FLAPS ON AN ACR MDT WOULD NOT EXTEND FOLLOWING A 2+ HOUR FLT WITH OAT -55 DEGREES C. AFTER WARM UP ON THE GND THE FLAPS OPERATED NORMALLY.

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ON FINAL APCH THE FLAPS DID NOT ACTIVATE. THE FLAPS ON THE MDT ARE ELECTRICALLY CTLED AND HYDRAULICALLY ACTIVATED. AS SUFFICIENT RWY WAS AVAILABLE; I CONTINUED THE APCH TO A NORMAL LNDG. ALL OTHER SYSTEMS USING THE SAME HYD SYS OPERATED NORMALLY. FLAP ACTIVATION FAILURES ON THIS ACFT ARE NOT UNCOMMON. AS WITH MANY OTHER SYS'S ON THIS ACFT; THE SYS MAY BE RETURNED TO NORMAL BY DEACTIVATING ALL ELECTRICS AND THEN CYCLING THE SYS. THIS PROC DID NOT REACTIVATE THE FLAPS. A CALL TO MAINT WAS INITIATED. DURING THE CALL MY F/O NOTICED THE FLAPS WERE NOW WORKING. ON PREVIOUS OCCASIONS THIS HAD OCCURRED AFTER LONG FLTS WHERE THE ACFT WAS SUBJECT TO LOW TEMPS. SINCE I HAD BEEN FLYING FOR OVER 2 HRS AT FL250 WITH AN OAT AT -55 DEGS C AND NOT IN VISIBLE MOISTURE; AND SINCE THE SYS NOW WORKED FINE; I ASSUMED THE COLD SOAK TO BE THE CAUSE. MAINT WAS NOT AVAILABLE ON SITE; SO I ELECTED NOT TO WRITE THE DISCREPANCY UP AS I CONSIDERED IT AN INTERMITTENT PROB THAT WARMER TEMPS HAD SOLVED. UPON RETURNING TO BASE; I WROTE THE PROB UP AS AN INFO ONLY ENTRY. FAA IS CONSIDERING CERTIFICATE ACTION BASED ON THEIR POS THAT I ACCEPTED FOR FLT AN UNAIRWORTHY ACFT. IN ADDITION; THEY CONSIDER THE WRITE UP UNACCEPTABLE. MY REMARKS CONCERNING THE FAA ARE HEARSAY AS I HAVE NOT BEEN APPROACHED BY THE FAA. MAINT CONFIRMED TO ME SUBSEQUENTLY THAT MY ASSESSMENT WAS CORRECT IN THAT APPARENTLY AN O-RING IN THE FLAP HYD DISTRIBUTION VALVE HAS A TENDENCY TO CONSTRICT ALLOWING HYD FLUID TO CIRCULATE THROUGH THE VALVE W/O ACTUATING THE FLAPS. THE MFR IS APPARENTLY AWARE OF THE PROB. THE CONSTRICTIONS ARE AS A RESULT OF COLD SOAK.