DC3 CARGO FLT ENCOUNTERS DSNDING AIR MASS RESULTING IN LOSS OF ALT.

Narrative

DURING CRUISE FLT AT 12000 FT; DUE TO WHAT I CAN ONLY GUESS AS A DSNDING AREA OF AIR; THE ACFT BEGAN TO LOSE AIRSPD WHILE TRYING TO MAINTAIN ALT. AFTER A FEW MINS OF THIS; THE ACFT HAD SLOWED FROM ABOUT 115 KTS TO ABOUT 90 KTS AND DSNDED TO ABOUT 11800 FT. IN HOPES OF SOON FLYING OUT OF THE DSNDING AIR; NO PWR WAS ADDED ABOVE THE 50 PERCENT CRUISE WHICH WAS ALREADY SET. THE WHOLE TIME; WE WERE FLYING IN AND OUT OF A SCATTERED LAYER OF CLOUDS. THEN WE FLEW THROUGH A CLOUD WHICH HAD IN IT MODERATE TURB OR WINDSHEAR; WE BEGAN TO LOSE MORE AIRSPD WHICH WAS CORRECTED FOR BY LOWERING THE NOSE; WHICH; IN TURN; SACRIFICED ANOTHER 200-300 FT OF ALT. BY NOW; ATC STARTED TO INQUIRE ABOUT OUR ALT (11500 FT) AND WARNED US OF ANOTHER ACFT THAT WAS BELOW US AT 11000 FT AND AT OUR 2-3 O'CLOCK POS; AND I THINK; 3 MI. AT ABOUT THIS TIME; WE EXITED THE CLOUD. THE CTLR THEN ASKED IF WE NEEDED TO DECLARE AN EMER. TO THIS I RESPONDED THAT YES; WE ARE DECLARING AN EMER. WE THEN ADDED MAX PWR AND WITHIN 1 MIN HAD CLBED BACK UP TO 12000 FT. AFTER NORMAL CRUISE AIRSPD WAS RECOVERED; WE SET NORMAL CRUISE PWR AND TOLD THE CTLR THAT THE PROB WAS SOLVED AND WE ARE RESCINDING THE EMER. ABOUT 20 MI FURTHER ALONG; AFTER WE HAD TIME TO REGROUP AND FIGURE OUT WHAT HAD HAPPENED; WE ISSUED A PIREP FOR TURB. THE FLT WAS CONTINUED TO OUR DEST WITHOUT FURTHER INCIDENT.