NTSB Factual Report

HISTORY OF FLIGHT

On June 2, 2002, at 2200 eastern daylight time, a Bombardier CL600-2B19, N849AS, registered to State Street Bank and Trust Company, and operated by Atlantic Southeast Airlines, Inc., as flight 4509, had the left main landing gear collapsed during landing on runway 26R, at the Atlanta Hartsfield International Airport, in Atlanta, Georgia. The airplane was being operated under the provisions of Title 14 CFR Part 121, and instrument flight rules. Visual meteorological conditions prevailed and an instrument rules flight plan was filed for the scheduled domestic passenger flight. The air transport pilot, first officer, flight attendant, and 50 passengers were not injured, and the airplane sustained substantial damage. The flight originated in Baltimore, Maryland, on June 2, 2002, at 1940.

The airplane landed on runway 26R at 2200. The airplane made an uneventful touchdown. During the landing roll at approximately 1000 feet down the runway at 110 knots and after the thrust reversers were deployed, the left landing gear collapsed. The left main wheel assembly remained attached to the side stay and was stuck between the fuselage and the inner edge of the inner wing flap. The flaps remained in the down position at 45-degrees and the flaps along with the flap actuator assemblies helped to support the wing when the airplane slid 4,000 feet down the center of the runway. The Captain was able to keep the airplane on the runway using rudder control, thrust reversers, right brakes, and the nose wheel steering system.

According to the flight crew, the approach to landing was uneventful. Upon landing the pilot deployed the thrust reversers and felt the airplane tilt to the left. At first he believed that he had a flat tire. However, as the aircraft continued down the runway he realized that the left main landing gear had collapsed. As the airplane began slowing the Captain notified the tower that he had a problem and ordered the first officer to contact the flight attendant to prepare the cabin for an emergency evacuation out the left entry door. After the airplane slid to a stop, the evacuation began. According to the flight attendant the evacuation progressed smoothly, with 40 passengers evacuating through the left main entry door and ten passengers evacuating through the right over wing exit.

PERSONNEL INFORMATION

A review of information on file with the Federal Aviation Administration (FAA) Airman's Certification Division, Oklahoma City, Oklahoma, revealed that the Captain held a commercial pilot certificate in airplane single engine land, and an Airline Transport Pilot certificate in Multi-engine land airplanes, with type ratings in the ATR-42, ATR-72, CL-65, and the EMB-120. A review of records on file with the FAA Aero Medical Records revealed the pilot held a first-class medical certificate issued on April 16, 2002, with a restriction that he must have available glasses for near vision. The pilot reported on his application for the medical certificate that he had accumulated 15,450 total flight hours.

A review of information on file with FAA Airman's Certification Division, Oklahoma City, Oklahoma, revealed that the First Officer held a private pilot certificate in airplane single engine land, commercial pilot certificate in rotorcraft-helicopter, and airline transport pilot certificate in airplane multi-engine land airplanes, with no type ratings listed. A review of records on file with the FAA Aero Medical Records revealed the pilot held a first class medical certificate issued on March 21, 2002, with no restrictions. The First Officer reported on his application for the medical certificate that he had accumulated 3,000 total flight hours.

AIRCRAFT INFORMATION

The Canadair Regional Jet (CRJ) CL600-2B19, Serial Number 7347, N849AS, was maintained by Atlantic Southeast Airlines, Inc., on a FAA approved continuous airworthiness inspection program. The airplane's most recent maintenance inspection was conducted on May 27, 2002. At the time of the accident, the airplane had accumulated a total time of 6,831 hours, and 5,584 cycles. Further review of the maintenance records found that the left and right main landing gears had been eddy current inspected 384 cycles prior to the accident. Bombardier Alert Service Bulletin A601R-32-079D Dated December 1, 2000, and Airworthiness Directive (AD) 2001-22-09, "Main Landing Gear - Repetitive Eddy Current Inspection of Left and Right MLG Fittings" requires that an eddy current inspection of the main landing gear fittings be repeated at every subsequent 500-flight cycles. The airplane's landing gear area had been inspected eight times prior to the accident.

METEOROLOGICAL INFORMATION

The nearest weather reporting facility at the time of the accident was Hartsfield International Airport, in Atlanta, Georgia. The 2153 surface weather observation was: Sky Clear, visibility 8 statute miles, temperature 25-degrees Celsius, dew point 21-degrees Celsius, wind 340-degrees at 7 knots, and altimeter 29.02. Visual meteorological conditions prevailed at the time of the accident.

WRECKAGE AND IMPACT INFORMATION

The airplane came to rest in an overrun area near taxiway B3. Emergency response vehicles arrived at the accident site and there were no signs of a fuel leak and there was no fire. The following morning on June 3, 2002 the airplane was towed from the runway by the use of a tug truck, flat bed trailers and air bags to a Delta maintenance hangar on the airport for further examination.

Examination of the left trunnion main fitting found a three-inch area of discolored material, which had penetrated through the cross section of the main fitting. The area surrounding the fracture appeared to have sheared off rapidly, showing signs of overload. The left main landing gear assembly was removed and sent to the NTSB Materials Laboratory in Washington, DC for further examination.

Examination of the left wing found the inner and outer flaps ground down as a result of the collapsed left main gear and dragging on the runway. There was leading edge damage and a detached runway light lodged in the leading edge of the left inner flap. The left wing tip had a two foot by one foot oval shape grind pattern on the under side of the wing near the wing tip. There was damage to the inboard inner flap area where the left main gear had become lodged after the collapse. The collapsed wheel assembly also damaged the electric hydraulic pump located in the wheel well. The wheel assembly remained attached to the airplane by the side stay. The level of Nitrogen gas and hydraulic fluid from the left landing gear strut could not be measured due to damage. The left main tires showed excessive wear and one tire was completely cut in a circular pattern around the tire rim.

Examination of the right wing and main landing gear showed little damage as a result of the accident. However, the right main landing gear assembly was removed and shipped to the NTSB Materials Laboratory for further examination. The nose gear showed no signs of damage. However, the left nose wheel tire showed excessive wear that indicated the airplane was leaning heavily on the left side as it slid down the runway.

The airplane's Flight Data Recorder and Cockpit Voice Recorder were recovered from the airplane and sent to the NTSB Recorder Laboratory in Washington, DC for examination. Examination of the FDR found the accident landing to be within normal limits, and the CVR found nothing noteworthy on the recording.

The airplane had 1,600 pounds of fuel remaining in the left fuel tank and 1, 710 pounds of fuel remaining in the right fuel tank. There was no fuel leaks observed.

SPECIAL TESTS AND RESEARCH INFORMATION

METALURGICAL EXAMINATION

Visual and bench binocular microscope examination of the upper fracture face of the left main landing gear revealed a discolored thumbnail fracture area typical of a preexisting crack that emanated from the aft face of the barrel at the shock strut radius run out area. This thumbnail region exhibited a dark brown tint typical of iron oxide. The thumbnail region intersected the outside and inside surface, and its width in the circumferential orientation measurement approximately 3 inches. The fracture outside the thumbnail region contained chevron marks that extended into the thumbnail region. The chevron marks extended all around the fracture and terminated at the forward end. The tips of the chevron markings were pointing toward the central portion of the thumbnail region, indicating that the fracture propagated away from the shock strut radius run out area. Closer examination of the thumbnail region revealed radial marks that emanated from the outside surface at the shock strut radius run out. The approximate center of the thumbnail region contained a ratchet mark that extended from the outside surface nearly to the inside surface. The ratchet mark split the thumbnail region into two fracture planes. The two fracture planes each contained multiple smaller thumbnail regions that emanated from the exterior surface. The presence of smaller thumbnail regions indicate that the crack initiated from multiple origins on the exterior surface. The fracture on the inboard side of the ratchet mark contained at least six thumbnail regions, exhibiting a light silver luster compared to the surrounding area. These thumbnails exhibited a flat morphology. The length of one thumbnail region measured 0.026 inch, and its width (portion of the crack that extent across the exterior surface of the part) measured 0.046 inch. The length of another thumbnail measured 0.023 inch and its width measured 0.057 inch. The area outside the flat thumbnails exhibited a much rougher morphology compared to areas inside the thumbnail region.

The fracture on the outboard side of the ratchet mark contained one thumbnail region. The length of this thumbnail measured 0.035 inch, and the width measured 0.087 inch. The area outside of this region also showed a rougher morphology compared to the area inside this region. Shear lips were noted on the internal and external wall of the barrel with the exception of the surface that was intersected by the smaller thumbnail regions. The 3-inch wide brown tinted thumbnail fracture region was cut from the remainder of the upper fracture face. The width of the excised fracture measured approximately 5 inches. The excised fracture was ultrasonically cleaned with Alconox, a commercial detergent. This cleaning procedure was partially successful in exposing closely spaced fatigue crack arrest positions at the periphery of the thumbnail regions. The fracture was cleaned with Endox 214, an electrolytic process that removes oxides from the surface of ferrous alloys. This cleaning process exposed finer fatigue crack arrest positions within and around several thumbnail regions and coarser crack arrest marks at the ends of the 3-inch wide thumbnail region.

Scanning electron microscope (SEM) examination of the outboard side of the ratchet mark revealed the area located inside of the 0.035-inch by 0.087-inch thumbnail region contained fatigue striations. The fatigue striations emanated at the exterior surface. A representative from Messier-Dowty, manufacturer of the landing gear, indicated that the surface of the component is prepared sequentially by shot peening, grit blasting, cadmium plating, epoxy priming and a topcoat of polyurethane. A layer of paint had peeled off the exterior surface in the area of the fatigue origin area. The exterior surface at the fatigue origin area showed features typical of a grit blasted surface, as required by the manufacturing process. No evidence of mechanical damage, such as gouging, was found at the fatigue origin area. The exposed portion of the exterior metal surface showed features consistent with shot peening, and analysis of the metal surface showed evidence of a layer of cadmium (verified by X-ray energy dispersive spectroscopy). The edges of the paint layer were exposed in the exterior surface of the excised fracture. The fracture face of the paint layer contained two layers. The inside layer was yellow consistent with primer and the outside layer was gray consistent with a topcoat of paint. The composition/type of the primer and paint was not verified.

The fatigue crack propagated outboard until the fracture mode changed to an intergranular fracture. The intergranular fracture area extended between 0.034 and 0.040 inch from the fatigue origin area and appeared as a band that was concentric with the crack arrest positions within the thumbnail region. The band was not continuous and the severity of the intergranular fracture features varied along the length of the band. Beyond the intergranular regions the fracture mode returned to fatigue cracking and extended to a position located approximately 0.09 inch from the fatigue origin. The fracture face between 0.09 inch and 1.7 inch from the origin exhibited mechanical damage that resulted from relative motion between mating fractures, in addition to areas that contained iron oxide scale (verified by X-ray energy dispersive Spectroscopy), and ductile dimples. No evidence of fine fatigue striations was found between 0.09 and 1.7 inch from the origin area. During SEM examination, the fracture was coated with a layer of gold-palladium, to prevent charging of the paint layer. Visual examination revealed that the gold-palladium deposit highlighted coarse crack arrest marks in the region between 0.09 and 1.7 inch from the fatigue origin area. The coarse crack arrest marks terminated at 1.7 inches away from the fatigue origin area. The coarse crack arrest marks were visible with the unaided eye and bench binocular microscope, but were hardly noticeable by scanning electron microscope examination. The area outside of the 1.7-inch boundary contained ductile dimple features typical of overstress separation. No evidence of crack arresting was found beyond 1.7 inch from the origin.

The distance between striations was determined for various areas within the brown tinted region, moving outboard from the origin area. No striations at all were noted between the fatigue origin and 0.028 inch from the fatigue origin area because of mechanical damage that resulted from relative movement between mating fractures and corrosion damage. The number of crack arrest marks between 0.09 inch and 1.7 inch from the radius area also could not be determined because of similar mechanical and corrosion damage Additional electrolytic cleaning did not reveal more striations or fracture features at the ends of the brown tinted region. Subsequent electrolytic cleaning steps produced minor cracks on the fracture surface. These minor surface cracks were not present on the face of the fracture prior to cleaning.

A longitudinal section was made through the origin of the 0.035-inch by 0.087-inch thumbnail. The section was polished and etched with nital. Examination of the section showed a microstructure of tempered martensite that is typical of a quench and tempered steel. No anomalies such as forging laps were noted in the microstructure.

Examination of the lower fracture surface found paint chip fragments peeled from the exterior surface near the shock strut radius area of the lower fracture surface. The exposed metal surface contained a gold tint typical of a cadmium-plating layer. The cadmium-plating layer is required after grit blasting. The lower fracture surface was excised and sent to Bombardier Aerospace, Montreal, Quebec, Canada, for comparison examination. A representative of the Transportation Safety Board of Canada supervised this examination. A follow-up meeting was held at the Safety Board Materials Laboratory to discuss the results of the examination. The discussion indicated that the lower fracture contained similar fracture features to those found on the upper fracture face, with the exception that a fragment of the fracture from the outboard side of the ratchet mark that contained the 0.035-inch by 0.87inch thumbnail was missing.

The main fitting for the right landing gear M-DT P/N 17002-112, S/N DCL358/99, was disassembled from the same accident airplane for comparison study. Visual examination of the main fitting by the Safety

NTSB Probable Cause

Failure of the left main landing gear fitting due to a fatigue crack that emanated from multiple origins at the external surface of the wall at the shock strut radius run out area for undetermined reasons.