On July 27, 2010, about 1215 Atlantic standard time, a Cessna 402C, N402VN, registered to and operated by Hyannis Air Service, Inc., dba Cape Air, as Flight 1284, experienced separation of the lower portion of the right main landing gear during the landing roll at the Luis Munoz Marin International Airport, San Juan, Puerto Rico. Visual meteorological conditions prevailed at the time and a company VFR flight plan was filed for the 14 Code of Federal Regulations (CFR) Part 135 scheduled domestic passenger flight from Antonio Rivera Rodriguez Airport, Isla de Vieques, Puerto Rico. The airplane sustained minor damage and the certificated airline transport pilot and one passenger were not injured. The flight originated from Isla de Vieques about 1148. The operator reported that the takeoff and en route portion of the flight was uneventful; however, during the landing roll after brake application, the pilot reported the airplane dipped to the right. At that time, the passenger informed the pilot that the lower portion of the right main landing separated from the airplane. The airplane came to rest upright on the runway. Inspection of the airplane revealed the barrel of the right upper barrel and trunnion assembly, part number 5141103-6, was fractured circumferentially thru the wall thickness immediately below the main gear actuator retract collar. The fractured upper barrel and trunnion assembly was retained and submitted to the Safety Board's Materials Laboratory for failure analysis. Additionally, a fractured upper barrel and trunnion assembly from another Cessna 402C airplane (N161TA) operated by Hyannis Air Service, Inc., dba Cape Air, and two upper barrel and trunnions from a Cessna 402C (N402BK) were also submitted by Hyannis Air Service, Inc., dba Cape Air, to the Safety Board's Materials Laboratory. Examination of the upper barrel and trunnion assembly from the incident airplane revealed two separate fatigue regions that originated at multiple origins on the outer surface of the barrel and propagated inward. The fatigue regions were located at the smaller diameter side of the transition radius. Optical measurements of the transition radius between the diameters ranged from 0.036 to 0.050 inch, though the engineering drawing does not specify a radius dimension. The radius appears smooth and where the full transition was visible, tangent to the smaller diameter region. The barrel outer surfaces showed some light surface corrosion but no significant corrosion or corrosion pitting was noted at the fatigue origin areas. The surfaces of both fatigue regions were heavily corroded leaving very little of the fine fracture details. At its maximum, the fatigue of one of the regions penetrated through about 90 percent of the wall thickness and measured 1.6 inches in length. The wall thickness immediately adjacent to the fracture measured between 0.1055 and 0.1065 (specification in that area is between 0.1125 and 0.1040 inch). The barrel material was identified as AISI3 4340 alloy steel as required by the engineering drawing, and hardness measurements on the outer diameter surface averaged 39.6 HRC (specification is 39 to 44 HRC). Examination of the upper barrel and trunnion assembly from N161TA which had previously been partially examined by an outside laboratory revealed a crack around the barrel in the aft outboard quadrant. The crack face exhibited features consistent with fatigue initiating at multiple locations on the outer diameter surface of the barrel. At its maximum, the fatigue crack penetrated through about 90 percent of the wall thickness in a slow growth manner on a normal plane to the barrel axis. The remaining wall thickness along the crack was oriented at 45 degrees to the barrel wall, and exhibited crack arrest marking consistent with rapid crack extension. In total, the fatigue crack measured 1.7 inches around the barrel circumference. The fatigue surfaces were heavily corroded leaving few fine details; however, the outer surface of the barrel at the fatigue crack was only lightly corroded and no significant corrosion pitting was present at the origin areas. The barrel material was confirmed as AISI 4340 using a hand held alloy analyzer, and the hardness of the barrel measured on the barrel exterior also met the drawing requirements of HRC 39 to 44. The two upper barrel and trunnion assemblies from N402BK were identified in the NTSB Materials Laboratory as "A" and "B." Inspection of both upper barrel and trunnion assemblies revealed visible cracks in the braze fillets at the top and bottom of the actuator collar. The actuator collar of the one marked "B" contained a visible crack in the outboard side of the actuator collar. Magnetic particle inspection of both revealed crack indications at the transition radii between the collars in both assemblies. Cracks indications were found in the forward inboard and aft outboard quadrants of the A assembly, and only in the aft outboard quadrant of the "B" assembly. Sectioning of both barrels revealed the surfaces contained features of multiple linked fatigue cracks. Each crack had multiple initiation points on the surface of the barrel with propagation into the barrel walls. The two cracks in the "A" barrel measured 0.90 inch and 1.65 inches long. The aft outboard crack was completely through the wall of the barrel while the forward inboard crack was 0.017 inch deep. The "B" barrel crack was 1.5 inches long and 0.037 inch deep. The "A" barrel surface had light or no corrosion in the area of the fatigue initiations, while the surface of the "B" barrel was heavily corroded. Further examination of the trunnion assemblies from N402BK revealed the wall thickness adjacent to the cracks to range from 0.845 inch to 0.860 inch for the "A" barrel, and 0.0855 inch to 0.0860 inch for the "B" barrel. The calculated wall thickness for this location from the engineering drawing was 0.0843 inch to 0.0885 inch. The "B" barrel was further sectioned which revealed multiple initiation fatigue features. The crack measured approximately 0.475 inch wide and 0.170 inches deep. During the sectioning to open the "B" barrel actuator arm crack, the collar to barrel braze joint separated under little or no load. The mating surfaces of the collar and barrel were covered by thick layers of corrosion products. Scrubbing the surfaces in soap and water removed the majority of deposits and revealed heavily corroded surfaces with some remnants of the braze metal. An approximate 1 inch wide section of the "A" barrel to actuator collar braze joint was removed; the joint line showed complete separation along one edge. Closer viewing found corrosion pitting into both the collar and the barrel. The joint line other side of the removed section appeared tighter and partially intact. Little force was necessary to pry the components apart at the joint line. The separated surfaces were mostly covered by corrosion products but a small section was copper colored indicating and intact area of braze. The barrel material of the two was confirmed as AISI 4340 using a hand held alloy analyzer. Hardness of the barrels measured on the barrel exteriors also met the drawing requirements of HRC 39 to 44. Historically, with respect to the upper barrel and trunnion assembly, personnel from Hyannis Air Service, Inc., dba Cape Air reported their first barrel failure was in 2002, and involved N618CA. The airplane total time at that time was reported to be 15, 663 hours. The next failure occurred in August 2003, involving N36911. The airplane total time at that time was reported to be 23, 754 hours. Since then excluding the incident airplane, they reported one other failure which involved N4652N, and occurred on August 31, 2009. The operator submitted the fractured upper barrel and trunnion assembly from that airplane to an outside laboratory for metallurgical analysis. Since 2002 in an effort to identify failure mode of the upper barrel and trunnion assembly, they submitted several cracked and/or fractured upper barrel and trunnion assemblies to an outside laboratory. Based on information provided by the outside laboratory indicating that cracking of the barrel originated on the inside surface, the operator generated a fleet campaign directive (FCD) "ATA 32-05-03-2010", that required cleaning and visual inspection of the barrels of the left and right upper barrel and trunnion assemblies of their airplanes for cracking and hydraulic leaks not associated with the lower seals. The FCD was recurring every 60 hours to coincide with their AAIP inspections, and was in place at the time of the incident. Review of the airplane maintenance records revealed FCD "ATA 32-05-03-2010" was last performed on July 10, 2010, at a recorded airplane total time of approximately 26,877 hours. The sheet associated with the FCD indicated no discrepancies were noted during the visual inspection. The airplane had accumulated approximately 62 hours since the inspection at the time of the incident. The airplane is maintained in accordance with (IAW) an FAA approved aircraft inspection program (AAIP) specified in 14 Code of Federal Regulations (CFR) Part 135.419. There are six operational inspection phases each performed at 60 hour intervals. The AAIP is based on Cessna's Progressive Inspection Program. The upper barrel and trunnion assembly is not serialized and is considered an on-condition item. The operator reported that there was no record that the upper barrel and trunnion from the incident airplane had been replaced since the airplane was manufactured in 1981. The airplane total time at the time of the incident was reported to be approximately 26, 939 hours. The operator reported accumulating 2 cycles per flight hour. Although Supplemental Inspection No. 32-10-04, titled "Main Gear Actuator Collar" and Supplemental Inspection No. 32-30-04, titled "Upper Barrel Main Gear" call for inspection of the area of the barrel, neither call for specifically inspecting the area of the barrel where the crack was located. Additionally, FAA personnel located in the Wichita Aircraft Certification Office reported it was not the intent of either supplemental inspection to specifically inspect the barrel for cracks. At the time of the incident, there was no Supplemental Inspection specified by Cessna for the incident make and model airplane to perform non-destructive testing to detect cracks of the barrel of the upper barrel and trunnion assembly. A review of FAA Service Difficult Reports (SDR's) by part number (5141103-6) revealed a total of three reports involving cracks in the barrel of the upper barrel and trunnion assembly. The NTSB has investigated 3 previous accidents or incidents involving fracture of the barrel of the upper barrel and trunnion assembly. The first and third events occurred in the U.S., and were designated NTSB case numbers MIA02LA089, and ERA09IA115. The second event occurred in the Bahamas, and was designated NTSB case number MIA04WA066. The NTSB Materials Laboratory was involved in the examination of the parts from MIA04WA066, and also ERA09IA115, while the Cessna Aircraft Company Materials and Process Engineering department examined the component from NTSB case number MIA02LA089. The NTSB Materials Laboratory Factual Reports concerning MIA04WA066 and ERA09IA115 indicated both barrels contained fatigue cracks that originated on the outer surface. The report from Cessna concerning MIA02LA089 indicated the presence of two fatigue cracks 180 degrees apart that also originated on the outer surface. Postincident, the operator developed with concurrence from Cessna Aircraft Company an ultrasonic inspection of the affected area. The inspection is performed every 2,500 landings on landing gear over 15,000 hours time in service since new. On landing gear above 25,000 hours, the upper barrel and trunnion are removed and replaced.