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Summary

Summary

The aircraft had just commenced descent, and the pilot was trimming the elevator for the descent, when the pilot heard a loud noise and the elevator control pitched down. The pilot was able to arrest the aircraft's nose-down pitch, but was unable to trim out the nose-down forces. He managed to control the aircraft sufficiently to carry out a straight-in approach and landing. After landing, it was found that the elevator trim tab was jammed in the nose-down position. The trim tab was removed for investigation. X-ray examination showed that the threads in both the male and female screw assemblies were severely worn. The shaft was bent over its entire length and had jammed in a position that corresponded to the nose-down position of the tab. It is suspected that the male threads had disconnected from the female threads when the pilot turned the elevator trim wheel. The elevator trim tab was then unrestrained and entered a flutter mode that only ceased when the flutter forces bent and jammed the male end of the rod. The actuator assembly is required to be inspected for condition and security at each 100-hour inspection, and overhauled at either 1,000 hours or 3 years, whichever is sooner. The investigation disclosed that the elevator trim actuator had last been overhauled at 738 hours and 2 years 4 months prior to the incident. The last periodic inspection was accomplished 3 weeks and approximately 40 hours prior to the incident. When the actuator was last overhauled, the repair agency replaced the male shaft due to "wear in the shaft". The manufacturer's maintenance manual states that, should one thread require replacing, both the male and female threads must be replaced as a pair. The local spare parts supplier advised that they sell the male and female threads as separate components and their recent stock records showed that they had sold 27 male ends and three female ends. Another supplier confirmed this purchasing imbalance. There is a large cost differential between the male and female threads. SAFETY ACTION Local safety action As a result of this occurrence, the Civil Aviation Safety Authority has published Airworthiness Advisory Circular AAC1-111, dated 8/99, detailing the flutter problems that can be associated with inadequate maintenance of trim tab systems. The information has been published in the CASA Flight Safety magazine in order to give it a wider distribution than is possible through the Airworthiness Advisory Circular. ATSB safety action As a result of the investigation into this occurrence, the Australian Transport Safety Bureau (formerly BASI) issues interim recommendation IR19990187 to the Cessna Aircraft Corporation on December 16,1999. It is recommended that Cessna Aircraft Corporation take appropriate action to ensure that Cessna elevator trim-tab actuator component parts affected by this safety deficiency are recoded so that they cannot be purchased separately. In addition, it is recommended that Cessna Aircraft Corporation review comparable designed trim-tab actuators to ensure that they are not affected by this safety deficiency. Additionally, the ATSB identified the following safety deficiency: A number of incidents have occurred where the elevator trim-tab actuator jackscrew thread contact area of Cessna 300/400 series aircraft has deteriorated to a point where the threads either disengaged or "jumped" threads. This event appears to be the result of non-adherence by operators to the instructions in the manufacturer's technical manuals. This condition could result in control surface flutter or a jammed control surface, with a resultant loss of flight control. The ATSB has issued the following safety advisory notices on December 16, 1999: SAN19990188 The Civil Aviation Safety Authority should note the safety deficiency identified in this document and take appropriate action. SAN19990189 The US Federal Aviation Administration should note the safety deficiency identified in this document and take appropriate action. SAN19990218 The US National Transportation Safety Board is advised of the recommendation made to the aircraft manufacturer and safety advisory notices issued to the Civil Aviation Safety Authority and the US Federal Aviation Administration.

 
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