Nashville, TN, USA
N8076U
McDonnell Douglas DC-8-71F
According to the pilot, the freighter flight was normal in all aspects until the landing gear were extended for landing at destination. The left main landing gear, (LMLG) indicated "unsafe", and all attempts, using the emergency/abnormal checklists and telephone/radio-relayed communications with company maintenance, failed to extend it. An emergency, LMLG retracted landing was performed with minimal damage to the aircraft. Postcrash investigation revealed that company maintenance installed a one-way check valve in the LMLG extend hydraulic lines instead of a restricted flow valve. The wrongly installed valve had no factory specification or part number attached, and the tag reportedly removed from it at installation possessed the wrong factory specification number, and the correct vendor' s part number. The company maintenance manual states that upon completion of the valve installation, a leak and operational test of the MLG retract/extension system be performed. The valve installation mechanic and the company inspector both stated that the finished job was leak and "ops" tested.
On April 26, 2001, at about 0629 central daylight time, a Boeing/McDonnell Douglas DC-8-71F, N8076U, registered to Emery Worldwide Airlines, Inc., operating as a Title 14 CFR Part 121 cargo flight, flight 228, sustained a left main landing gear, (LMLG) up landing at Nashville International Airport, Nashville, Tennessee. Visual meteorological conditions prevailed and an IFR flight plan was filed. The airline transport-rated pilot, airline transport-rated copilot, and flight engineer were not injured, 63,951 pounds of freight were not damaged, and the aircraft sustained minor damage. The flight departed Dayton, Ohio, about 44 minutes before the incident. According to the pilot, all operations were normal from the time of departure at Dayton until commencement of a visual straight-in approach to Nashville's runway 31. Upon gear extension, the LMLG did not indicate down, and a go-around was executed. The crew accomplished the appropriate emergency/abnormal procedures checklist, as well as communicating with company maintenance for additional trouble-shooting procedures. After performing a low flyby near the tower, and confirming with tower personnel that the LMLG was completely up, on their third attempt the crew performed a LMLG up landing on runway 31. The aircraft came to rest about 400 feet from the end of the 11,000 foot runway about 4 feet left of centerline on the nose gear, the RMLG, and the bottom of the engine cowlings of engines number one and two. The crew evacuated the aircraft by use of the emergency ropes/tapes. Examination of the aircraft maintenance logs for aircraft N8076U revealed that prior to the flight, on April 25, 2001, the LMLG hydraulic system had undergone a valve replacement by Emery maintenance at Dayton. The valve replaced was a two-way, filtered, hydraulic flow restrictor valve that directs full, 3,000 lb. aircraft system hydraulic pressure to retract the MLG, but restricts the full 3,000 lb. pressure from slamming the MLG to the extended position. The wording extracted from the maintenance log page number 09856-14, for aircraft N8076U, under "discrepancy" was, "Robbed fluid restrictor vlv from LH M/L/G Retract cylinder-lines temporarily capped". The "corrective action" write up was, "Installed LT MLG filtered restrictor as required IAW United [Airlines] MM 32-31-19 pg 202 (B) 6 plus note. Lk and Ops checks good". The proper hydraulic fluid restrictor valve possessed an aircraft manufacturer's, (Douglas) specification number, of 4776708-5503, and a vendor's, (Crissair, Inc. of El Segundo, Calif.) part number of 6F2020-1. The part number of the installed replacement valve was not displayed anywhere on the valve, however, the maintenance log page for N8076U of April 25th had the number "4776708-5503" filled in the block titled, "part no. on". A copy of the identification tag reportedly affixed to the replacement valve, supplied by Emery maintenance, shows the part number of the valve was 4776708-5503A. This part number could not be found by the aircraft manufacturer or the FAA, using Emery's illustrated parts catalog. The hydraulic valve installation mechanic and the installation inspector both stated that the finished job was leak and "ops" tested, as required by United Airlines maintenance manual 32-31-19, pages 203, 204, subparagraph C, "Adjustment/Test". A copy of the maintenance log page, the United Airlines maintenance manual pages referenced, and statements from the mechanic and inspector who performed the work and made the log entries are attachments to this report. Post incident examination of the aircraft revealed damage was limited to the abraded undersides of the numbers one and two cowlings and CFM56-2 engines due to the grinding action of the grooved concrete runway surface. Markings and paint transfer on the runway revealed the pilot kept the left side engines from contacting the runway until the last 3,700 feet of landing rollout. The number two cowling was ground through to the constant speed drive/generator case, the accessory gearbox, and the drain mast. The number one cowling was ground through enough to slightly open a hydraulic line routed under the engine. The fully extended flaps did not contact the runway. No structural damage was detected using examination data supplied by Boeing's Service Engineering/Technical and Fleet Support Division. The maintenance/recovery crew had to bypass the LMLG extension side hydraulic valve to get the LMLG to extend on the ground. When the hydraulic tubing to the valve was unsecured and opened by maintenance personnel, the LMLG free fell and locked down. For added confirmation that the LMLG extension hydraulic valve had malfunctioned, the RMLG extension hydraulic valve was removed from its location and installed in the LMLG system and normal hydraulic extension operation of the LMLG and LMLG door occurred. The NTSB took custody of the hydraulic valve removed from the LMLG and transported it to an FAA certified hydraulic repair station for further examination. The valve was bench tested for pressure and flow. The valve exhibited normal flow of hydraulic fluid to 3,000 pounds in one direction only. The part number 4776708-5503A could not be found in the repair station's comprehensive data sources, nor could it be referenced by telephone call to the vendor. Both the repair station and the vendor confirmed that the proper factory specification number should be 4776708-5503 and the vendor part number should be 6F2020-1. The cockpit voice recorder, (CVR) and flight data recorder, (FDR) were removed from the aircraft and shipped to the NTSB Recorders Laboratory for analysis. The CVR was found to be non-operational due to an internal malfunction and the tape contained no useful information. No NTSB recorder analysis group was convened, nor was a transcript prepared. The CVR factual report is an attachment to this report. The FDR was read out and the last 100 seconds for all parameters except control column position and elevator deflection was plotted. The plot is an attachment to this report. The removed hydraulic valve was sent to the FAA, FSDO, Cincinnati, on May 29, 2001, at the request of the FAA Principal Maintenance Inspector for Emery Worldwide Airlines, reportedly to pursue a "suspected unapproved parts" case. The NTSB form 6120.15, "Receipt of Parts" was signed by the FAA Assistant Principal Maintenance Inspector for Emery on May 31, 2001. Both the CVR and the FDR were returned to the airline and the NTSB form 6120.15 was signed on October 12, 2001 by the airline's Flight Safety Supervisor.
The failure of company maintenance personnel to install the correct hydraulic landing gear extension component, and the failure of company maintenance inspection personnel to comply with proper post maintenance test procedures, resulting in the impossibility of the LMLG to extend, and the subsequent LMLG up landing. A factor in the accident was the improper identification tag marking on the replacement component, and no marking on the component, itself.
Source: NTSB Aviation Accident Database
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