MLG ON PWRBACK HAS MAIN GEAR LEAVE RAMP AND SINK INTO DIRT.

Narrative

WE INITIATED A NORMAL PWRBACK. THE GUIDEMAN INFORMED US THE PWRBACK WOULD BE WITH THE TAIL TO OUR L. AS WE WERE ROLLING BACK -- THE CAPT INITIATED A TAIL L TURN ON THE GUIDEMAN'S COMMAND. THE GUIDEMAN THEN GAVE A STRAIGHT BACK SIGNAL. AFTER SEVERAL SECONDS I WAS BEGINNING TO GET UNCOMFORTABLE WITH HOW LONG WE HAD BEEN ROLLING BACKWARDS. AS I WAS ABOUT TO MENTION THIS; THE CAPT STARTED TO BRING THE THRUST REVERSERS FORWARD. I THEN QUICKLY GLANCED AT THE GUIDEMAN ON MY SIDE AND AT THE FRONT GUIDEMAN. NEITHER PERSON APPEARED AT ALL CONCERNED WITH OUR POS. AS I WAS LOOKING AT THE FRONT GUIDEMAN FOR SIGNALS; THE CAPT BROUGHT THE ENGS OUT OF REVERSE TO STOP OUR BACKWARD ROLL. ALMOST SIMULTANEOUSLY WITH THE THRUST REVERSERS CLOSING; THE FRONT GUIDEMAN TURNED AROUND TOWARD THE TERMINAL AND WE FELT THE R SIDE OF THE ACFT SINK. THE CAPT WAVED ONE OF THE WING-WALKERS OVER AND ASKED WHAT HAD HAPPENED. THE WING-WALKER STATED THAT THE R MAIN GEAR HAD GONE INTO THE DIRT. MY RECOMMENDATION TO AVOID THIS HAPPENING FOR THE 7TH TIME IS TO INITIATE FORWARD THRUST AS SOON AS THE ACFT NOSE CROSSES THE BIG WHIT LINE THAT DIVIDES THE RAMP AREA FROM THE TAXI AREA. THIS LINE IS APPROX 100 FT FROM THE JETBRIDGE AND WOULD PROVIDE PLENTY OF CLRNC FROM RAMP EQUIP AND OTHER ACFT. ALSO THE MAIN GEAR WOULD NEVER BE WITHIN 100 FT OF THE UNSTRESSED ASPHALT AREA.