WING FLAPS EXTENDED DURING FLT WITH THE FLAP HANDLE UP. COULD NOT BE RETRACTED.

Narrative

AS WE LEVELED AT 6000 FT THE ACFT STARTED TO ACCELERATE TO CRUISE SPD BUT THEN STARTED TO DECELERATE. FLAPS; GEAR AND PWR POS WAS CONFIRMED TO BE NORMAL. THEN THE FLAP STARTED TO MOVE TO 20 DEG POS. WE SLOWED THE ACFT TO FLAP SPD AND REQUESTED TO RETURN TO ST LOUIS. AS WE WERE TURNING FROM DOWNWIND TO BASE FLAPS EXTENDED TO 35 DEGS. WE LANDED AT ST LOUIS UNEVENTFULLY. ALL FLAP EXTENSIONS WERE UNCOMMANDED AND UNCTLABLE. WE WOULD LIKE TO RECOMMEND TO LOOK INTO THE POSSIBILITIES OF THIS HAPPENING AGAIN. WE DO BELIEVE THIS COULD HAVE BEEN EASILY A FATAL ACCIDENT IF THIS WAS TO HAPPEN DURING THE CRITICAL PHASE OF THE FLT. SUPPLEMENTAL INFO FROM ACN 242519: AT FIRST; FLAP AND GEAR INDICATED UP AND LOCKED; THEN FLAPS EXTENDED TO 20 DEGS. WE REQUESTED VECTORS TO STL AND SLOWED TO FLAP SPDS. ON DOWNWIND TO RWY 12R AT STL FLAPS EXTENDED TO 35 DEGS. IF THIS HAD OCCURRED DURING TKOF OR LNDG THE RESULTS COULD HAVE BEEN FATAL. RECOMMEND POSSIBLE CHKLIST CHANGE TO INCLUDE EMER GEAR AND FLAP RETRACTION PROCS; AND POSSIBLE SYS SAFEGUARDS TO PREVENT THIS FROM OCCURRING AGAIN. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR SAID THAT A VALVE; PROBABLY THE FLAP CTL VALVE; WAS FOUND TO BE LEAKING AND THE CAUSE OF THE UNCOMMANDED WING FLAP EXTENSION. THE FLAPS COULD NOT BE RETRACTED AT ANY TIME; EVEN AFTER LNDG. RPTR'S RECOLLECTION OF FLAP POS DURING THE ENTIRE FLT ARE HAZY BUT IT SEEMS THAT THE FLAPS EXTENDED IN STAGES. AT ONE TIME; THEY WERE ABLE TO MAINTAIN 3000 FT FOR A PERIOD. FULL FLAPS ARE 45 DEGS ON THIS AIRPLANE. HE ONLY REMEMBERS THE FLAPS GETTING TO 35 DEGS. HE HAD NO TROUBLE MAKING THE ARPT BUT IS CONCERNED THAT IF THIS HAD HAPPENED AFTER TKOF ON A HOT DAY IT WOULD HAVE BEEN CATASTROPHIC. HE SAID HE WAS TOLD THAT THIS HAS HAPPENED A NUMBER OF TIMES ON THIS ACFT.