WOOD DALE, IL, USA
N530KF
Boeing B-727-61
A U. S. Marshals Service employee, who was seated on the left side, middle section of the airplane's passenger cabin, said, "I was observing the ground area looking down and aft across the wing. The wing flaps were partially deployed when I observed a large piece of the flap area to rise and leave the a/c and disappear. The flaps continued down and I could not see the particular area the piece left from, from my position any longer." On landing, it was discovered that a 6-foot long, 16-inch wide outboard section of the aft segment of the left inboard, segmented fowler flap was missing. The segment was located at a residence 2 miles west of the airport. An examination of the flap segment revealed that the anodized layer was incomplete on the segment spar and a required oxide layer was not present. The area showed corrosion and the degrading of the bond resulting in separation between the honeycomb and the aft side of the segment spar. The flap segment had been in service since its overhaul in October 1993 and had approximately 5,000 hours. Overhaul facility instructions did not specify a maximum area of the flap segment's honeycomb core that could be debonded during the overhaul. Manufacturer's repair manuals did specify a maximum size for repair of the honeycomb core in the inboard and outboard trailing edge fore flap, but none of the figures in the manual that was in effect at the time of the overhaul covered repair to the honeycomb core for the aft flap segment. A current version of the manufacturer's repair manuals specifies the maximum size for the repair of the trailing edge aft flap segment that includes the honeycomb core.
On October 17, 2000, at 1320 central daylight time, a Boeing 727-61, N530KF, operated as Department of Justice, United States Marshals Service Flight JUD530, was damaged when a portion of the airplane's left inboard flap departed the wing and struck a faring beneath the airplane's fuselage. The airplane was on approach to runway 09R (10,141 feet by 150 feet, dry, concrete) at O'Hare International Airport, Chicago, Illinois, at the time of the incident. The airplane landed without further incident. The 6-foot long, 16-inch wide, flap segment fell into the back yard of a residence in Wood Dale, Illinois. Visual meteorological conditions prevailed at the time of the incident. The 14 CFR Part 91, non-scheduled, public use domestic flight was operating on an IFR flight plan. The captain, first officer, flight engineer, 19 U. S. Marshals Service employees, and 86 federal prisoners reported no injuries during the incident. No injuries were reported by persons on the ground. The cross-country flight originated at Detroit, Michigan, at 1145, and was en route to Chicago, Illinois. A U. S. Marshals Service employee, who was seated on the left side, middle section of the airplane's passenger cabin, said, "I was observing the ground area looking down and aft across the wing. The wing flaps were partially deployed when I observed a large piece of the flap area to rise and leave the a/c and disappear. The flaps continued down and I could not see the particular area the piece left from, from my position any longer." Another U. S. Marshals Service employee, in the airplane's passenger cabin, said the airplane was approximately 500 feet above the ground preparing to land when he heard a loud thump on the left side of the airplane. "About five seconds later a prisoner advised me that part of the wing fell off. I then looked and saw that he was correct, and tried to signal for someone in the middle of the aircraft. I was advised that they were aware of the situation. We then landed with no further incident." On landing, it was discovered that a 6-foot long, 16-inch wide outboard section of the aft segment of the left inboard, segmented fowler flap was missing. A 10-inch long, 7-inch wide hole was found at a fairing location in the bottom left side of the airplane fuselage, below the left engine. A 5-inch long crack in the fuselage extended aft from the hole. The flap segment impacted into a woodpile in the back yard of a residence at 444 North Grove Street, in Wood Dale, approximately 2 miles west of O'Hare International Airport. The woodpile was approximately 100 feet west of the residence. A second piece of the flap, approximately 12 inches long and 6 inches wide, was located near the woodpile. The flap pieces were recovered by Federal Aviation Administration inspectors and taken to O'Hare International Airport. The recovered flap pieces and the remaining portion of the left inboard, segmented fowler flap were retained for further examination. The flap segment was examined at the National Transportation Safety Board Materials Laboratory, Washington, DC, on May 16, 2001. The examination revealed that the aft honeycomb wedge and upper and lower skins separated from the remainder of the flap by (1) separation of the bond between the honeycomb and the aft side of the spar, (2) separation of the bond between the upper and lower skins and the upper and lower spar flange surfaces, (3) fracture from the closure rib, (4) fracture of the honeycomb wedge and lower skin on the inboard side of the piece, and (5) peeling of the upper skin from the inboard section of the flap. The outboard closure rib was fractured at approximately mid-chord (location "F1") and at the inboard side of the trailing edge (location "F2") near the trailing edge attach point for the separated aft panel piece. The fracture features at locations "F1" and "F2" were examined using scanning electron microscopy. The fracture surfaces had many features corresponding to microstructural details mixed with some dimpled features, features typical of overstress fracture of a cast magnesium alloy. No evidence of fatigue or any other progressive crack growth was observed. The overall deformation adjacent to the fracture at "F1" was consistent with upward bending of the trailing edge relative to the leading edge. The overall deformation and fracture at "F2" was generally consistent with bending, where the upper surface was in tension. The bond was broken between the forward side of the separated aft panel piece and the aft side of the spar, and the honeycomb core and lower skin were fractured at location "F3". At the forward end of the separated aft panel piece, the lower skin was deformed downward. The forward edge of the upper skin was also deformed downward at locations near the outboard end and upward near the inboard end. The entire chordwise width of the upper skin aft of the spar at the inboard end was debonded from the honeycomb core and the upper flange and was deformed into a curl. The outboard end of the separated aft panel piece was crushed and soiled consistent with ground impact. The lower skin and honeycomb debonded from the lower and aft sides of the spar over a length of approximately 68 inches (out of a total skin span length of 172 inches) at the outboard end of the spar. The upper skin debonded from the upper side of the spar at the outboard end up to a seam located 134.5 inches from the outboard end. Portions of the spar surface in the debonded areas had a light gray shiny appearance. Other areas of the spar had a darker appearance with white deposits on the surface, features typical of corrosion of aluminum alloys (Figure 4). The corroded area was wider closer to the lower side of the spar. The debond area was dark across most of its surface, indicating corrosion of the spar in these areas (Figure 5). Adhesive remained attached to the spar in only a few areas, such as the area indicated in Figure 5. The debonded areas on the aft honeycomb wedge panel piece corresponding to the areas shown in Figures 4 and 5 are shown in Figure 6. Most of the foaming adhesive (green) between the honeycomb and the aft side of the spar remained attached to the honeycomb on the aft panel piece, but isolated areas of foaming adhesive remained attached to the spar, as shown in Figure 4. Measured at the cut, the total thickness of the foaming adhesive was approximately 0.25 inches. According to a representative of the manufacturer, the typical thickness of the foaming adhesive based on the Structural Repair Manual (SRM) is currently 0.05 to 0.2 inches. A few isolated areas of red-colored adhesive between the skin panels and the upper and lower surfaces of the spar were observed on the upper and lower surface of the spar, as shown in Figure 5, but most of the adhesive remained attached to the skins as shown in Figure 6. A closer view of the faying surface of the lower skin is shown in Figure 7. A line of rubbing marks was observed on the bonding surface of the lower skin as shown by the unlabeled arrows. This damage corresponds to rubbing contact with the lower aft edge of the spar, with motion of the skin in the forward and aft direction relative to the spar. Examination of the similar area where the upper skin had been bonded to the front spar revealed no evidence of rubbing. Non-uniform sanding marks, generally in arc forms, were observed along the entire length of the exposed upper, lower, and aft sides of the spar. A closer view of the aft surface of the spar showing typical marks is shown in Figure 8. The sanding marks appeared to be replicated on the foaming adhesive that remained with the honeycomb on the aft panel piece. Small areas of semi-transparent, yellow-colored primer were observed in some areas of the aft side of the spar, as shown in Figure 8. However, most of the primer appeared to remain bonded to the foaming adhesive on the aft panel piece. A yellow-colored material with a bubbled texture (appearing different from the yellow-colored primer) was observed around the exposed rivet and bolt heads on the exposed aft side of the spar. Arc-shaped sanding marks were not found in these areas with bubbled texture around the rivet and bolt heads. A portion of the spar where the green foaming adhesive remained attached to the spar was sectioned, mounted, polished, and examined using an optical metallograph. The mounted cross-section was next sputter-coated with gold and palladium and examined using scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS). The resulting cross-sections at the aft side of the spar are shown in Figures 9 and 10 as viewed using SEM. The resulting EDS spectra for each layer labeled in Figures 9 and 10 were as follows. The layer labeled "spar" had a major peak of aluminum and minor peaks of copper and magnesium. The layer labeled "oxide" had a major peak of aluminum and minor peaks of oxygen, copper, and carbon. The layer labeled "primer" had a major peak of carbon and minor peaks of aluminum and silicon. The layer labeled "foaming adhesive" had a major peak of silicon and minor peaks of sulfur and carbon. An oxide layer (indicative of anodization) of varying thickness was observed in some areas at the aft surface of the spar, but the oxide layer generally appeared fractured and debonded from the spar surface. For most of the cross-section, no oxide layer was observed as shown in Figure 10, and the primer appeared to be directly bonded to the spar surface. According to the airplane's maintenance records, the flaps were overhauled by Chromalloy Aircraft Structures, Wichita, Kansas, on October 9, 1993, and had approximately 5,000 hours time-in-service between the time of the overhaul and the time of the accident. The record states the flap was "disassembled and overhauled in accordance with SRM (Boeing Structural Repair Manual) 57.50.2 and HHR (a Chromalloy Aircraft Structures repair specification manual) 82.8." The repair specification HHR 82.8, dated August 19, 1993, is titled "Repair of Boeing 727 OUTB'D Fore Flap Assembly P/N 65-21631. In the current version of the Boeing SRM, SRM 57.50.2 is a parts list for the flaps. In the current version of the SRM (and in the version that was in effect at the time of the overhaul) the repair of the inboard and outboard trailing edge flaps are located in SRM 57.50.4. The specifications in HHR 82.8 describe the method for disassembly and reassembly of the flap, including debonding the skins, doublers, and the honeycomb core. After disassembly, the bonding surfaces should be abraded with aluminum oxide Scotch Brite metal conditioning pads to remove most of the adhesive. If aluminum surfaces are exposed in the abrading process, that portion must be treated by the phosphoric acid non-tank anodize method (corrosion protection). This specification does not indicate a maximum size of an area that can be debonded during the overhaul. However, a maximum size for repair of the honeycomb core in the inboard and outboard trailing edge fore flap was specified by Boeing SRM 57.50.4, figure 4 (the version in effect at the time of the overhaul), last revised January 1, 1981. According to that figure, the maximum suggested size of a repair that includes the honeycomb core is 21 inches spanwise and half the chordwise width of the flap. Also included in the figure, an alternative maximum suggested repair size is up to 2 inches chordwise across the entire span. None of the figures in SRM 57.50.4 that were in effect at the time of the overhaul cover repair to the honeycomb core for the aft flap segment. In the current version of SRM 57.50.4, the maximum size for the repair of the trailing edge aft flap segment that includes the honeycomb core (describer in figure 12) is 20 inches spanwise and half the chordwise width. No alternative sizes are suggested. The NTSB Materials Laboratory Factual Report is included as an addendum to this report.
Inadequate overhaul of the flap segment that allowed for corrosion and debonding that resulted in the flap segment departing the airplane during the approach. Factors relating to this incident were insufficient information provided to the overhaul facility by the airplane manufacturer.
Source: NTSB Aviation Accident Database
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