RWY EXCURSION.

Narrative

WX AT LNDG: APPROX 20 OVCST 1/2 SF R22 RVR 55 TD 45 RO. WIND ESTIMATED 300 13620. RWY 22 BRAKING ACTION RPTED POOR BY MLG. APCH AND LNDG UNEVENTFUL. ACFT BROUGHT TO STOP WITH 2500 FT REMAINING. MEDIUM AUTOBRAKES AND REVERSE EPR 1.5 WERE USED. AT ABOUT 40 KTS FO DISENGAGED AUTOBRAKES AND MANUALLY BROUGHT ACFT TO A STOP. BRAKING ACTION WAS INDEED POOR. RWY SURFACE APPROX 60 FT EITHER SIDE OF CTRLINE WAS 1-2 INCHES OF SNOW OVER PATCHY ICE. CAPT TOOK ACFT TO MAKE 180 DEG TURN ON RWY AS DIRECTED BY TWR. HE TAXIED SLOWLY TO R EDGE OF RWY; THOUGH IT WAS DIFFICULT TO JUDGE JUST WHERE THAT WAS. THERE WAS 1-2 FT OF SNOW DRIFTS (UNPLOWED) COVERING THE 10-20 FT CLOSEST TO RWY EDGE LIGHTS. CAPT BEGAN L 180 DEG TURN AT MIN PWR AND TRAVELING VERY SLOWLY. TURN WAS NORMAL THROUGH 90 DEGS OF TURN. ACFT WAS MOVING SO SLOWLY CAPT FELT HE NEEDED PWR TO PREVENT ACFT FROM SIMPLY COMING TO A STOP. HE ADDED SMALL AMOUNT OF PWR ON R ENG. BY THE TIME THIS THROTTLE MOVEMENT RESULTED IN INCREASED THRUST ACFT HAD COMPLETED ABOUT 120 DEGS OF TURN. THE EFFECT OF THE ADDITIONAL THRUST WAS TO PROPEL THE ACFT ALONG THE LONGITUDINAL AXIS RATHER THAN ASSIST IN THE L TURN. THIS WAS BECAUSE BY NOW THE NOSE WHEEL HAD ENCOUNTERED 1-2 FT OF SNOW DRIFT ALONG THE OPPOSITE SIDE OF THE RWY. TRACTION UNDER THIS SNOW WAS EVEN WORSE THAN ON THE RWY CTRLINE; AND THE NOSE WHEEL TIRES COULD NOT HOLD THE TURN WITH THE ADDITIONAL THRUST. THE NOSE WHEELS DEPARTED THE RWY EDGE AFTER 120 DEGS OF TURN; COMING TO REST 10-15 FT FROM THE RWY EDGE. I BELIEVE IF THE RWY SURFACE UNDERNEATH THE DEEPER SNOW AT RWY EDGE HAD BEEN AS GOOD AS THE CTR PART OF THE RWY; THE TURN WOULD HAVE BEEN COMPLETED WITHOUT INCIDENT. WE HAD NO WAY OF KNOWING HOW SLIPPERY THE SURFACE WAS AT THE RWY EDGE. RWY DOWNSLOPING CROWN AND XWIND COULD HAVE BEEN CONTRIBUTORY FACTORS. THERE WERE NO PAX INJURIES AND THE ACFT WAS NOT DAMAGED. LESSONS: EXTREME CARE SHOULD BE USED WHEN ADDING PWR ON VERY SLICK SURFACES. MIGHT BE BETTER TO SHUT DOWN AND BE TOWED TO THE GATE. ALSO; IT MIGHT BE PRUDENT AS A PRECAUTION TO MAKE THE 180 DEG TURN ABEAM AN EXIT TAXIWAY IN CASE TRACTION IS LOST IN THE TURN. IF IT IS; ACFT MAY STOP ON HARD SURFACE WHERE IT CAN BE TOWED BACK ON RWY AND TO GATE.