LOS ANGELES, CA, USA
N470EV
Boeing 747-273C
The crew felt a rumble as the aircraft was accelerating in the climb-to-cruise segment, and subsequently found that the right wing inboard wing panel had been partially torn away. The captain elected to return to the airport where a landing was made with the inboard flaps, which showed secondary damage, retracted. An inspection showed trailing edge portions of the composite right inboard wing panel had separated and damaged the fore and mid right inboard flaps. Maintenance records revealed that cracking was detected in the forward outboard corner during the last C check. The crack was repaired, inspected, and repainted. The previous C check also found disbonding and delamination in the right wing panel. Boeing has received a total of 245 operator reports involving disbonding of this panel, which has separated from the aircraft in flight in 95 instances. The wing flaps are susceptible to secondary damage as a result of the panel separations. A service bulletin with five revisions has been issued. Operators have been directed to replace the coin tap sound inspection with an ultrasound inspection of the panels to improve the detection likelihood of a disbonding that has occurred below the panel surface. Prior to this incident, the operator chose to retain the coin tap sound inspection method rather than adopt the ultrasonic inspection.
On May 19, 1996, about 0440 hours Pacific daylight time, Quantas Flight 7554, a Boeing 747-273C freighter, N470EV, with a crew of three, experienced an in-flight composite skin disbonding of the right inboard wing fixed trailing edge upper panel while over the Pacific ocean. The aircraft sustained minor damage and the flight crew were uninjured. The aircraft was operated by Evergreen International Airlines, Inc., as a scheduled cargo flight under contractual agreement with Quantas Airlines, under 14 CFR Part 121 when the incident occurred. The flight originated from Los Angeles, California, at 0429. During departure, the crew felt a rumble as the aircraft was accelerating through 330 knots. The flight engineer checked his panel and was unable to identify the source of the rumble. After level off, the rumble decreased. The flight engineer attempted to locate the source of the rumble, which he localized behind the R3 door on the main cargo deck. When the sun was rising an hour later, the captain walked back to see if he could find the source of the problem. As he looked at the right wing through the aircraft window, he saw that the composite inboard wing panel had been partially torn away. The captain elected to return to Los Angeles International Airport where a successful heavy weight landing was made at 0916 with the inboard flaps retracted. An on-scene examination of the aircraft by the Safety Board revealed that trailing edge portions of the composite right inboard wing panel, part number 65B11623, had separated. The panel separation further damaged the retracted fore and mid right inboard flaps. The inboard rod assembly and outboard rod assembly fitting fractured and separated. Two of the four bolts attaching the fitting to the landing air support beam had separated. The web of the number 3 adjustable rib exhibited horizontal and vertical tears. A review of the aircraft maintenance records revealed that the wing panel had been inspected during the fifth phase of a 6 phase C check at a British Airways maintenance facility. At the time of the inspection, cracking was detected in the forward outboard corner. On May 7, 1995, the crack was repaired, the required inspection items were carried out, and the panel was repainted. The C check was completed on July 30, 1995. The previous C check was conducted by HAECO, and during the fourth phase of that check on August 4, 1993, disbonding and delamination were found in the right wing panel. The panel was repaired on December 8, 1993. (The work cards for both repairs are appended to this report.) The records also showed a B check was completed on May 8, 1996, with no non-routine entries for the wing panel. A review of the structural history of the 747 fixed trailing edge upper wing panel was conducted. According to Boeing, 245 operator reports have been received involving disbonding of the panel from June 6, 1970, through the date of this incident. In 95 of those reports a section of the panel separated from the aircraft in flight. The data also shows that wing flaps are susceptible to secondary damage as a result of in-flight panel separations. During this period Boeing made a series of improvements consisting of core changes, increased plies and ply orientation, revised rigging instructions, and improvements to eliminate water ingression. These production revision records (PRR) applied to panel part number 65B11623. In 1977, the panel was redesigned as part number 65B22845 starting with aircraft production line number 353. In 1993, the panel was redesigned a second time as part number 113U1011. The incident aircraft, aircraft production line number 131, was equipped with the original design panel, part number 65B11623. Boeing stated that there have been no reports of disbonding involving part number 113U1011 in the 40 sets of panels that have been installed since mid 1994 to date. (A chronological history of the panel is appended to this report.) Boeing published service bulletin (SB) 747-57-2261 on March 7, 1991, which is still in effect and now contains five subsequent revisions. In revision 3, issued on April 15, 1993, the SB directed the operator, in part, to replace the coin tap sound inspection with an ultrasound inspection of the panels. The SB does not apply to aircraft equipped with panel part number 113U1011, or to aircraft production line numbers 1036, 1037, 1040 and later. (The summary page from the SB and each subsequent revision is appended to this report.) On July 13, 1994, Boeing issued a service letter to all operators with aircraft production line numbers 1 through 1035, 1038, and 1039. In the letter Boeing summarized the redesign of the wing panel and informed operators of a program that offers operators a 50 percent reimbursement for the new panel. (A copy of the service letter is appended to this report.) Boeing noted that the SB coin tap sound inspection, and now the ultrasonic inspections with revision 3, are intended to detect disbonding that has occurred below the panel surface. Boeing developed the following background information regarding this problem in 1995. They said that: "the presence of water inside the panel honeycomb core has proven to be a factor in panel losses. The expansion and contraction of waters freeze and thaw cycle breaks the bond between the skin and core, causing disbonds. Water typically enters at the fasteners common to the internal titanium doublers and support ribs. Water can also enter through a damage site." Boeing also stated that "the number 1 and 2 adjustable ribs are designed to flex, allowing the panel to deflect upward for the following conditions: The inboard trailing edge flaps impart an upload to the panel in the ground static mode, deflecting the panel upward at the side of body approximately 2 inches." Continuing, they stated "when the inboard trailing edge flaps are extended or retracted they push up on the panel." Prior to this incident, the operator chose to retain the coin tap sound inspection method rather than adopt the ultrasonic inspection prescribed in revision 3 to the SB.
the operator's failure to adopt a manufacturer's stipulated ultrasonic inspection procedure, which led to the in-flight separation of the wing panel.
Source: NTSB Aviation Accident Database
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