Philadelphia, PA, USA
N326EN
DeHavilland DHC-8-311
The airplane was on approach for landing. When the landing gear was selected to the extended position, the main landing gear extended properly but the nose landing gear did not. The crew performed a go-around, exited the airport airspace, and unsuccessfully attempted to extend the nose landing gear using several checklists and in consult with ground-based maintenance personnel. A subsequent fly-by of the air traffic control tower confirmed that the nose landing gear doors were open, but that the nose landing gear was not extended. The captain subsequently landed the airplane with the nose landing gear retracted and delayed lowering the nose until the slowest speed possible. The nose of the airplane contacted the runway and the airplane slid for about 525 feet before it stopped. There was no fire, and the passengers deplaned normally at the scene. On-site examination of the airplane and a laboratory examination of the nosegear components revealed that the steering links had been circumferentially loaded in tension beyond their load-carrying capacity. Failure of the steering links allowed the nosewheels to rotate in the wheel well and become wedged within the structure. Hardness testing satisfied the manufacturer's minimum requirements and no determination could be made as to when the overload occurred.
HISTORY OF FLIGHT On November 16, 2008, about 0934 eastern standard time, a deHavilland DHC-8-311, N326EN, operated by Piedmont Airlines as USAirways flight 4551, sustained minor damage when it landed with the nose landing gear retracted at Philadelphia International Airport (PHL), Philadelphia, Pennsylvania. The 2 flight crewmembers, 1 cabin attendant, and 35 passengers were uninjured, and visual meteorological conditions prevailed. The scheduled passenger flight was operating on an instrument flight rules flight plan from Lehigh Valley International Airport (ABE), Allentown, Pennsylvania, to Philadelphia, under the provisions of Title 14 Code of Federal Regulations Part 121. During a telephone interview, the captain stated, and was collaborated by the first officer, that on the morning of the flight, they performed a preflight inspection of the airplane while it was parked, started the engines, and then taxied a short distance to the gate. The captain stated that during taxi to the runway, everything appeared normal. They completed all checklists; the captain maneuvered the airplane onto the runway, and transferred control to the first officer, who was to be the flying pilot. While on initial climbout the captain noted that the nose gear took about "three to four seconds longer to retract than the main landing gear." During the interview, both crewmembers reported that the flight was uneventful until the base leg of the approach. At that point, after the landing gear handle was selected to the extended position, the main landing gear extended properly, but the flight crew observed cockpit annunciations indicating that the nose gear and doors were not properly configured for landing. The flight crew performed a go-around; left the landing gear extended, and departed the area to attempt to rectify the situation. The first officer transferred airplane control to the captain, and performed the alternate landing gear extension checklist. However, the anomalous indications remained, and the nose landing gear remained retracted for landing. The flight crew then flew the airplane past the air traffic control tower (ATCT) to enable the ATCT personnel to determine the status of the nose landing gear. ATCT personnel reported that the nose gear doors were open, but that the nose landing gear was not visible. The flight crew then made several attempts to extend the nose gear by using the alternate gear extension checklist, and conferring with the airline's maintenance personnel, but were unsuccessful. The crew subsequently elected to return to the airport to perform a nose-gear-up landing on runway 27L. During the landing, and after the airplane's main landing gear touched down, the captain held the nose of the airplane off the runway until the slowest speed possible. After the nose contacted the runway, the airplane slid on it for about 525 feet before coming to a stop. There was no fire. The passengers deplaned via the main cabin door, and were taken to the terminal by a bus. PERSONNEL INFORMATION The captain held an airline transport pilot certificate with a rating for airplane multi-engine land, a type rating in the incident airplane make and model, and a commercial pilot certificate with a rating for airplane single-engine land. He reported 7,336 total hours of flight experience, including 2,726 hours as pilot in command (PIC). He reported 6,253 total hours of flight experience in the incident airplane make and model, including 1,832 hours as PIC. His most recent Federal Aviation Regulation (FAR) Part 121 annual check ride was completed on September 24, 2008, and his most recent Federal Aviation Administration (FAA) first-class medical certificate was issued on August 13, 2008. The first officer held a commercial pilot certificate with ratings for airplane single-engine land, multi-engine land, instrument airplane, and several type ratings, including the incident airplane make and model with second-in-command privileges. She reported 2,553 total hours of flight experience, including 2,005 hours in the incident airplane make and model. Her most recent FAR Part 121 annual check ride was completed on June 18, 2008, and her most recent FAA first-class medical certificate was issued on January 21, 2008. AIRCRAFT INFORMATION According to FAA records, the airplane was manufactured in 1990 and was equipped with two Pratt & Whitney PW123 turboprop engines. According to the operator, the airplane was certificated for a maximum gross weight of 43,200 pounds, and at the time of the incident, it weighed 37,996 pounds. The airplane's most recent inspection was completed on October 1, 2008 in accordance with an FAA-approved continuous airworthiness inspection program. At the time of the incident, the airplane had accumulated 33,224 total hours in service. METEOROLOGICAL INFORMATION The 0954 recorded weather observation at PHL included winds from 280 degrees true at 24 knots with gusts to 30 knots, 10 miles visibility, scattered clouds, temperature 9 degrees C, dew point minus 3 degrees C, and an altimeter setting of 29.70 inches of mercury. AIRPORT INFORMATION Runway 27L was 10,506 feet long and 200 feet wide. The runway surface was asphalt, and dry. A foam agent was applied to the airplane immediately after coming to rest by Airport Rescue and Fire Fighting (ARFF). WRECKAGE AND IMPACT INFORMATION According to the FAA inspector who responded to the accident, the airplane came to rest on the runway centerline, between taxiways U and Y. The nose landing gear doors, hinges, and the skin on the underside of the fuselage sustained scrape damage due to contact with the runway surface. The damage was confined to the nose gear doors and adjacent skin surfaces. No structural damage was reported. Airport personnel and mechanics utilized air bags to lift the nose of the airplane off the runway surface. The FAA inspector then looked into the nose gear well, and found the nose wheels canted at an angle, which wedged them against the nose wheel well structure. Personnel used a pry bar to free the nose wheels from the wheel well, and the nose gear was then swung freely to its normal extended position, and locked in the down position. The airplane was towed to a maintenance hangar for additional examination. The links on top of the steering column were found to be fractured and pushed upward. The nose wheel over steering or frangible fuse pin was still intact. Approximately 1 week after the incident, another of the operator's DHC-8-102 airplanes was inspected, and the links on top of the steering column of that airplane were also found fractured. The steering links from both airplanes were sent to the National Transportation Safety Board Materials Laboratory for examination. There was no determination as to when the damage occurred to either airplane. TEST AND RESEARCH Nose Wheel Steering Links According to the Safety Board's Material Laboratory Factual Report, the steering link assembly attached to the nose landing gear steering cylinder, and it served to convert linear movement of the steering cylinder to rotary motion of the nose gear strut, which then provided nosewheel steering capability. Examination of the incident airplane's steering link revealed that the inner ring portion, that normally encircled the center bushing, had fractured, and the rings had been deformed outwards. Manipulation revealed that the telescoping links could be moved inwards and outwards but could not be extracted. The retainers were examined and were found to be correctly wire-locked. Examination of the fracture faces revealed shear lip features oriented on a 45-degree slant plane and a reduction in cross section adjacent to the fracture face (necking), consistent with the inner rings being circumferentially-loaded in tension beyond their load-carrying capacity. The steering link from the other airplane was in similar condition as the incident steering link and exhibited similar fractures. The manufacturer's design drawing indicated that the swivel link was to be produced from low-alloy "4340" steel. Note 7 on the drawing required "heat treat to 180-200 ksi [thousand pounds per square inch] per DCMP-1303 in a protective atmosphere furnace;" 180-200 ksi converts to 40-43 Hardness, Rockwell "C" scale (HRC). Hardness testing was performed by the Safety Board's Material Laboratory on the swivel link lug, and results ranged from 38.3 to 40.0 HRC, which converted to tensile strengths of approximately 173 ksi to 180 ksi, respectively. According to a representative of the airplane manufacturer, the failure of the nose wheel steering links would "allow the [nose] wheel to roll-over/tilt and get caught on the [nose gear wheel well] structure." Frangible Fuse A Frangible Fuse (Towing Fuse) that was located on top of the nose wheel strut was designed to prevent damage to the nose wheel steering mechanism during an over-travel event while being towed. According to a representative of the airplane manufacturer and the operator's maintenance personnel, a rotation of the nose wheel to the left or right, beyond the 120 degree maximum castering range, would allow stops on the retaining flange/locating ring to contact and break the frangible fuse, which could indicate damage to the steering mechanism. The frangible fuse, which would not have been visible during a normal preflight inspection, was examined after the incident and did not indicate that any castering limitation had been exceeded.
The mechanical overload of the nosewheel steering links for undetermined reasons, which resulted in nose landing gear rotation and its subsequent wedging within the wheel well structure.
Source: NTSB Aviation Accident Database
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