AN SF340 FO RPT ABOUT BEING UNABLE TO MAINTAIN 16000 FT DUE TO RIME ICING AND LEAVING ASSIGNED ALT WITHOUT ATC CLRNC SE OF DJB; OH.

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I WAS THE PF ON FLT PIT TO LAN. WE WERE ASSIGNED 16000 FT MSL AS A FINAL ALT. BOTH THE CAPT AND I NOTED THE POOR CLB PERFORMANCE AND ALSO THE DETERIORATING AIRSPD ONCE LEVELED AT 16000 FT. MODERATE RIME ICE WAS BUILDING ON THE WING BLADES; HOWEVER; THE WING DEICE WAS KEEPING THE WINGS CLEAN. WE REQUESTED 14000 FT. ATC RESPONDED 'UNABLE' DUE TO OTHER TFC; AND ASSIGNED 15000 FT. ONCE AIRSPD BEGAN TO DROP AGAIN; 14000 FT WAS REQUESTED. WE ADDED PWR TO NO AVAIL. THE CAPT THEN STATED TO ATC; 'WE NEED LOWER.' AT THIS POINT THE ACFT BEGAN BUFFETING. AT THE CAPT'S COMMAND; I PUSHED THE YOKE FORWARD TO DECREASE THE ANGLE OF ATTACK AND DSNDED. WHILE THIS WAS HAPPENING; THE CAPT AGAIN STATED THAT WE WERE IN NEED OF A LOWER ALT AND DSNDING. ONCE LEVELING AT 14000 FT; AIRSPD INCREASED TO A MORE ACCEPTABLE LEVEL. THE FLT PROCEEDED WITHOUT INCIDENT. IN RETROSPECT; I BELIEVE THE CAPT MADE THE CORRECT DECISION WHEN SHE TOLD ME TO DSND. I WOULD HAVE DONE THE SAME THING. WE SHOULD HAVE REQUESTED LOWER SOONER AND MORE DIRECTLY BUT THE ONSET OF THE BUFFETING WAS QUICK AND OCCURRED WITHOUT ANY STALL WARNING INDICATIONS -- IT DID STOP ONCE THE DSCNT BEGAN; HOWEVER. I HAVE NEVER EXPERIENCED THAT TYPE OF SIT BEFORE IN ANY ACFT. WHILE IT APPEARED THE MAIN WINGS WERE CLR OF ICE; I DON'T KNOW WHAT THE TAIL SECTION LOOKED LIKE. ON OUR RETURN TRIP WE STAYED BELOW 14000 FT AND HAD NO PROBS. IT SHOULD BE NOTED THAT ANOTHER SF340 CREW HAD SIMILAR PROBS WHILE RETURNING FROM FNT TO PIT ON THE SAME DAY DURING THE SAME TIME FRAME. IN THE FUTURE; I WILL PAY MUCH CLOSER ATTN TO ICING CONDITIONS ON HOT; HUMID DAYS WITH HIGH GROSS WTS. I WILL ALSO ENSURE ATC CLRLY UNDERSTANDS AN IMMEDIATE NEED FOR AN ALT CHANGE AND WORK WITH THEM TO THE BEST OF MY ABILITY. SUPPLEMENTAL INFO FROM ACN 476511: IN THE FUTURE; I WOULD REQUEST A COURSE CHANGE PRIOR TO WAITING FOR ATC TO BE ABLE TO DSND ME AND I WOULD NOT ALLOW MY ACFT TO BECOME SO SLOW.