Inappropriate Pitch Commands, Uncommanded Disconnects Noted

The FAA has issued a Special Airworthiness Information Bulletin due to an intermittent signal processing error in the Garmin GAD 43e being received from the Honeywell KFC 275 or KFC 325 autopilot system.

This only occurs in configurations of the G500/G600 Integrated Flight Deck System interfaced with the Honeywell KFC 275 or KFC 325 autopilot, utilizing a Garmin GAD 43e. Other configurations utilizing the GAD 43e do not appear to be susceptible. Also, G500/G600 configurations not utilizing a Garmin GAD 43e do not appear to be susceptible.

This issue affects the operation of the altitude capture (ALTC) and/or (VS) modes of the autopilot, potentially causing inappropriate pitch commands and/or unintentional disengagement of these modes. Other autopilot modes (including altitude hold or glideslope/glidepath) are not affected.

Garmin has determined the following behaviors may occur when altitude capture or vertical speed modes are in use:

• Premature engagement of altitude capture mode (ALTC) resulting in inappropriate pitch up or pitch down commands.
• Pitch commands in vertical speed (VS) mode that are not appropriate for the selected vertical speed.
• Unintentional disengagement of altitude capture (ALTC) or vertical speed (VS) modes.

The SAIB notes that altitude capture modes are annunciated on the PFD as white “ALT” or green “ALTC” text and vertical speed mode as green “VS” text.

Garmin has provided a solution to this concern with a software upgrade. This solution can be implemented by incorporating Garmin Mandatory Service Bulletins No. 1571 and No. 1570.

The FAA recommends incorporating the software fix per Garmin STC Service Bulletin 1570 and Garmin TSO Service Bulletin 1571. Prior to incorporating the software fix, the FAA recommends that the following precautions are taken as summarized from Garmin Service Advisory No. 1555.

Pilots are advised to:

• Monitor the autopilot/flight director for proper operation at all times.
• If inappropriate autopilot/flight director pitching is experienced, immediately disengage the autopilot/flight director and hand-fly the airplane.

Until the software update is completed to address this issue, the FAA recommends minimizing the use of VS and/or altitude capture (ALTC) modes as follows:

• Use pitch attitude hold or Indicated Airspeed mode (IAS) as an alternative to VS mode for climbs and descents.
• Avoid climbs or descents with altitude capture mode armed. Hand-fly the airplane to level off at the desired altitude, then engage altitude hold (ALT) mode at the desired altitude.
• Do not use VS/ALTC (altitude capture) on coupled, non-precision approaches. (ILS and LPV coupled approach operation is not affected by this issue).

FMI: SAIB