On August 7, 1999, about 1345 central daylight time, a McDonnell Douglas DC-10-10F, N68058, registered to Wilmington Trust Company, operated by Federal Express Corporation as flight 3206, experienced separation of the left inboard trailing edge flap and flap vane while on final approach to land at the Memphis International Airport, Memphis, Tennessee. Visual meteorological conditions (VMC) prevailed at the time and an IFR flight plan was filed for the 14 CFR Part 119 domestic cargo flight. The airplane was substantially damaged and there were no reported injuries to the captain, first officer, second officer, or to the two jumpseat occupants. The flight originated about 0845 pacific daylight time from the Los Angeles International Airport, Los Angeles, California. Use your browsers 'back' function to return to synopsisReturn to Query Page
The first officer was flying the airplane, and while executing a Category 111B approach to runway 18L in VMC with the flaps fully extended, a loud "jolt" was heard. The autopilot was disconnected and the first officer reported having to use 1/2 to 3/4 right aileron input to counter the left banking tendency. The first officer later reported that while on a stabilized approach about 1,000 to 1,500 feet, about 3 miles from the airport, with the flaps extended to 50 degrees and both autopilots activated for a practice Category IIIB approach, he heard a loud sound from the rear of the airplane and the autopilot "struggled to keep the aircraft straight and level." He disconnected the autopilot system and reported that the airplane "wanted to roll left. I countered with right wing down control inputs. About 3/4 of max kept the aircraft straight and level." He continued the approach and landed. After clearing the runway he noted that the flap disagree on the control surfaces instrument, and all engines were secured. The captain later reported that after hearing the jolt, the Category IIIB approach was aborted. The landing was firm with less "controllability", but the rollout was normal.
According to a transcription of communications with Memphis Air Traffic Control Tower (ATCT), the first report by the flight crew to ATCT personnel occurred after the airplane had landed. A copy of the transcriptions of communications is an attachment to this report.
Examination of the airplane by FAA personnel revealed that the left inboard trailing edge flap and vane separated from the airplane. The separated left inboard trailing edge flap and vane were found in a residential area approximately 4 miles north of the airport. The vane was noted to be missing an approximate 2-foot segment, which was found several days after the accident about 1 block from where the flap and vane were found. Further examination of the airplane revealed that the four bolts (1 each P/N BM 3306-7-44 and 3 each P/N BM3306-8-44) of the forward attach point of the outboard hinge of the left inboard trailing edge flap, were failed. Six segments of these failed four bolts were recovered and marked "1-4", and also marked whether it was from the flap or hinge side. The two bolts (1 each P/N BM 3306-7-56 and 1 each P/N BM3306-7-54) of the aft attach point of the outboard hinge of the left inboard trailing edge flap, were fractured. Four segments of these fractured two bolts were recovered and marked "5-6", and also whether it was from the flap or hinge side. The left inboard actuator of the left inboard trailing edge flap was extended 3 inches; the right inboard actuator of the right inboard trailing edge flap was extended 15 inches. Damage to seven frames on the left side of the fuselage starting at fuselage station 1561 and ending at fuselage station 1681 was noted; also six longerons were damaged. The NTSB retained the recovered 10 segments of the six failed or fractured bolts, the flight data recorder, and the inboard flap actuator for the left inboard trailing edge flap. The cockpit voice recorder was not retained for readout.
Metallurgical examination of the failed and fractured bolts was performed by the NTSB Materials Laboratory in Washington, D.C. The results of analysis of the fractured segments of bolts that were marked Nos. 5 and 6 which were from the aft attach point of the outboard hinge of the left inboard trailing edge flap revealed they exhibited fracture features associated with "overstress bending/tensile separations." The results of analysis of the failed segments of bolts that were marked as Nos. 1 through 4, which were from the forward attach point of the outboard hinge of the left inboard trailing edge flap showed minimal deformation. Intergranular stress corrosion cracking that initiated from corrosion pits was detected on three of the four bolts; stress corrosion cracking was detected on all of the four bolts marked 1-4. Additionally, X-ray energy dispersive spectroscopy (EDS) of the fracture origin area of bolt Nos. 1, 2, and 3 indicated the presence of chromium (Cr). No determination was made as to the reason the chromium was present in the fracture origin areas. No nickel was detected in the fracture origin area of bolt No. 1. The NTSB Materials Laboratory did not prepare a metallographic cross section of any of the bolts. A copy of the Materials Laboratory Factual Report is an attachment to this report.
Metallurgical examination of the failed and fractured bolts was also performed by Materials and Process Engineering division of Boeing-Long Beach. The results of analysis of examination of the six segments of bolts marked 1-4 indicate the failure was due to stress corrosion cracking. The results of analysis of examination of the four segments of bolts marked 5-6 indicated failure was due to ductile overload. The report also indicates no evidence of material or processing discrepancies were found. SEM analysis of a metallographic specimen mount in plastic was performed on the bolt marked No. 1. The examination determined there were two distinct layers. The inner layer was comprised of a high concentration of Nickel (Ni), and the outer layer was comprised of a high concentration of Cadmium (Cd). No evidence of a layer of chromium plating was found. The report confirms the NTSB's report of the finding of Chromium (Cr) in the fracture origin of bolt No. 1. According to Boeing personnel, they do not allow or approve reworking of the accident type bolts. A copy of the report is an attachment to this report.
On October 16, 1990, McDonnell Douglas issued an All Operator Letter (AOL) 10-2008B to operators of DC-10 and KC-10 type airplanes stating that H-11 steel tension bolts have been susceptible to stress corrosion failures and McDonnell Douglas had conducted a design review; the AOL provided locations where H-11 steel tension head bolts are installed. The AOL indicated that, "Based on this evaluation, certain structural joint locations were identified where a change in bolt material to one made from a corrosion resistant steel was recommended." The AOL also indicated that, "In H-11 steel bolt/nut applications in which a single failure could affect aircraft safety or where in-service history indicates a need for replacement, Service Bulletins and AOL's have been issued to inform operators of each specific problem and to provide recommendations." The AOL listed the location of the failed and fractured H-11 bolts. At the time of the accident, there was no Service Bulletin in effect issued by either McDonnell Douglas or Boeing requiring the replacement of the H-11 bolts with Inconel type bolts in the failed or fractured flap bolt locations.
According to personnel from Federal Express, review of their FAA approved maintenance schedule pertaining to the inspection of the failed or fractured bolt locations revealed no inspection of the four forward hinge bolts of the outboard hinge of the inboard flap. Inspection of the lower two hinge bolts of the outboard hinge of the inboard flap occurs at the 2C interval.
The flight data recorder which was retained was read out by the NTSB Vehicle Recorders Laboratory in Washington, D.C. The read out indicated that seven takeoffs and eight landings including the accident landing, were recorded. The readout determined in part that the flap extension speed was exceeded for a total of 13 seconds during landing No. 1 when the flaps were extending between approximately 16 and 20 degrees. The time between the flap overspeed and the flap separation was approximately 21.8 hours. The read out also determined that after the flap separation, the maximum left roll was 8.09 degrees. At that time the right outboard aileron was deflected up 9.05 degrees and the left inboard aileron was deflected down 10.99 degrees. According to Boeing personnel, the inboard and outboard aileron travel is 20.2 and 20.0 degrees, respectively. Flap position was only recorded for the right flap. A copy of the Flight Data Recorder Factual Report is an attachment to this report.
According to the airplane Flight Crew Operating Manual pertaining to the flap system, "When the flaps are extended between 20 and 50 degrees, the system provides automatic retraction (or prevents extension), to protect structural integrity in the event that selected flap position airspeed limitations are exceeded. As airspeed is reduced, the flaps automatically return to their original selected position. The system has a manual override capability in the event of malfunction."
Functional testing of the inboard actuator of the left inboard trailing edge flap trailing was performed with no discrepancies noted. Disassembly of the actuator was performed which revealed no evidence of side loading of the piston against the barrel. A copy of the report is an attachment to this report.
Additional participants to the investigation were Ms. Ricka Jain, of Moog Aircraft Group, Torrance, California; and Mr. Louis DeBiase of Federal Express Corporation, Los Angeles, California.