Narrative:

Cleared to intercept ILS runway 12, descend to 2000 ft. Aircraft on glide path, gear down 240 KIAS. Passing 3000 ft, we entered an area of tcb just north of a rain shower on a 100 degree heading. Entered light rain and turbulence. However, descent became a slight climb and airspeed increased approximately 30 KTS for about 1-2 mi. When clear of clouds, position was slightly north of localizer and full GS deflection high, 2000 ft. Field in sight. I turned to 150 degrees to avoid one additional cell, see if visual approach parameters could be attained. We had not been able to talk to approach control because of radio congestion to inform of our situation. As I turned back north and began thinking missed, approach control came on the radio to alert us of an aircraft 2-3 mi south, headed at us, avoiding same storm. We had TA warning, but no evasive actions were given or taken. We accomplished a missed approach, vectored left 270 degrees to visual approach and uneventful landing. During this 8-10 mins, the cell had grown from a 1 mi wide shower to a 5-6 mi wide level 4 thunderstorm. During missed, controller asked us what we had been doing. First officer responded captain using emergency authority/authorized to avoid WX. I should have been at 210 KIAS or so with some flaps down, when we hit the WX, but in hindsight would have overspd flaps. This storm looked like houston summer tcb's that we were avoiding the rain, but by no means expecting the upside of what I'm sure became one intense microburst. Supplemental information from acn 556026: tried to contact ATC to deviate right of course, but was unable due to frequency congestion. Captain exercised emergency authority/authorized and started a right turn to avoid WX.

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Original NASA ASRS Text

Title: A B739 CREW, ON APCH TO CANCUN, (MMUN) EXECUTED A GAR AFTER AVOIDING TSTM ACTIVITY, WHICH LEFT THEM HIGH AND THE RECIPIENT OF A TCASII TA.

Narrative: CLRED TO INTERCEPT ILS RWY 12, DSND TO 2000 FT. ACFT ON GLIDE PATH, GEAR DOWN 240 KIAS. PASSING 3000 FT, WE ENTERED AN AREA OF TCB JUST N OF A RAIN SHOWER ON A 100 DEG HDG. ENTERED LIGHT RAIN AND TURB. HOWEVER, DSCNT BECAME A SLIGHT CLB AND AIRSPD INCREASED APPROX 30 KTS FOR ABOUT 1-2 MI. WHEN CLR OF CLOUDS, POS WAS SLIGHTLY N OF LOC AND FULL GS DEFLECTION HIGH, 2000 FT. FIELD IN SIGHT. I TURNED TO 150 DEGS TO AVOID ONE ADDITIONAL CELL, SEE IF VISUAL APCH PARAMETERS COULD BE ATTAINED. WE HAD NOT BEEN ABLE TO TALK TO APCH CTL BECAUSE OF RADIO CONGESTION TO INFORM OF OUR SIT. AS I TURNED BACK N AND BEGAN THINKING MISSED, APCH CTL CAME ON THE RADIO TO ALERT US OF AN ACFT 2-3 MI S, HEADED AT US, AVOIDING SAME STORM. WE HAD TA WARNING, BUT NO EVASIVE ACTIONS WERE GIVEN OR TAKEN. WE ACCOMPLISHED A MISSED APCH, VECTORED L 270 DEGS TO VISUAL APCH AND UNEVENTFUL LNDG. DURING THIS 8-10 MINS, THE CELL HAD GROWN FROM A 1 MI WIDE SHOWER TO A 5-6 MI WIDE LEVEL 4 TSTM. DURING MISSED, CTLR ASKED US WHAT WE HAD BEEN DOING. FO RESPONDED CAPT USING EMER AUTH TO AVOID WX. I SHOULD HAVE BEEN AT 210 KIAS OR SO WITH SOME FLAPS DOWN, WHEN WE HIT THE WX, BUT IN HINDSIGHT WOULD HAVE OVERSPD FLAPS. THIS STORM LOOKED LIKE HOUSTON SUMMER TCB'S THAT WE WERE AVOIDING THE RAIN, BUT BY NO MEANS EXPECTING THE UPSIDE OF WHAT I'M SURE BECAME ONE INTENSE MICROBURST. SUPPLEMENTAL INFO FROM ACN 556026: TRIED TO CONTACT ATC TO DEVIATE R OF COURSE, BUT WAS UNABLE DUE TO FREQ CONGESTION. CAPT EXERCISED EMER AUTH AND STARTED A R TURN TO AVOID WX.

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.