Can Experience Inadvertent Disengagement Without Clear Annunciation

The FAA has issued a Special Airworthiness Information Bulletin (SAIB) concerning for certain Honeywell/Bendix King autopilots (AP). According to the SAIB, the autopilots can experience inadvertent disengagement without clear annunciation to the pilot and may also fail to engage with conflicting annunciation to the pilot. These failure conditions are the result of a resistor that can fail open in the autopilot computer.

The SAIB is the result of an open resistor (R259) found in the KCP 220 AP computer causing improper AP operation and annunciation to the pilot. Honeywell has determined that AP computers KC 190/191/192/193 will have the same failure effects as the KCP 220. These autopilot computers are associated with the KAP 100, KAP 150, KAP 150H, KFC 150, KFC, 275, and KFC 325 autopilot systems.

The following describes the potential effects when the resistor (R259) in any of the respective AP computers fails open:
If AP is not engaged with an opened resistor (R259); No Effect; If resistor (R259) fails open while the AP is engaged. pitch and roll servos will disengage, and AP Annunciator on the control panel will continue to illuminate and to indicate AP is engaged; If installed, AP Annunciator on the optional remote Annunciator Panel or EFIS EADI will continue to illuminate and to indicate AP is engaged. Aural Disengage Warnings will activate.

If the pilot engages the AP with resistor (R259) already failed open, pitch and roll servos will not engage;
AP Annunciator on the control panel will continue to illuminate and to indicate AP is engaged; If installed, AP Annunciator on the optional remote Annunciator Panel or EFIS EADI will continue to illuminate and to indicate AP is engaged; Aural Disengage Warning will not activate; Flight Director commands will indicate a deviation if the aircraft drifts away from the selected mode targets (lateral and/or vertical), and; Effects of open resistor R259 can be felt by pilot during preflight test. Pilot (s) will not feel any friction or resistance on the Yoke/Control Wheel.

Honeywell has issued alert service bulletins to inspect and modify the KC 190/191/192/193 and KCP 220 AP computer.

The FAA recommends that owners, operators, and maintenance personnel of the airplanes installed with the affected AP computer incorporate the appropriate Honeywell Bendix King Service Bulletin.

FMI: SAIB