What was found
FLIGHT GUIDANCE PANEL ON GOING SAFETY OF FLIGHT ISSUE CONCERNING INADEQUATE DESIGN/DURABILITY OF CERTAIN COMPONENTS. FLIGHT GUIDANCE PANEL(FGP) EQUIPPED AIRCRAFT.OUR AIRCRAFT. OUR ORIGINAL FGP, ELECTRONIC ROTARY HDG,/SPEED/ALT. PRESELECT SWITCHES STARTED HAVING ISSUES JULY 2024 AS AN OEM UNIT, IT WAS REPLACED WITH A REPAIRED UNIT FROM THE MANUFACTURER SEPT. 2024, THEN AGAIN IN SEPT. 2025 WITH REPAIRED UNIT FROM THE MANUFACTURER DUE TO THE SAME CONDITION. THE PROBLEM WE EXPERIENCED WAS SHIFT LOWER IN ALTITUDE PRESELECTED ALTITUDE FROM A SET FL370 TO FL360. SECOND UNIT PRESELECT PRIOR TO DEPARTURE, AFTER RECEIVING IFR CLEARANCE TO 2000', WHILE WAITING FOR THE PASSENGERS WE WERE SEATED IN THE CABIN AND HEARD THE ALTITUDE ALERT WARNING CHIME. UPON CHECKING IT OUT WE FOUND THE PRESELECT HAD SHIFTED TO 1000' BY ITSELF SETTING OFF THE "ONE TO GO" DECENT WARNING. THE NEWER CHALLENGER 3500 ARE EQUIPPED WITH AUTO-THROTTLES WHICH ARE CONTROLLED BY THE SPEED CONTROL KNOB WHICH IS OF THE SAME DESIGN. CUSTOMER REPLACED FGP WITHIN LAST YEAR AND IS HAVING ISSUES WITH HIS HDG AND ALT SELECT KNOB AGAIN CAUSING ISSUES IN FLIGHT CONTROLLING THE AP. IS THERE ANY RELIABILITY IMPROVEMENT FOR THESE PANELS? IS IS A KNOWN ISSUE THAT COLLINS HAS BEEN WORKING WITH ROTARY SWITCH SUPPLIER. IN ADDITION, THIS IS A TOPIC ON BOMBARDIER EXECUTIVE MEETING. IMPLEMENTED PREVENTATIVE MAINTENANCE? TO REPLACE ALL 3 ROTARY SWITCHES? AT EACH REPAIR TO ELIMINATE NFF’S?.