What was found
ONCE ESTABLISHED IN CRUISE FLIGHT WITH ALTITUDE HOLD ENGAGED, THE S-TEC SYSTEM 55X AUTOPILOT FREQUENTLY BEGINS TO GENTLY PORPOISE IN PITCH. THIS GETS PROGRESSIVELY WORSE UNTIL THE AUTOPILOT CAUSES SERIOUS PITCH EXCURSIONS OF SEVERAL HUNDRED FEET PER MINUTE UP AND DOWN. THE ONLY SOLUTION FOUND IS TO DISENGAGE THE ALTITUDE HOLD MODE AND SELECT VERTICAL SPEED EQUALS ZERO. THE OTHER MAJOR PROBLEM NOTICED WITH SYSTEM 55X IS THAT DURING THE APPROACH TO LANDING, THE AUTOPILOT WILL PITCH DOWN ABRUPTLY WHEN THE FLAPS AND/OR GEAR ARE DEPLOYED. THIS CAN BE ALARMING, ESPECIALLY DURING AN INSTRUMENT APPROACH IN INSTRUMENT METEOROLOGICAL CONDITIONS.