thunderstorms ADR2 is reselected and all indications are normal except CAT III is displayed in the FMA.