B727 SUFFERS DAMAGE TO ITS 'A' HYD SYS FROM AN UNDETERMINED SOURCE AFTER LNDG AT DFW; TX.

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FERRY FLT TUL-DFW. 1 DEADHEADER ON BOARD. A TUL FLT CHK AIRMAN. UNEVENTFUL FLT TO DFW. FLEW ILS RWY 17C. WX 5000 FT BROKEN; 5 MI VISIBILITY. NORMAL APCH AND LNDG. AFTER CLRING THE RWY AND TAXIING A HOSE OR SOMETHING FROM THE L WHEEL AREA; WE HAD NORMAL COCKPIT INDICATIONS SO WE ELECTED TO CONTINUE TAXIING. WHILE WAITING TO CROSS RWY 17R THE FE SAID SYS 'A' HYD WAS LOW BUT STILL ABOVE THE MINIMUM 2.5 GALLONS. WE ONLY HAD A SHORT TAXI TO THE GATE AND EVERYTHING WAS NORMAL; SO WE ELECTED TO TAXI TO GATE. WE WROTE UP THAT SYS 'A' NEEDED SVCING. IT WAS STILL ABOVE THE MINIMUM 2.5 GALLONS AND WE WROTE UP THAT IT WAS RPTED THAT WE HAD A HOSE OR SOMETHING TRAILING THE ACFT. BEFORE WE COULD GET OUT OF THE SEAT; A MECH STUCK HIS HEAD IN THE DOOR AND ASKED IF WE HAD HIT A BIRD. WE SAID NO; AND WENT DOWN TO LOOK AT THE L WHEEL WELL. WE SAW THAT THE HYD SVC DOOR HAD BEEN CAVED IN AND THAT THERE WAS INDEED A HOSE TRAILING BEHIND THE ACFT. THERE WERE NO BIRD REMAINS AND THERE WAS NO DIFFERENT COLORED PAINT. THE ONLY THING WE HEARD THE ENTIRE FLT WAS A LOUD NOISE AT ABOUT 500 FT BUT WE THOUGHT IT WAS THE DEADHEADER SLAMMING THE LAVATORY DOOR. I CAN ONLY GUESS THAT THE SVC DOOR OPENED ON TOUCHDOWN THEN SLAMMED SHUT CAUSING THE DAMAGE.