Aircraft Porpoising, Wing Rock and Rudder Kick could be a Servo Tach Problem

July 2013

The autopilot (A/P) gets blamed for many squawks, such as aircraft porpoising, oscillating reaction and wing rock, among others. However, the A/P may just be reacting to false readings it is receiving from the computers due to servo tach problems.

Collins 334C – ( ) & Sperry SM200 – ( ) servos are installed on Hawker, Beechcraft, Gulfstream and Falcon airframes. It is not uncommon for them to experience tach problems due to oil, dirt and brush dust collecting around the tachometer. This contamination is caused by hydraulic leaks, migrating grease from servo bearings and the dust that is produced by tach or motor brushes. Over time, this buildup causes the servos to send false readings to the computer resulting in A/P squawks.

Duncan Aviation can have these servos easily cleaned and squawks cleared in one day. For more information, contact your Duncan Aviation accessories customer account rep.


Jerry Cable Accessory Tech Rep Astra, Challenger, Citation, Embraer, Falcon, Global, Gulfstream, Hawker, King Air, Learjet, Westwind Lincoln, NE (LNK) +1 402.479.8112

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