Aero-Tips!

A good pilot is always learning -- how many times have you heard this old standard throughout your flying career? There is no truer statement in all of flying (well, with the possible exception of "there are no old, bold pilots.") It's part of what makes aviation so exciting for all of us... just when you think you've seen it all, along comes a scenario you've never imagined.

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Look for our daily Aero-Tips segments, coming each day to you through the Aero-News Network. Suggestions for future Aero-Tips are always welcome, as are additions or discussion of each day's tips. Remember... when it comes to being good pilots, we're all in this together.

Aero-Tips 03.01.06

The FAA has issued Special Airworthiness Information Bulletin (SAIB) SW-06-21 concerning "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..."

NOTE:

• Turn coordinator-referencing autopilots include those built by S-TEC.
• The KAP 140 autopilot, very common and installed in current-production Cessna and Diamond airplanes, also references the turn coordinator.
• Older autopilots may also use a turn coordinator source.
• "Glass cockpit" airplanes with these autopilots use a hidden turn coordinator-like gyroscope that may suffer the same failure modes.

Check the Pilot's Operating Handbook autopilot supplement for the airplane you fly.

SAIB SW-06-21 warns that a failed turn coordinator or something as simple as a loose grounding strap from the turn coordinator's power supply can cause the autopilot command a roll.  This has ramifications also for flight if the turn coordinator becomes inoperative or its circuit breaker pop.

The warning continues:  "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 SAIB lists three recommendations, condensed here:

1. Perform a preflight test to check all modes of the autopilot as contained in the Pilots Operating Handbook autopilot supplement.
2. Disconnect the autopilot and hand-fly the airplane if you see any deviation from expected flight path. 
3. Check the turn coordinator power reference for proper grounding during annual inspection.

I’d add carefully observing proper turn coordinator operation during taxi, and active instrument cross-check in flight.

Aero-Tip of the day:  Know how your autopilot works, how to test it, and its failure modes.  Never let an autopilot take you anywhere you can’t immediately take over and hand-fly.

FMI: Aero-Tips