While taxiing for departure, the crew of the Boeing 747 selected the flaps to the take-off position. As the flaps extended, the left outboard aileron deflected to the full down position. The aircraft returned to the gate for rectification.
Investigation revealed that the left aileron cable (AA-11), which connected the inboard aileron quadrant to the aileron cable drum at wing station (WS) 776.98, had failed immediately outboard of the cable drum. The adjacent cable (AB-13), which connected the outboard aileron quadrant to the aileron control drum, was frayed at a location consistent with having been in contact with the other cable. The aileron cable drum had four grooves to accommodate the four separate aileron cables which ran inboard and outboard from the drum and connected to the inboard and outboard aileron quadrants. Markers were installed at the WS767 and WS780 locations to provide visual guidance for the routing and attachment of the aileron cables to specific grooves on the cable drum.
The aileron control drum forward guide pin was bent and displayed evidence of abrasion from interference by the cables. There was also abrasion to the top two grooves of the cable drum. Further examination revealed that the two aileron cable markers (decals) attached to the aileron drum's inboard and outboard mounting brackets at WS767 and WS780 were installed incorrectly. The marker for WS767 was fitted at WS780 and vice versa.
The aircraft had been manufactured in 1983 and had operated 62,399 hours to the time of the incident. Since 2 June 1997, when both cables were changed due to wear, the aircraft had operated 1,022 hours. The appropriate dual certifications, for the aileron control system, had been carried out at that time.
The investigation determined that another aircraft in the operator's fleet had had the same aileron cables changed, due to fraying and wear, about 12 months prior to the incident. This aircraft was inspected and it was found that the aileron cable markers at WS767 and WS780 were also transposed.
The investigation also found that eight other aircraft, from various operators, had aileron cable markers incorrectly installed at the WS767 and WS780 locations. However, it was not possible to determine if the markers had been transposed during, or after, aircraft manufacture.
The transposition of the markers provided incorrect guidance for the installation of the cables to the control drum. The two top cables then interfered with each other and resulted in an accelerated wear rate and subsequent cable failure. As the cable control drum area was difficult to see, and the aileron system operated correctly during funtional testing, incorrect installation of the cables was difficult to detect.
- The markers which provided visual guidance for the installation of the aileron control cables were transposed.
- The aileron control cables were incorrectly installed.
- Accelerated wear of the cables resulted in premature failure.
As a result of the investigation into this occurrence, the Bureau of Air Safety Investigation issued the following Interim Recommendation on 16 September 1997:
The Bureau of Air Safety Investigation recommends that Boeing Commercial Airplane Group issue service information and appropriate corrective action to applicable B747 operators requiring fleet checks for:
- correct placement of aileron control cables on the cable drum located at WS776.98; and
- correct installation of aileron control position decals at WS767 and WS780".
The Boeing Commercial Airplane Group response, dated 13 November 1997 stated:
"The reference cover letter provided a copy of your report of an incident involving a 747-300 airplane, Serial Number 23029. The report indicates that one of the left aileron cables was broken during taxi prior to departure. The report stated that the aileron cable failures may be due to misrouting which may occur as a result of incorrectly installed markers. A check by the Australian Bureau of Air Safety Investigation found eight airplanes with incorrectly installed markers.
Your report recommended that Boeing initiate a fleet check for proper placement of the aileron control cables at Wing Station (WS) 776.98 and correct installation of aileron cable markers at WS767 and 780.
It has not been determined whether the incorrect marker installations occurred during or after manufacture. As a result of these findings, we plan to issue a service bulletin to 747 operators recommending fleet checks of all 747 airplanes prior to Line Position 1130, except Line Position 1122, for
- Correct routing of aileron control cables on the aileron cable drum located at wing station 777
- Correct installation and replacement as required, of aileron cable position markers at wing station 767 and wing station 780
Airplane Line Position 1130 was delivered in September 1997. Airplane Line Position 1122 was checked at Boeing. A review of the applicable engineering drawings shows that the marker installation and cable installation drawings are correct, and have always been correct. In addition, a check of airplanes in the factory and on the flight line indicates that both the markers and cables are currently being installed per drawing".
BASI response status: Closed - Accepted
Subsequent safety action
Boeing issued Service Bulletin 747-27-2367 (Flight controls - aileron - aileron control cable inspection and control cable marker inspection, replacement) on 25 June 1998.
As a result of the Bureau's interim recommendation, the US National Transportation Safety Board (NTSB) conducted an independent review of the circumstances of this occurrence and subsequently issued recommendation A-98-6 to the US Federal Aviation Administration (FAA) on 3 February 1998. This recommendation stated:
"The National Transportation Safety Board recommends that the Federal Aviation Administration:
Issue an airworthiness directive to require operators of Boeing 747 airplanes, produced before production line number 1130, to conduct a one-time inspection of the aileron control system to ensure correct routing of the aileron control cables on the aileron cable drum located at wing station (WS) 776.98 and correct installation of aileron cable decals at WS767 and WS780 at the earliest possible inspection interval".
|Date:||20 August 1997||Investigation status:||Completed|
|Time:||0940 hours EST|
|State:||Queensland||Occurrence type:||Flight control systems|
|Release date:||01 February 1999||Occurrence category:||Incident|
|Report status:||Final||Highest injury level:||None|
|Aircraft manufacturer||The Boeing Company|
|Type of operation||Air Transport High Capacity|
|Damage to aircraft||Minor|