THE PLT OF A C550 WITH THE FMS REMOVED UTILIZED A VFR ONLY GPS FOR NAV AND TURNED TO AN INCORRECT HDG WHEN CLRED DIRECT TO A FIX.

Narrative

DEPARTING MMU; WE WERE VECTORED TO INTERCEPT THE COL VOR RADIAL LEADING TO WHITE INTXN. WE WERE SUBSEQUENTLY CLRED DIRECT WHITE. OUR NORMAL NAV SYS FMS WAS OUT FOR REPAIRS. HENCE WE WERE 'SLANT ALPHA EQUIP' (VOR/DME ONLY). UP UNTIL THIS POINT; ON PRIOR LEGS; WE WERE REPEATEDLY REMINDING CTLRS WE WERE 'SLANT A' AND HAD LIMITED ABILITY TO GO DIRECT; BUT COULD TAKE HDGS. AFTER A WHILE; THIS BECAME ANNOYING AND CTLRS WERE FRUSTRATED WITH US; SINCE THEY TEND TO ASSUME ALL JETS CAN GO DIRECT ANYWHERE. I DECIDED TO USE OUR BACK-UP GPS WHICH IS PANEL MOUNTED; BUT APPROVED ONLY FOR VFR USE. I DIALED UP WHITE INTXN AND PRESSED 'DIRECT.' I EITHER PULLED UP THE WRONG INTXN OR READ THE BEARING WRONG; AND TOLD THE PF TO TURN TO A WESTERLY HDG. AS WE TURNED; I REALIZED MY ERROR AND TOLD HIM TO BEGIN TURNING BACK SOUTHWARD. SIMULTANEOUSLY; THE CTLR TOLD US TO TURN TO 170 DEGS AND ASKED WHY WE'D TURNED W. HE GAVE NO TFC ALERTS AND WAS CALM. ALSO; TCASII GAVE NO WARNINGS AND SHOWED NO TFC IN OUR VICINITY. STILL I LOST SITUATIONAL AWARENESS AND IT COULD HAVE BEEN A PROB. CONTRIBUTING FACTORS WERE LACK OF NORMAL FMS CAPABILITY AND RELIANCE ON LONG NEGLECTED VOR/DME SKILLS; FATIGUE (EARLY MORNING WAKE-UP FOR FIRST LEG); AND RELIANCE ON A BACK-UP VFR GPS. I AM NOT USED TO USING A POSSIBLY OUT-OF-DATE DATABASE. ULTIMATELY; I SHOULD HAVE CONTINUED TO QUERY THE CTLR FOR HDGS UNTIL ABLE TO NAV WITH OUR 'SLANT A' LIMITATIONS (VOR/DME). CTLRS SHOULD ALSO NOTE EQUIP CODES AND TREAT ACFT ACCORDINGLY; LEST THEY ARE TEMPTED TO USE EQUIP THEY REALLY SHOULD NOT.