Narrative:

Thunderstorms were approaching iad from the west as we began an approach to runway 1L. We received a windshear report from a B777 in front of us on the approach when we were approximately 3000 ft and descending on the GS. The shear reported was in excess of 20 KTS and we elected to abandon the approach. We were above 2000 ft and iad approach gave us a heading and a clearance to 2000 ft for the miss. After initially accepting the clearance we asked for 3000 ft to avoid descending in the missed approach. We were given 3000 ft altitude clearance and the first officer selected autoplt on to reduce workload. The flight director FMA showed altitude after we had selected 3000 ft and pulled the altitude knob. The aircraft (in an apparent updraft) climbed to 3300 ft before first officer disconnected the autoplt and recaptured the cleared altitude manually. It was a very high workload time and we were deciding to divert to avoid the encroaching WX. Some of my concerns raised by this event: the 2500 ft GPWS alert was so loud it interrupted the first pilot report of the shear -- I had to request the controller to repeat the report. This cost valuable time and radio xmissions. The high volume of the GPWS alerts block out all radio traffic. The clearance to miss at 2000 ft seemed inappropriate given the height we initiated the miss. I should not have accepted the altitude portion of the clearance and immediately requested 3000 ft. Even a 'soft go around' is a handful when you are initiating a missed approach so close to the assigned altitude. The autoplt failing to hold altitude after what we both believed to be an appropriate selection and altitude maneuver added to our workload.

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Original NASA ASRS Text

Title: A320 FLT CREW RECEIVES A WINDSHEAR ALERT; EXECUTES A GAR; HAS AN ALTDEV AND DIVERTS TO ANOTHER ARPT.

Narrative: TSTMS WERE APCHING IAD FROM THE W AS WE BEGAN AN APCH TO RWY 1L. WE RECEIVED A WINDSHEAR REPORT FROM A B777 IN FRONT OF US ON THE APCH WHEN WE WERE APPROX 3000 FT AND DSNDING ON THE GS. THE SHEAR REPORTED WAS IN EXCESS OF 20 KTS AND WE ELECTED TO ABANDON THE APCH. WE WERE ABOVE 2000 FT AND IAD APCH GAVE US A HDG AND A CLRNC TO 2000 FT FOR THE MISS. AFTER INITIALLY ACCEPTING THE CLRNC WE ASKED FOR 3000 FT TO AVOID DSNDING IN THE MISSED APCH. WE WERE GIVEN 3000 FT ALT CLRNC AND THE FO SELECTED AUTOPLT ON TO REDUCE WORKLOAD. THE FLT DIRECTOR FMA SHOWED ALT AFTER WE HAD SELECTED 3000 FT AND PULLED THE ALT KNOB. THE ACFT (IN AN APPARENT UPDRAFT) CLBED TO 3300 FT BEFORE FO DISCONNECTED THE AUTOPLT AND RECAPTURED THE CLRED ALT MANUALLY. IT WAS A VERY HIGH WORKLOAD TIME AND WE WERE DECIDING TO DIVERT TO AVOID THE ENCROACHING WX. SOME OF MY CONCERNS RAISED BY THIS EVENT: THE 2500 FT GPWS ALERT WAS SO LOUD IT INTERRUPTED THE FIRST PLT REPORT OF THE SHEAR -- I HAD TO REQUEST THE CTLR TO REPEAT THE REPORT. THIS COST VALUABLE TIME AND RADIO XMISSIONS. THE HIGH VOLUME OF THE GPWS ALERTS BLOCK OUT ALL RADIO TFC. THE CLRNC TO MISS AT 2000 FT SEEMED INAPPROPRIATE GIVEN THE HEIGHT WE INITIATED THE MISS. I SHOULD NOT HAVE ACCEPTED THE ALT PORTION OF THE CLRNC AND IMMEDIATELY REQUESTED 3000 FT. EVEN A 'SOFT GAR' IS A HANDFUL WHEN YOU ARE INITIATING A MISSED APCH SO CLOSE TO THE ASSIGNED ALT. THE AUTOPLT FAILING TO HOLD ALT AFTER WHAT WE BOTH BELIEVED TO BE AN APPROPRIATE SELECTION AND ALT MANEUVER ADDED TO OUR WORKLOAD.

Data retrieved from NASA's ASRS site as of January 2009 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.