Easyjet A320 near Brest on Jul 27th 2016, jammed trimmable horizontal stabilizer
Last Update: April 13, 2017 / 16:23:26 GMT/Zulu time
The French BEA reported the investigation into the serious incident has been delegated to the AAIB of the United Kingdom.
The aircraft remained on the ground for 29 hours before resuming service.
On Apr 13th 2017 the AAIB released their bulletin reporting that the aicraft was enroute at FL340 when a pitch oscillation occurred followed by the autopilot disconnecting and a master caution associated with an ECAM message "STAB JAM". The first officer took manual control of the aircraft while the captain carried out the ECAM actions. Following the ECAM actions requiring to move the stabilizer manually until the elevator was in a neutral position the captain moved the stabilizer trim wheel a little, the first officer stated however the aircraft was in trim, hence no further movement of the stabilizer trim wheel was done. As result of the fault the control law degradated to "Alternate Law", which reduced levels of protection and lost the use of the autopilot. The crew descended the aircraft below RVSM airspace and continued to destination. As designed the control law changed to "Direct Law" upon gear down selection, the crew continued for a safe landing.
The AAIB reported that according to the flight data recorder a handover between ELAC1 and ELAC2 (ELevator Aileron Computer) happened in rapid succession at the time of the pitch oscillation, the stabilizer position changed to 0.6 degrees more nose down compared to the setting prior to the upset and the elevator position changed to 2 degrees nose up. The stabilizer moved again 70 seconds after the upset, probably as result of the manual trim, but not sufficient to return the elevator to 0 degrees. The fact, that the stabilizer did move however suggests, that the stabilizer was not physically jammed.
The Trimmable Horizontal Stabilizer Actuator (THSA) was sent to the manufacturer for further analysis. The AAIB wrote: "They discovered that the mini reduction gear assembly was not driving the COM transducer, so the COM transducer could not sense the commanded stabiliser position. About 2 ml of water was found inside the mini reduction gear liner/cavity, which was an unusual finding. The output shaft of the mini reduction gear was found to be jammed and disassembly of the unit revealed damage to the gears. The COM transducer was opened which also revealed the unexpected presence of water. Corrosion was also found within the transducer housing. The seals and sealant used to seal the component were in satisfactory condition. However, a leak check of the seal was not conducted before the transducer was opened as the manufacturer was not expecting water to be present. According to the manufacturer this was the first instance of water ingression into this component."
The AAIB analysed:
According to the THSA manufacturer and the aircraft manufacturer the failure of the THSA was most probably due to water ingress into the THS COM transducer which then migrated into the mini reduction gear. The water in the mini reduction gear probably froze during flight and movement of the THSA caused torque on the gear to damage the mini reduction gear. The stabiliser would have moved as commanded and been sensed by the MON transducer but there would have been no movement sensed by the COM transducer; this discrepancy was detected and the system logic then prevented any further electrical commands to the THSA.
Although the source of the water and how it entered the transducer could not be determined, the aircraft manufacturer stated that the worst case scenario from water in this component is the one experienced by G-EZWX. Due to the system monitoring no increased attitude deviations would be expected, and the aircraft would remain fully controllable in ‘Alternate Law’ with manual stabiliser trim remaining available.
This article is published under license from Avherald.com. © of text by Avherald.com.
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