Autopilot Altitude Hold Oscillation
A helicopter is equipped with a two-axis autopilot. During a test flight, the autopilot in altitude hold mode causes the helicopter to oscillate vertically with an amplitude of ±50 feet at a frequency of 0.5 Hz. The problem occurs only when the outside air temperature is above 30°C. The autopilot system uses a barometric altitude sensor and pitch/roll gyros. The maintenance manual provides a troubleshooting flowchart and a list of acceptable sensor tolerances. The helicopter is due for its annual inspection in 10 days. The operator reports that the autopilot was recently serviced for a pitch axis issue.
1. What could be causing the altitude hold oscillation, and how would you troubleshoot it?
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The oscillation could be due to a faulty barometric altitude sensor, a sluggish servo, or incorrect gain settings in the autopilot computer. The temperature dependency suggests a sensor or component that is sensitive to heat. Troubleshooting: 1) Check the barometric altitude sensor for proper output and compare with a known good source. 2) Test the servo response and check for binding or excessive play. 3) Verify the autopilot gain settings per the maintenance manual. 4) Since the pitch axis was recently serviced, check for any rigging errors or loose connections in the pitch control system that might affect altitude hold. 5) Perform a ground test with a heat source applied to the sensor to simulate high temperature.
2. If the sensor is found to be within tolerance but the oscillation persists, what other factors should be considered, and what action would you take?
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Other factors: 1) The servo motor may have a dead band or backlash that is temperature-sensitive. 2) The autopilot computer may have a software issue or incorrect configuration. 3) The helicopter's static system may have a leak that is affected by temperature. Actions: 1) Perform a detailed inspection of the servo and its linkages. 2) Check the static system for leaks using a pitot-static tester. 3) Consult the manufacturer for any known issues or software updates. 4) If no fault is found, consider adjusting the gain settings slightly, but only if permitted by the maintenance manual. 5) Document all findings and if the issue persists, the autopilot may need to be removed for bench testing.