As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE ENROUTE FD WAS PITCHING UP AND DOWN CAUSING PLANE TO PORPISE. ALTITUDE VARIED BY AS MUCH AS +-150 FEET. TROUBLESHOT PROBLEM TO FLAP/FLIGHT DIRECTOR BIASING POTENTIOMETER. REMOVED RT. WING ACCESS PANEL TO REMOVE POTENTIOMETER. REMOVED OLD UNIT AND INSTALLED NEW UNIT. INSTALLED AND ADJUSTED IAW HAWKER 800XP MAINTENANCE MANUAL CHAPTER 34-22-60-501. FLIGHT DIRECTOR COMMAND BARS OPS CHECKED GOOD.