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FAA Issues Warning on Autopilots

February 13, 2006

The FAA has issued Special Airworthiness Information Bulletin (SAIB) SW-06-21 to "owners and operators of certain autopilot components that could exhibit a potential failure of the Turn Coordinator causing the [autopilot's] roll computer to drive the servo when not commanded...."

This SAIB warns that a failed turn coordinator or something as simple as a loose grounding strap from the turn coordinator's power supply can drive the autopilot to force the airplane into an uncommanded roll.  This also has ramifications for flight should the turn coordinator become inoperative or its circuit breaker pop.

The warning continues:  "If the power reference siginal is disconnected from the Turn Coordinator, the autopilot's roll computer produces drive signal....  When this failure occurs the autopilot might (if equipped) display...an "AP Ready" annunciation even though the roll axis is in a failed or misleading condition.  This failure may result in loss of control of the aircraft."

The FAA has "field data to support that when the Rate Gyro of the Turn Coordinator losses [sic] its power reference signal, the Roll Flight Guidance Computer would continuously drive the servo in a non-commanded direction.  The field data...identifies that this failure may occur in various autopilots."  The most common turn coordinator-referencing autopilots in Beechcraft are built by
S-TEC.

The SAIB lists three recommendations, condensed here:

Perform a preflight test to check all modes of the autopilot as contained in the Pilots Operating Handbook autopilot supplement.

Disconnect the autopilot and hand-fly the airplane if you see any deviation from expected flight path. 

Check the turn coordinator power reference for proper grounding during annual inspection.

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