West Atlantic ATP at Jersey on Aug 18th 2020, difficulties with roll control
Last Update: April 19, 2021 / 10:30:52 GMT/Zulu time
On Apr 19th 2021 the UK AAIB released their bulletin concluding:
As the aircraft levelled-off at 2,000 ft and 200 KIAS, there were two uncommanded left rolls and the autopilot automatically disengaged. The crew found it harder to turn the control wheels to the right, but they maintained control of the aircraft and, although the flight director failed during the approach, they made an uneventful landing in Guernsey.
Extensive testing on the aircraft did not identify the cause, but the operator replaced several components as a precautionary measure. Subsequent component testing found no anomalies that could be definitively associated with the incident, although it did identify issues relating to equipment maintenance and testing. The operator has addressed these through appropriate safety action, and they reported that there been no recurrences since the aircraft returned to service.
The AAIB described the aircraft examination:
Initial examination and system testing was carried out under the supervision of the AAIB.
There was no evidence of anything untoward when the aircraft was checked externally and, with electrical power switched off, the control wheels were free to operate throughout their entire range of movement without any restrictions. The ailerons, balance tabs, and aileron trim were all observed to operate normally.
The rear avionics compartment, where the No 2 autopilot is located, was checked and found to be dry. All the equipment in the compartment was found to be secure.
The aileron control system mechanical runs were visually checked and operated throughout their range of movement. No anomalies were identified, and the control cable tensions were confirmed to be correct.
The autopilot, SCS and AHRS were functionally tested and no faults were apparent, although the right aileron synchro required a small adjustment to bring it into the limits defined in the Aircraft Maintenance Manual.
The operator decided to remove the following components as a precautionary measure before the aircraft was returned to flight:
- Aileron servo (serial number 391)
- Autopilot computer No 1 (serial number 311)
- Autopilot computer No 2 (serial number 218)
- Autopilot controller (serial number 158)
- Accelerometer (serial number 152)
After reviewing the FDR data and the CVR audio recording, AHRU 1 (serial number 1124) and AHRU 2 (serial number 1281) were also removed. In the meantime, the aircraft had been returned to service and flown without further incident.
The AAIB described that the laboratory examination of the components found no anomalies except for autopilot computer #2. The unit had been repaired in 2001 after a problem with overshooting selected headings. It then had been stored for 3 years, then fitted to another aircraft and in September 2010 fitted to SE-MAO. The AAIB wrote: "It was fitted to SE-MAO in September 2010 and the operator’s maintenance history showed that it had been transposed with the No 1 computer several times whilst troubleshooting various autopilot anomalies." The AAIB annotated that the overhaul agency tested the autopilot computer at normal ambient (room) temperatures and certify the unit airworthy if the tests were passed. Tests of the unit under supervision by the AAIB also did not find any malfunction.
The AAIB subsequently wrote with respect to the laboratory examination:
The printed circuit boards (PCBs) were removed and visual examination found localised corrosion, white translucent staining, and an accumulation of detritus on three of them. The affected PCBs were associated with the SCS logic and autopilot servo operation.
The manufacturer responsible for the autopilot design reviewed photographs of the PCBs and stated that, in their opinion, the corrosion was probably caused by trapped flux, possibly left after a previous repair. They noted an area adjacent to one corrosion site where there was a visible difference in the conformal coating. The repair records from 2001 were no longer available so the detail of the work carried out is unknown.
There was no requirement to check the internal condition of a computer returned for test or repair, so, under the normal test regime, the computer from SE-MAO would have been recertified as airworthy because it passed the requirements of the automated test.
The AAIB analysed:
Initial “jolts” and autopilot disengagement
The FDR data showed that the two unexpected left rolls occurred whilst the No 2 autopilot was engaged. Both rolls were preceded by the ailerons deflecting and the aircraft manufacturer determined that the rate of aileron deflection matched the performance capability of the aileron servo. When the second roll occurred, the autopilot disengaged with an audible continuous cavalry charge. This indicates that the autopilot logic detected a condition that met the requirements for the autopilot to disengage automatically. It was not possible to determine the reason for the automatic disengagement because no faults were identified during the investigation and the autopilot computer does not record a ‘fault log’.
Increased force when turning the control wheel to the right
The crew reported that it was harder to turn the control wheels to the right than to the left after the autopilot disengaged. They perceived that this force decreased as the aircraft slowed down, which might indicate an aerodynamic influence, but no anomalies were found in the aileron control system and the ailerons, balance tabs, and trim worked normally.
The flight data showed that the autopilot disengaged when the continuous ‘cavalry charge’ started, so the aileron servo clutch should have de-energised at that point. If the clutch failed to de-energise, the aileron servo would have remained connected to the control system, but any increased resistance caused by this should have been apparent in both directions of control wheel movement. The crew reported that the aileron out-of-trim indicator remained neutral after the autopilot disengaged, indicating that electrical power had probably been removed from the autopilot servo as designed.
When the servo was tested and dismantled, there was no evidence of any fault that could have caused or contributed to the incident, and the reason for the increased force to turn the control wheel could not be established.
Loss of the flight director
Flight director status and autopilot modes are not recorded on the FDR. No anomalies were identified during functional testing after the incident and the No 2 autopilot computer passed the automated test in accordance with the recertification requirements. The reason for the loss of the flight director modes during the approach to land at Guernsey could not be established. The operator reported that there have ‘not been any significant autopilot issues’ since the aircraft was returned into service after the incident.
No 2 autopilot computer and corrosion
The No 2 autopilot computer was active when the incident occurred and would have been responsible for automatic flight control, the SCS, and the flight director functions. The computer passed the requirements of the recertification test which was conducted at ambient temperature.
This article is published under license from Avherald.com. © of text by Avherald.com.
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