Narrative:

I was captain and PF with line check airman (captain) in the right seat on MMMX chkout. Autoplt was on and we had been level at 16000 ft for several mins and deviating for numerous thunderstorms around the airport. We were being vectored to intercept the arrival east of ptj and given a descent when incident occurred. With autoplt on, descent to 12000 ft initiated. Aircraft was in 'open descent,' but climbed initially 200 ft. I noted this to PNF and monitored situation. Aircraft leveled off after initial 200 ft climb and started slight descent. Almost instantaneously, large updrraft occurred and aircraft pitched up to 10 degrees nose high and started to climb again. I immediately turned off the autoplt and forced the aircraft into a descent. At largest point of deviation, aircraft reached 16800 ft. Aggressive descent initiated and descent resumed with no other problems. Supplemental information from acn 623359: deviating around WX en route to ptj on arrival. Autoplt on in descent airplane climbed 900 ft. Disconnected autoplt, still unable to stop climb. Unable to notify ATC due to frequency congestion. WX was developing quickly with precipitation at the airport. No traffic conflicts and ATC did not contact us about the altitude deviation. We were not able to contact ATC until we were descending again on the arrival.

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Original NASA ASRS Text

Title: ALTDEV ALT EXCURSION OF ABOUT 900 FT WHEN AN A320 ON APCH DURING A NIGHT OP ENCOUNTERS UNFAVORABLE WX CONDITIONS W OF MMMX.

Narrative: I WAS CAPT AND PF WITH LINE CHK AIRMAN (CAPT) IN THE R SEAT ON MMMX CHKOUT. AUTOPLT WAS ON AND WE HAD BEEN LEVEL AT 16000 FT FOR SEVERAL MINS AND DEVIATING FOR NUMEROUS TSTMS AROUND THE ARPT. WE WERE BEING VECTORED TO INTERCEPT THE ARR E OF PTJ AND GIVEN A DSCNT WHEN INCIDENT OCCURRED. WITH AUTOPLT ON, DSCNT TO 12000 FT INITIATED. ACFT WAS IN 'OPEN DSCNT,' BUT CLBED INITIALLY 200 FT. I NOTED THIS TO PNF AND MONITORED SIT. ACFT LEVELED OFF AFTER INITIAL 200 FT CLB AND STARTED SLIGHT DSCNT. ALMOST INSTANTANEOUSLY, LARGE UPDRRAFT OCCURRED AND ACFT PITCHED UP TO 10 DEGS NOSE HIGH AND STARTED TO CLB AGAIN. I IMMEDIATELY TURNED OFF THE AUTOPLT AND FORCED THE ACFT INTO A DSCNT. AT LARGEST POINT OF DEV, ACFT REACHED 16800 FT. AGGRESSIVE DSCNT INITIATED AND DSCNT RESUMED WITH NO OTHER PROBS. SUPPLEMENTAL INFO FROM ACN 623359: DEVIATING AROUND WX ENRTE TO PTJ ON ARR. AUTOPLT ON IN DSCNT AIRPLANE CLBED 900 FT. DISCONNECTED AUTOPLT, STILL UNABLE TO STOP CLB. UNABLE TO NOTIFY ATC DUE TO FREQ CONGESTION. WX WAS DEVELOPING QUICKLY WITH PRECIP AT THE ARPT. NO TFC CONFLICTS AND ATC DID NOT CONTACT US ABOUT THE ALTDEV. WE WERE NOT ABLE TO CONTACT ATC UNTIL WE WERE DSNDING AGAIN ON THE ARR.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.