NTSB Factual Report

On August 18, 2004, at 1214 mountain daylight time, a Cessna 750 (Citation X) twin-engine business jet, N961QS, sustained minor damage when the right main landing gear failed during the landing roll at Jackson Hole Airport, Jackson, Wyoming. The captain, first officer, and two passengers were not injured. The airplane was registered to Neil S. Hirsch Inc., Oklahoma City, Oklahoma, and operated by NetJets, Columbus, Ohio. Visual meteorological conditions prevailed, and an instrument flight rules flight plan was filed for the 14 Code of Federal Regulations Part 135 flight. The flight departed San Jose International Airport, San Jose, California, at 1100, and was destined for Jackson, Wyoming.

According to the captain, the visual approach to runway 19 and landing touchdown were normal. During landing roll, approximately 10-15 miles per hour, prior to executing a left turn onto taxiway A1, the right wing suddenly dropped, and the airplane came to a stop. No visual display warnings or aural warnings were observed or heard prior to the anomaly. The captain secured the airplane and instructed the passengers and first officer to evacuate the airplane. The airplane came to rest approximately 100 feet from the end of runway 19.

The Federal Aviation Administration (FAA), Cessna Aircraft Company representatives, and NetJets representatives responded to the incident.

Examination of the airplane (serial number 750-0061) by the airport operations manager and local maintenance personnel revealed the right main landing gear trailing link and wheel assembly separated from the airplane, and a portion of the upper trailing link and axle assembly remained attached to the main trunnion. The #4 tire was blown and a flat spot was noted on the tire. Examination of the runway revealed a tire skid mark approximately 445 feet from the airplane's resting point. The separated trailing link assembly was removed by Cessna Aircraft Company personnel, and was retained by the FAA for further examination by the National Transportation Safety Board Materials Laboratory in Washington, DC.

After repairs were completed at Jackson Hole, the aircraft was ferried to the Wichita Citation Service Center, Wichita, Kansas. During the ferry flight, the flight crew reported to mechanics a "parking brake on" Crew Alerting System message. According to Cessna, the parking brake was found out of rigging specifications. This condition could cause trapped pressure in the #4 brake line, resulting in a locked wheel. The wheel speed transducer was tested by the manufacturer and no anomalies were noted in the speed measurement capability.

According to NetJets maintenance personnel, at the time of the incident, the landing gear assembly had 5,228.5 hours total time since new, and 3,358 total cycles since new. The trailing link and axle assembly was certified for 44,400 total cycles.

A review of the maintenance records by NetJets' FAA Principal Maintenance Inspector revealed that on July 7, 2004, a Cessna Citation Service Center completed Cessna Service Bulletin (SB) 750-32-17R1 "Landing Gear - Main Landing Gear Trunnion Replacement," and other approved airworthiness inspection program items. During the completion of the SB, the trailing link and oleo strut were removed from the trunnion. No anomalies were noted with the trailing link during compliance with the SB. At the time of the SB compliance, the landing gear assembly had 5,138.0 hours total time, and 3,301 cycles.

On August 23 and 24, 2004, at the NTSB Materials Laboratory in Washington, DC, the NTSB investigator-in-charge, a NTSB senior metallurgist, a NTSB structures engineer, representatives from Cessna Aircraft Company, and a representative from NetJets examined the fractured components. The examined components included the trailing link and axle assembly, the oleo strut assembly, and the connecting hardware between the trailing link and trunnion. The trailing link was a machined steel forging made of 300M alloy steel heat treated to 280 to 300 ksi ultimate tensile strength. Examination of the trailing link revealed the link was circumferentially fractured through the cylindrical section approximately 6 to 7 inches aft of the trunnion attached clevis. The aft portion of the link also contained a 10-inch long longitudinal crack extending aft of the circumferential fracture along the top centerline of the link. For most of its length, the crack ran in or immediately adjacent to the forging flash line. The crack was visible for about 4 inches aft of the oleo strut attachment lugs and 6 inches forward of the lugs to the circumferential crack. Rust and corrosion were visible on the exterior along much of the crack. Borescope inspection of the inner diameter of the link established that the crack penetrated completely through the wall for its entire length. The internal inspection revealed corrosion on the inside surface adjacent to the crack and at several locations on the bottom inside surfaces of the link. Magnified optical examinations of the fracture surfaces displayed features consistent with overstress, and closer examination revealed darkly oxidized regions at the intersection of the circumferential and longitudinal crack. Features within the oxidized region were indicative of a preexisting and progressive cracking mode propagating out of the crack.

The longitudinal crack was opened by cutting the trailing link and axle assembly. The crack faces contained heavy deposits of corrosion; however, macroscopic features were sufficient to indicate that the crack initiated at a geometric transition on the interior surfaces of the link. The geometric transition was an interior corner formed by the intersection of the two major inner diameters of the link. Crack arrest marks indicated that the crack grew upward through the wall thickness before turning at both sides to propagate forward and aft. The arrest lines indications were consistent with fatigue progression.

Several craters in the surface were present on the interior surface along the inside corner on both sides of the crack initiation location. The craters were generally circular in shape, some with raised edges and visibly penetrated into the surface on the link. With the primer removed, the surfaces of the craters had smooth appearances with wrinkles and features consistent with previously molten and solidified metal reminiscent of the surfaces of a weld bead. Small globular formations were present within some craters and many small particles were found attached to the surrounding surfaces, reminiscent of weld splatter. The bottoms of most of the craters were covered with titanium cadmium (TiCad) plating. The plating appeared smooth and uniform covering the splatter and crater bottoms.

Microstructures and micro hardness values at the craters and into the underlying material were consistent with a short duration high temperature event at the surface of the link, such as an electrical arc or weld that locally melted the material after the final production heat treatment.

The NTSB metallurgist conducted an experiment to show the affects of an electrical arc on the microstructure of the link. An arc was struck between a piece of the link and a low carbon electrode (a common nail). The arc was established using 110 volt alternating current and allowed to end when the electrode welded itself to the link piece. Except for the scale of the damage (the experiment damage area was much larger) the microstructure changes and characteristics were very similar to those at the craters on the incident link piece.

A Cessna metallurgist also conducted an experiment in an attempt to duplicate the crater features present on the incident trailing link. According to Cessna, the crater feature was duplicated by subjecting a sample of quenched and tempered 300M material to direct contact with a stainless steel electrode during a simulated plating process. The simulation resulted in a series of craters into the steel, similar in appearance (visually as well as microstructurally) as that present at the longitudinal crack origin of the trailing link. Cessna reported the incident craters had formed some time after machining and prior to final primer coating of the inner diameter of the trailing link, possibly just prior to or during the TiCad plating process.

On August 27, 2004, Cessna Aircraft Company issued an Alert Service Letter (ASL), ASL750-32-19 "Landing Gear - Main Landing Gear Trailing Link." The ASL provided instructions to complete a visual inspection of the main landing gear trailing link for the possibility of craters present on the inner wall of the trailing link. The ASL compliance was listed as MANDATORY and must be accomplished within 50 landings for airplanes serial numbers -0044 thru -0079 and airplanes with more than 3,000 landings, and within 100 landings for airplanes with less than 3,000 landings. As of March 8, 2005, Cessna report 99 percent of the Cessna 750 worldwide fleet had been inspected. During the ASL inspections, two additional trailing links with craters were found and the trailing links were replaced.

NTSB Probable Cause

the failure of the right main landing gear trailing link due to fatigue cracking initiated by an undetected manufacturing defect. A contributing factor was the improper plating process by unknown manufacture personnel.