Narrative:

Picked up passenger in natchez, ms, for a leg to oxford, ms. Had to overfly same SIGMET area. Called FSS for radar update, little rock radar OTS, no help. Planned to fly same route I just flew. After establishing contact with ARTCC asked for highest altitude available, either FL330 or FL350. Finally given FL290 and told it would be the final. Controllers said FL290 reported best ride, most acrs were going around rather than over. Storms had intensified, however, there was an area I felt I could circumnav and go over just as I had 30 mins earlier. No radar returns ahead and splitting 2 small WX areas 12-15 NM either side of the aircraft. All of a sudden, altitude hold wasn't holding and we began to climb. At 300 ft above assigned altitude I cancelled altitude hold, informed the ARTCC we were in the top of one (poor phraseology) and climbing. Controller immediately assigned FL310. Climbed slowed at FL303 and we continued to FL310. Within 5 mins, same occurred. I reported climbing and was immediately assigned block altitude FL310 through FL330. Out of FL320 normal climb established and had a smooth ride at FL330 over the rest of the area nnebound. After leveling at FL330 controller asked if I had climbed on my own to avoid WX. I said no (negative). I had no control of the situation. Radar indicated I was over the tops, obviously not high enough and I get caught in updrafts above the build-ups. ATC issued FL290 as final. Although I was topping the storm, I should have insisted on higher since I had just flown over the area. ATC immediately cleared higher in both instances without interfering with other aircraft. Question: why wasn't I given FL330? Why is ATC so quick to assign another altitude in unusual flight conditions? If I had been still climbing at FL310 I would not have attempted a level off. Not recommended procedure. ATC continual resistance to assign altitudes above FL290 to turboprop aircraft in any flight/WX conditions.

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Original NASA ASRS Text

Title: ALTDEV ALT EXCURSION WHILE IN PROX TO TSTM ACTIVITY IN A NIGHT OP.

Narrative: PICKED UP PAX IN NATCHEZ, MS, FOR A LEG TO OXFORD, MS. HAD TO OVERFLY SAME SIGMET AREA. CALLED FSS FOR RADAR UPDATE, LITTLE ROCK RADAR OTS, NO HELP. PLANNED TO FLY SAME RTE I JUST FLEW. AFTER ESTABLISHING CONTACT WITH ARTCC ASKED FOR HIGHEST ALT AVAILABLE, EITHER FL330 OR FL350. FINALLY GIVEN FL290 AND TOLD IT WOULD BE THE FINAL. CTLRS SAID FL290 RPTED BEST RIDE, MOST ACRS WERE GOING AROUND RATHER THAN OVER. STORMS HAD INTENSIFIED, HOWEVER, THERE WAS AN AREA I FELT I COULD CIRCUMNAV AND GO OVER JUST AS I HAD 30 MINS EARLIER. NO RADAR RETURNS AHEAD AND SPLITTING 2 SMALL WX AREAS 12-15 NM EITHER SIDE OF THE ACFT. ALL OF A SUDDEN, ALT HOLD WASN'T HOLDING AND WE BEGAN TO CLB. AT 300 FT ABOVE ASSIGNED ALT I CANCELLED ALT HOLD, INFORMED THE ARTCC WE WERE IN THE TOP OF ONE (POOR PHRASEOLOGY) AND CLBING. CTLR IMMEDIATELY ASSIGNED FL310. CLBED SLOWED AT FL303 AND WE CONTINUED TO FL310. WITHIN 5 MINS, SAME OCCURRED. I RPTED CLBING AND WAS IMMEDIATELY ASSIGNED BLOCK ALT FL310 THROUGH FL330. OUT OF FL320 NORMAL CLB ESTABLISHED AND HAD A SMOOTH RIDE AT FL330 OVER THE REST OF THE AREA NNEBOUND. AFTER LEVELING AT FL330 CTLR ASKED IF I HAD CLBED ON MY OWN TO AVOID WX. I SAID NO (NEGATIVE). I HAD NO CTL OF THE SIT. RADAR INDICATED I WAS OVER THE TOPS, OBVIOUSLY NOT HIGH ENOUGH AND I GET CAUGHT IN UPDRAFTS ABOVE THE BUILD-UPS. ATC ISSUED FL290 AS FINAL. ALTHOUGH I WAS TOPPING THE STORM, I SHOULD HAVE INSISTED ON HIGHER SINCE I HAD JUST FLOWN OVER THE AREA. ATC IMMEDIATELY CLRED HIGHER IN BOTH INSTANCES WITHOUT INTERFERING WITH OTHER ACFT. QUESTION: WHY WASN'T I GIVEN FL330? WHY IS ATC SO QUICK TO ASSIGN ANOTHER ALT IN UNUSUAL FLT CONDITIONS? IF I HAD BEEN STILL CLBING AT FL310 I WOULD NOT HAVE ATTEMPTED A LEVEL OFF. NOT RECOMMENDED PROC. ATC CONTINUAL RESISTANCE TO ASSIGN ALTS ABOVE FL290 TO TURBOPROP ACFT IN ANY FLT/WX CONDITIONS.

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.