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 Technician's Bench:Troubleshooting a Roll Oscillation David Carabajal Tech. Rep. Troubleshooting an autopilot roll oscillation or wingrock can be a very involved process. There are several variables to consider when taking on such a task. The cause of such a problem can be attributed to anything from the autopilot computer, to the aircraft cabling, or anything in between. The most common cause of roll oscillations is a malfunctioning roll servo, which I will discuss in detail. The servo is not problematic, however the motor commutator of the servo tends to "carbon up" over time. Over time can be characterized as four to six years on average. Fortunately, this situation can be easily diagnosed and remedied. The most accurate way to check the functionality of a servo motor is to check the voltage at which the motor starts to rotate. This can be accomplished by applying DC voltage from a variable power supply to pins three and four of the servo. If the polarity of the voltage is reversed, it will only reverse the rotation of the motor. The voltage is then slowly brought up from zero until the servo motor starts to rotate. This can be observed by removing the servo cover and watching the shaft on the back of the motor. The capstain will not rotate because there is no voltage being applied to the solenoid. A 28 volt motor should start to turn at < 2 volts D.C. A 14 volt motor should start to rotate at < 1.5 volts D.C. Like all things, there is some tolerance in these values. If the motor turns at 300mv higher than the specification then it is out of tolerance, but probably not the source of the problem. One common misconception that needs clarification is when roll oscillation only occurs in the NAV mode, the servo is not the source of the problem. This is opposite of the truth. The autopilot computer sends smaller correction voltages to the servo motor in NAV mode than in any other mode. Therefore, if a servo has a high start up voltage it will first be observed in the NAV mode of the autopilot. |
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