Atlanta, GA, USA
B18701
BOEING 747
The airplane experienced a partial separation of the right inboard foreflap during approach to the airport after the crew selected flaps 30. The crew performed a go-around and landed uneventfully. Examination of the outboard fitting that attaches the right inboard foreflap to the sequence carriage revealed a fracture in the lug. A portion of the fracture had a slow growth region consistent with fatigue that caused the lug to fail under normal flight conditions. The failure of the lug resulted in a skewed condition of the right inboard foreflap and subsequent partial separation of the foreflap. Examination of the sequence carriages for both the right and left inboard foreflaps revealed several discrepancies with the bearings including incompatible parts, flat spots, play, binding, and roughness of operation. Due to several other prior events, the manufacturer previously recommended specific maintenance tasks on the foreflaps and the FAA issued an airworthiness directive for mandatory inspections. A review of the airplane's maintenance records revealed that the foreflap components had been inspected or lubricated three times in the preceding six months, the most recent occurring two weeks prior to the incident. The records showed that the airplane was in compliance with all required and recommended maintenance practices for the flaps. However, maintenance personnel failed to notice the fatigue cracking in the lug and the anomalies with the bearings on the sequence carriages during the dedicated foreflap maintenance tasks.
HISTORY OF FLIGHT On May 19, 2013, about 1245 eastern daylight time, China Airlines flight 5254, a Boeing 747-400F, registration number B18701, experienced a partial separation of the right inboard foreflap during approach to runway 27L at Hartsfield-Jackson International Airport, Atlanta, Georgia (ATL). The flight crew performed a go-around and landed uneventfully on runway 28 about 1258. Portions of the right inboard foreflap were recovered on buildings and the ground approximately 4 to 5 miles east of the airport. None of the 3 crewmembers on board, or anyone on the ground was injured. The airplane was registered to China Airlines, Ltd., and operated as a 14 Code of Federal Regulations Part 129 scheduled international cargo flight. Visual meteorological conditions prevailed for the approach, which operated on an instrument flight rules flight plan. The flight originated from Ted Stevens Anchorage International Airport (ANC), Anchorage, Alaska, about 0250 Alaska daylight time and was destined for ATL. DAMAGE TO AIRPLANE Examination of the airplane after landing revealed that most of the right inboard foreflap departed the airplane during the event. The pieces of the foreflap were recovered in the area below the approach path. The right side of the fuselage sustained 3 separate punctures. The forward puncture was located above the wing-to-body fairing and just below the lower side skin lap joint in the main cargo area at body station (BS) 1620. The center puncture was located above the aft end of the wing-to-body fairing in the lower cargo area at BS 1720 and had a section of the flap structure embedded in it. The aft puncture was located above the cargo door at the upper side skin lap joint in the main cargo area at BS 1750. The right horizontal stabilizer leading edge also had a puncture, but there was no indication of any spar damage to the stabilizer. The damage to the airplane did not qualify as substantial damage under NTSB regulations. AIRCRAFT INFORMATION The B747-400F is equipped with two trailing edge flaps and 14 leading edge flaps on each wing for lift augmentation at lower airspeeds. The inboard trailing edge foreflaps are attached to the main flap tracks at two locations and to the midflap at three locations. At the flap track attach points (inboard and outboard) there is a sequence carriage attach fitting installed on the foreflap. The sequence carriage attach fitting is bolted to the aft end of a sequence carriage through a bearing. The sequence carriages roll along the flap tracks during flap operation. A toggle assembly is bolted to the forward end of the sequence carriage that engages a detent in the flap track at certain flap positions. Collectively the unit is known as a carriage toggle assembly. Boeing published service information states that skewed operation of the carriage toggle assemblies on the inboard foreflaps may cause damage to the foreflap and associated components in flight. This skewed operation is likely caused when the two carriage toggle assemblies per foreflap operate out of phase with each other. History indicates that the skewed condition can be caused by worn or broken detent rollers or binding of the various rollers on the carriage toggle assemblies or on the foreflap track rollers in the mid flap. In some instances, the damage caused by skewed operation may result in separation of the foreflap from the airplane. METEOROLOGICAL INFORMATION The ATL automated weather observing station was reporting winds from 130 at 9 knots, visibility of 7 statute miles and light rain at the time of the incident. The clouds were scattered at 1,000 feet above ground level (agl) and overcast at 10,000 feet agl. FLIGHT RECORDERS The cockpit voice recorder (CVR) and flight data recorder (FDR) were removed from the airplane and downloaded by the Aviation Safety Council of Taiwan. Based on the CVR transcript, the airplane was cleared to land about 1242 and the crew selected flaps 20 and lowered the landing gear. About 1245, the crew selected flaps 30 and about 6 seconds later, a low frequency roar was heard consistent with the separation of the right inboard foreflap. The crew performed a go-around and discussed the change in handling qualities of the airplane. About 1247, the control tower informed the crew that following aircraft had observed parts falling from the incident airplane. Air traffic control vectored the airplane around and the aircraft landed on runway 28 about 1258. Examination of the FDR data confirmed the timing of the event and showed no abnormalities with the airplane prior to the flap separation. WRECKAGE AND IMPACT INFORMATION The right outboard (No. 6) sequence carriage attach fitting on the foreflap was fractured at the bearing lug where it attaches to the No. 6 carriage toggle assembly. The forward clevis of the toggle assembly installed on the right inboard (No. 5) carriage toggle assembly was fractured where the detent roller was installed. The No. 6 sequence carriage attach fitting, No. 5 carriage toggle assembly, and the No. 6 carriage toggle assembly were retained for further investigation. During the repair of the airplane, China Airlines performed an inspection of the left inboard foreflap. Inspection revealed discrepancies with some of the bearings on the left outboard (No. 3) and left inboard (No. 4) carriage toggle assemblies. The No. 3 and No. 4 carriage toggle assemblies were removed from the airplane and retained for further investigation. TESTS AND RESEARCH The removed flap components from the airplane were examined under the direction of the NTSB at the Boeing Equipment Quality Analysis laboratory with representatives from Boeing and China Airlines present. The No. 3 carriage toggle assembly was intact with no obvious damage or deformation. The detent roller exhibited some lateral play but no binding when rotated and one of the toggle rollers exhibited some minor flat-spots. The No. 4 carriage toggle assembly was mostly intact, but the clevis arms at the detent roller exhibited damage and deformation on two distinct planes on the forward side. The assembly was marked in paint with "ASSY P/N 65B11598-45 S/N B18209". The detent roller exhibited flat-spots, significant play between the inner and outer races, and was seized preventing rotation. Two of the flight load rollers and one of the deadweight rollers exhibited some minor flat spots. One of the deadweight rollers was initially resistant to rotation but became free as it was worked. The No. 5 carriage toggle assembly had several areas of damage. The clevis arms at the detent roller were fractured and separated from the assembly along with the roller. The clevis fracture faces on the toggle were examined under a microscope and exhibited features consistent with ductile overstress separation. The detent roller was separated from the assembly but did not exhibit any noticeable damage. The outer races of the two flight load rollers were fractured and portions were separated. The remainder of the rollers (bearings) remained attached to the assembly. The attachment lugs for the two lower flight load rollers were both deformed outward. One of the deadweight rollers was fractured and the outer race was missing. The No. 6 carriage toggle assembly was intact with no obvious damage or deformation. The detent roller exhibited some lateral play and a portion of the seal was missing. The toggle rollers and the two flight load rollers exhibited some flat-spots. One deadweight roller and one toggle roller exhibited binding or roughness when rotated. One of the deadweight rollers exhibited some lateral play. One of the deadweight rollers was not the correct part number for the carriage toggle assembly. The No. 6 sequence carriage attach fitting was fractured through the lug that attaches to the detent roller on the carriage toggle assembly. The forward and aft fracture faces had considerable mechanical damage. The forward fracture face contained a "slow growth region consistent with fatigue spanning the width of the lug to a depth of approximately 0.30 inches". The remainder of the forward fracture and the aft fracture exhibited features consistent with ductile overstress separation. The fracture details indicated that the fracture began at the lug bore and progressed forward but the damage precluded identification of the specific initiation point. There was considerable corrosion pitting throughout the lug bore. There was fresh BMS 3-33 grease present in the lube passage and fitting. Material testing showed that the fitting was manufactured from 7075-T73 aluminum alloy in accordance with the drawing specifications. ADDITIONAL INFORMATION There is a long history of issues with the foreflaps, sequence carriages, and attach fittings on the B747-400 that have resulted in dedicated maintenance procedures, service letters, service bulletins (SB) and a Federal Aviation Administration (FAA) airworthiness directive (AD). On December 12, 1975, the FAA issued AD 75-20-05 to inspect and lubricate the foreflap mechanisms, and bearings to prevent separation of the foreflaps. China Airlines performed the most recent AD inspection and lubrication on January 28, 2013. Boeing recommended maintenance practices for the inboard foreflap carriage toggle assemblies and sequence carriage attach fittings include SB 747-27-2366 R2, 747-400 Maintenance Planning Data (MPD) Section 1-Item 12-050-00, MPD Section 1-Item 27-051-24, and MPD Section 2-Item 57-610-00. Examination of the maintenance records provided by China Airlines showed that two of the three recommended SB work packages and all of the MPD tasks had been performed on the incident airplane during three separate maintenance visits since November 2012. The most recent maintenance on the inboard foreflaps was the MPD lubrication task 12-050-00 performed on May 5, 2013. The SB work package 2 instructions, last performed in November 2012, outline specific attention to inspection of the sequence carriage attach fittings for cracks and migrated bearings. During the last MPD 57-610-00 inspection performed in November 2012, the No. 5 detent roller was found to be "worn out". As a result, the No. 5 carriage toggle assembly was replaced with an overhauled unit. Records provided by China Airlines indicated that the No. 4 carriage toggle assembly was replaced on the incident airplane in August 2005, for unknown reasons. The installed assembly was an overhauled unit and had the P/N and S/N painted on it during the overhaul. There were no records of replacement of the No. 3 and No. 6 carriage toggle assemblies since manufacture of the airplane. After the incident, the Civil Aeronautics Administration (CAA) of Taiwan performed an audit of the China Airlines maintenance program. Based on the audit, the CAA determined that the China Airlines maintenance program was in compliance with the Boeing MPD, all three work packages in SB 747-27-2366 R2 had been complied with, and the mechanics and inspectors were properly certificated in accordance with CAA regulations and in compliance with the approved China Airlines manuals. They did note a discrepancy in the recording of the overhaul on the No. 5 carriage toggle assembly although the overhaul was performed in accordance with the component maintenance manual.
Failure of the right inboard foreflap outboard fitting due to fatigue and bearing anomalies on the sequence carriages which resulted in the partial separation of the right inboard foreflap. Contributing to the incident was the failure of maintenance personnel to detect the fatigue cracking and bearing anomalies during dedicated inspections.
Source: NTSB Aviation Accident Database
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