WESTWIND II ANTI-SKID FAILS ON ONE WHEEL. ACFT VEERS OFF RWY.

Narrative

WE FLEW A VISUAL APCH AND BACKED UP WITH THE ILS TO RWY 16 AT PWK. EVERYTHING WAS NORMAL; WE TOUCHED DOWN ON THE CTRLINE IN THE TOUCHDOWN ZONE JUST BELOW OUR VREF SPD OF 128 KTS; HOWEVER; WHEN THE BRAKES WERE APPLIED THE ACFT VEERED STRONGLY TO THE L. THE CAPT ATTEMPTED TO CORRECT USING THE NOSEWHEEL STEERING TILLER; BUT THE ACFT DEPARTED THE RWY TAKING OUT 2 RWY LIGHTS. I SECURED BOTH ENGS AND WHEN WE STOPPED; I EVACED THE ACFT WITH NO INJURIES. DAMAGE TO THE ACFT WAS MINOR. THE SUBSEQUENT INVESTIGATION REVEALED THAT THE R ANTI-SKID UNIT HAD BROKEN OFF INSIDE THE AXLE HUB AND WAS SENSING A LOCKED WHEEL. IN THIS SIT; NO BRAKE PRESSURE CAN BE APPLIED TO THAT WHEEL. WITH HVY BRAKING ON ONE WHEEL AND NONE ON THE OTHER THE WESTWIND II IS DIFFICULT TO CTL. HOWEVER; THERE IS MORE. THERE ARE PROCS IN THE WESTWIND MANUAL FOR DEALING WITH BRAKE FAILURES; BUT ONLY IF YOU KNOW YOU'VE HAD A FAILURE. THE ANNUNCIATOR SYS FOR THE ANTI-SKID SHOWED NO FAILURE ON THE BEFORE LNDG CHKLIST. THE INDICATOR LIGHTS ARE CHKED EVERY TIME THE GEAR IS LOWERED. HAD WE KNOWN THERE WAS A FAILURE IN THE ANTI- SKID SYS; WE WOULD HAVE LANDED ON THE LONGER RWY AT ORD. AFTER DISCUSSING THIS INCIDENT AT LENGTH WITH THE CAPT WE DECIDED WE WERE LUCKY IT HAPPENED WHEN AND WHERE IT DID. IF THIS HAD HAPPENED DURING AN ABORTED TKOF ON A SHORT RWY AT HVY WT; THE RESULTS COULD HAVE BEEN CATASTROPHIC. WESTWIND PLTS AND ALL PLTS BEWARE; WHEN IT HAPPENS; IT HAPPENS FAST!