Catastrophic Structural Failure

One accident that failed to make the front pages occurred on May 9, 2005, when a student pilot and instructor were flying in the Kissimmee, Fla., area for a half-hour aerobatics lesson. The instructor had been demonstrating various maneuvers before the North American SNJ-6 airplane was seen entering a spin, descending rapidly and hitting the ground. Both occupants were killed. The airplane's right wing had separated in flight. Investigators discovered that the outboard right-wing lower attachment bracket had failed because of fatigue cracking, resulting in separation of the wing. The FAA issued an Emergency Airworthiness Directive requiring fluorescent-dye-penetrant inspections of the wing-attach flanges at 200-hour intervals.

At the other end of the attention spectrum was the December 19, 2005, accident involving Chalk's Ocean Airways flight 101, a Grumman Turbo Mallard amphibian. It had taken off from the Miami Seaplane Base in Florida on a regularly scheduled passenger flight to Bimini in the Bahamas. There were two pilots and 18 passengers on board. The seaplane base is on an island about two miles east of Miami. The water runway area is 15,000 feet long and 200 feet wide. The terminal on the island used by Chalk's Ocean Airways was built in 1926.

About one minute after takeoff, the right wing separated, and the airplane crashed into a shipping channel adjacent to the Port of Miami. All on board were killed. The NTSB recently completed its investigation into the accident, which paid a lot of attention to the age and specialized nature of the aircraft, the operator's maintenance programs and practices, and FAA oversight.

Things being the way they are in the world today, one of the first issues addressed was whether the crash involved terrorism or some other form of sabotage. A video taken by a witness who was on a beach showed the airplane in a nose-down attitude of between 35 and 45 degrees after the wing separated. It didn't show smoke or debris coming from the airplane, but did show a cloud of fire and smoke behind it. A video taken by a U.S. Coast Guard surveillance camera showed the airplane passing over Miami Harbor and moving away from the camera's position. As the airplane moved away, it also moved toward the center of the camera's field of view and became progressively smaller. About nine seconds after the airplane had gone too far away for the camera to pick up, a bright flash appeared. Black smoke could be seen in the area where the flash appeared, and a smoke trail fell toward the water. Pieces of the recovered wreckage were sent to the FBI's laboratory in Quantico, Va., and were screened for explosives and explosive residue. None were found.

The airplane originally was manufactured by Grumman in May 1947. At that time, it was certificated as a model G-73 and was equipped with two radial engines. Grumman stopped building the model in 1951. It was designed to carry two pilots and 10 passengers, with a maximum gross weight of 12,750 pounds. Chalk's Ocean Airways acquired the airplane in 1980. In July 1981, the airplane was modified in accordance with an FAA-approved Supplemental Type Certificate to become a G-73T. The engines were changed to Pratt & Whitney PT6A-34 turbopropeller models, and were equipped with constant-speed, three-blade propellers. The maximum gross weight was increased to 14,000 pounds and additional seats were installed, bringing the passenger seating capacity to 17. On the accident flight, three of the 18 passengers were infants held on the laps of adults. At the time of the accident, the airplane had accumulated 31,226 total flight hours and 39,743 cycles. (A cycle is one complete takeoff and landing sequence.) At the time of the accident, the left engine had 7,515 total hours of which 1,154 were since the last overhaul. The right engine had 9,036 hours with 3,037 since overhaul.

The captain was 37 and held an ATP certificate with ratings for airplane multi-engine land and sea and a type rating for the G-73T. Her first-class medical was current and required her to wear corrective lenses. She had logged 2,820 hours, including 1,630 in the G-73T and 430 as a G-37T pilot in command. She also served as the company's Director of Safety.

The first officer, 34, held a commercial-pilot certificate and was rated for airplane single-engine and multi-engine land and sea. His first-class medical was current with no limitations. His total time was 1,420 hours with 71 hours in the G-37T.

First to respond to the accident scene were lifeguards who patrolled Miami Beach on foot and jet skis. Most of the wreckage was submerged in about 30 feet of water along a rock jetty. The airplane broke up when it struck the water, and the debris field measured about 200 feet by 200 feet. A Coast Guard helicopter and rescuers were among the personnel to arrive within minutes of Miami's emergency dispatch center being notified of the accident.

Safety Board investigators interviewed four pilots who were employed by Chalk's at the time of the accident, one of whom was the husband of the captain on the accident flight. They also interviewed three pilots who had left the company before the accident. The investigators heard that some pilots had concerns about maintenance, including visible cracks, fuel leaks and degraded rivets. One former captain said he experienced two loss-of-engine-power events during flights in 2005 and quit because of his concerns with company maintenance issues. A second pilot told investigators that he also quit because of maintenance concerns and had given the company a five-page letter containing specifics.

The investigation determined that the right wing separated because of preexisting fatigue fractures and cracks in a stringer, the lower skin and rear lower spar cap. They said that this fatigue damage reduced the residual strength capability of the right wing structure, leading to the failure. Examination of the wreckage disclosed that a major repair had been made to the lower right-wing spar at the location where the wing separated from the fuselage. Maintenance records provided to the Safety Board, however, didn't contain any entry for this repair.

There had been a history of fuel leaks near the area where the wing failed, and the investigators said these leaks were indicators of structural damage inside of the wing. Had the leaks been properly addressed, the extent of the problem could have been discovered, they said. However, that would not necessarily have made the airplane safe, according to the investigators. Metallurgical examination showed significant fatigue cracks affecting the airplane's left wing, including one crack on the left-wing front spar lower cap that had extended from an area of corrosion damage and had begun to progress fairly rapidly. The Safety Board said that had the airplane not experienced the catastrophic failure of the right wing, the crack in the left wing likely would have led to a catastrophic failure. The Board said that the company's maintenance program was ineffective at identifying and correcting the long-standing structural problems that led to the in-flight separation of the right wing. The company's Director of Operations was quoted in the NTSB's report as defending the safety of airplanes in its fleet and pointing to the company's 86-year unblemished safety record before the time of the accident. The NTSB report referenced seven accidents or incidents involving Chalk's aircraft before the crash of flight 101.

The Safety Board noted that the FAA inspector who was responsible for overseeing the company's maintenance said it met all FAA requirements, and he was comfortable with the maintenance being performed on Chalk's airplanes. He had conducted an aging-aircraft inspection and records review only a few weeks before the accident. The Safety Board said he should have raised a red flag about the recurring fuel leaks on the accident airplane.

The Safety Board said there were some other clues that should have caused the FAA to look more closely at maintenance on Chalk's aircraft. Among these were: paperwork from the Department of Transportation expressing concern about the airline's financial condition; an incident in November 2004, when an airplane's elevator cable failed in flight and the airline submitted a Service Difficulty Report to the FAA; and a landing-gear-fatigue failure incident in 2001. The NTSB concluded that the Chalk's Ocean Airways maintenance program plan was inadequate to maintain the structural integrity of its aircraft fleet.

The NTSB determined that the probable cause of this accident was the in-flight failure and separation of the right wing during normal flight, which resulted from (1) the failure of the Chalk's Ocean Airways maintenance program to identify and properly repair fatigue cracks in the right wing and (2) the failure of the FAA to detect and correct deficiencies in the company's maintenance program.

Peter Katz is editor and publisher of NTSB Reporter, an independent monthly update on aircraft accident investigations and other news concerning the National Transportation Safety Board. To subscribe, write to: NTSB Reporter, Subscription Dept., P.O. Box 831, White Plains, NY 10602-0831.