Narrative:

Were approaching mth from cre. At this point, our IFR clearance was 'direct mth' at 8000 ft MSL, and heading 190 degrees. Our WX storm scope was indicating the presence of convective activity in the area of the airport, the exact location of which was difficult to determine, but it appeared to be to the north of the airport. We requested a southwest deviation from ZMA, which was approved at our discretion. As we approached west of the mth area, which was where our deviation had taken us, I called ZMA to advise that we intended to continue on our southwest heading until west of mth and then turn into mth, and that I was not sure of the exact location of the storm -- that it was probably north of mth, but could possibly be over mth. He repeated that deviations at our discretion were approved. Progressively, we had been stepped down to 2000 ft MSL, and were then cleared discretion to 1500 ft. Since I saw marathon, and since no lightning had flashed in a while, I turned east, advising the controller of the turn. As we leveled, I noticed an obscuration of the island lights to the southeast, the cause of which I assumed to be rain. As we approached closer to mth, we encountered heavy rain and a downdraft that dropped us to 1200 ft MSL before I could stop the descent. Fearing a second and more severe downdraft, I initiated a left turn, away from the precipitation that I had seen to my right before we entered the rain, and initiated a climb to recover mth and land at eyw. We were still in heavy rain and turbulence, as I called mia to advise him that we needed to land at eyw. I was too busy trying to control the aircraft to give details such as my turn, and my flight conditions, and my altitude, which was now 2300 ft. He offered, and I accepted, a vector of 220 degrees to eyw. As we came out of the precipitation and turbulence, ZMA handed us to key west approach, who immediately challenged our altitude as being a deviation from what was assigned and to immediately descend to 1500 ft. I responded that we had just been dealing with some serious WX, implying that control of the aircraft had been difficult. He responded that I only had to advise the controller of the change. I only acknowledged the transmission, deciding not to get into the fact that I had called mia to advise my new intentions, and that beyond that, I was completely involved in controling the aircraft. Key west approach had told me that miami requested him to discuss the altitude with me, and I agree that a more complete report of my situation and responses to it was appropriate, but at the time, my complete attention was focused on controling the aircraft, and since 1500 ft is already close to the water in those circumstances, I wanted to make sure that I was ready to deal with whatever next happened.

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Original NASA ASRS Text

Title: PLT OF AN SMA SEL ACFT DEVIATED FROM ASSIGNED ALT DUE TO LACK OF ACFT CTL IN TSTM WX CONDITIONS.

Narrative: WERE APCHING MTH FROM CRE. AT THIS POINT, OUR IFR CLRNC WAS 'DIRECT MTH' AT 8000 FT MSL, AND HDG 190 DEGS. OUR WX STORM SCOPE WAS INDICATING THE PRESENCE OF CONVECTIVE ACTIVITY IN THE AREA OF THE ARPT, THE EXACT LOCATION OF WHICH WAS DIFFICULT TO DETERMINE, BUT IT APPEARED TO BE TO THE N OF THE ARPT. WE REQUESTED A SW DEV FROM ZMA, WHICH WAS APPROVED AT OUR DISCRETION. AS WE APCHED W OF THE MTH AREA, WHICH WAS WHERE OUR DEV HAD TAKEN US, I CALLED ZMA TO ADVISE THAT WE INTENDED TO CONTINUE ON OUR SW HDG UNTIL W OF MTH AND THEN TURN INTO MTH, AND THAT I WAS NOT SURE OF THE EXACT LOCATION OF THE STORM -- THAT IT WAS PROBABLY N OF MTH, BUT COULD POSSIBLY BE OVER MTH. HE REPEATED THAT DEVS AT OUR DISCRETION WERE APPROVED. PROGRESSIVELY, WE HAD BEEN STEPPED DOWN TO 2000 FT MSL, AND WERE THEN CLRED DISCRETION TO 1500 FT. SINCE I SAW MARATHON, AND SINCE NO LIGHTNING HAD FLASHED IN A WHILE, I TURNED E, ADVISING THE CTLR OF THE TURN. AS WE LEVELED, I NOTICED AN OBSCURATION OF THE ISLAND LIGHTS TO THE SE, THE CAUSE OF WHICH I ASSUMED TO BE RAIN. AS WE APCHED CLOSER TO MTH, WE ENCOUNTERED HVY RAIN AND A DOWNDRAFT THAT DROPPED US TO 1200 FT MSL BEFORE I COULD STOP THE DSCNT. FEARING A SECOND AND MORE SEVERE DOWNDRAFT, I INITIATED A L TURN, AWAY FROM THE PRECIPITATION THAT I HAD SEEN TO MY R BEFORE WE ENTERED THE RAIN, AND INITIATED A CLB TO RECOVER MTH AND LAND AT EYW. WE WERE STILL IN HVY RAIN AND TURB, AS I CALLED MIA TO ADVISE HIM THAT WE NEEDED TO LAND AT EYW. I WAS TOO BUSY TRYING TO CTL THE ACFT TO GIVE DETAILS SUCH AS MY TURN, AND MY FLT CONDITIONS, AND MY ALT, WHICH WAS NOW 2300 FT. HE OFFERED, AND I ACCEPTED, A VECTOR OF 220 DEGS TO EYW. AS WE CAME OUT OF THE PRECIPITATION AND TURB, ZMA HANDED US TO KEY WEST APCH, WHO IMMEDIATELY CHALLENGED OUR ALT AS BEING A DEV FROM WHAT WAS ASSIGNED AND TO IMMEDIATELY DSND TO 1500 FT. I RESPONDED THAT WE HAD JUST BEEN DEALING WITH SOME SERIOUS WX, IMPLYING THAT CTL OF THE ACFT HAD BEEN DIFFICULT. HE RESPONDED THAT I ONLY HAD TO ADVISE THE CTLR OF THE CHANGE. I ONLY ACKNOWLEDGED THE XMISSION, DECIDING NOT TO GET INTO THE FACT THAT I HAD CALLED MIA TO ADVISE MY NEW INTENTIONS, AND THAT BEYOND THAT, I WAS COMPLETELY INVOLVED IN CTLING THE ACFT. KEY WEST APCH HAD TOLD ME THAT MIAMI REQUESTED HIM TO DISCUSS THE ALT WITH ME, AND I AGREE THAT A MORE COMPLETE RPT OF MY SIT AND RESPONSES TO IT WAS APPROPRIATE, BUT AT THE TIME, MY COMPLETE ATTN WAS FOCUSED ON CTLING THE ACFT, AND SINCE 1500 FT IS ALREADY CLOSE TO THE WATER IN THOSE CIRCUMSTANCES, I WANTED TO MAKE SURE THAT I WAS READY TO DEAL WITH WHATEVER NEXT HAPPENED.

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.