SUBJECT: Boeing 747 Aileron Cable Failure
OCCURRENCE SUMMARY
The crew of the B747 reported that, when taxiing for departure,
the left outboard aileron deflected to the full down position when
the flaps were selected to the take-off setting. The aircraft
returned to the gate where investigation revealed that one of the
left aileron cables was broken and another was frayed.
ANALYSIS
The Boeing B747 aileron control system comprises a cable loop
system located on the wing rear spar (refer figure 1).
In this occurrence involving aircraft Serial Number 23029, the
left aileron control cable (AA-11) failed and an adjacent cable
(AB-13) was severely worn and frayed (see figure 2). It was
apparent that the two control cables were misaligned on the cable
drum (Part Number 65B80583-1) located at wing station WS776.98 on
the rear spar. The forward cable drum guide pin exhibited signs of
wear consistent with being abraded by a control cable. The aircraft
had completed 1,022 flight hours since both cables had been
replaced on 2 June 1997, approximately 11 weeks prior to the
occurrence.
Another of the operator's aircraft, Serial Number 23028, had the
AB-13 cable replaced due to excessive wear on 23 July 1996. This
cable was frayed down to one remaining strand (see figure 3).
The investigation has revealed that the two aileron control cable
decals on the cable drum inboard and outboard mounting brackets at
WS767 and WS780 on both aircraft had been incorrectly fitted. The
placard for WS767 was fitted at WS780 and vice versa. This
transposition results in incorrect cable location information at
the cable drum and may lead to incorrect cable positioning during
installation (see figures 4 and 5).
The investigation has also revealed that to date, eight aircraft
from various operators have had the cable decals incorrectly
fitted. Boeing in-service activities Report 92-05, (2 April 1992)
details similar cable wear occurrences to three other aircraft, and
attributes the cable wear to mis-routing of the cables. No
information is given on the aileron control cable decals.
It has not been determined whether the incorrect cable drum decal
installation had been carried out during or after aircraft
manufacture.
Additionally, Boeing issued Service Letter 747-SL-27-98-A (6 May
1991), which covered incorrect aileron control cable decals at
WS1336.971. It is therefore possible that incorrect decals are
located throughout the aileron cable control system.
SAFETY DEFICIENCY
The incorrect placement of the B747 aileron control cable location
decals on the cable drum located at WS776.98 has the potential to
cause misalignment of the aileron control cables during
installation, which can lead to a cable failure and a reduction in
the effectiveness of the aileron control system.
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:
(i) correct placement of aileron control cables on the cable drum
located at WS776.98; and
(ii) correct installation of aileron cable position decals at
WS767 and WS780.
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 misroutng 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 WS 767
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
1, Correct routing of aileron control cables on the aileron
cable drum located at wing station 777
2. 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.
Airlane 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.
If you have any questions, please do not hesitate to contact me
at any time.
ATSB Note: Airworthiness directive 99 - 11- 15 was issued by the
FAA which implemented the suggested action.
