as they hit the water. On examination of the wreckage, investigators discovered that, in addition to the external doublers, internal doublers had also been affixed to the root area of both wings. However, maintenance records for these repairs were not available. Investigators also concluded that the cracked stringer that initiated the wing loss had probably failed completely some considerable amount of time prior to the accident, leading to a substantial weakening of the wing structure. Age of fleet The accident aircraft had been built in 1947 and the manufacturer, Grumman (now Northrop Grumman), had produced only 60 examples of this type. Grumman ceased production of the aircraft in 1951, leaving operators with no source of new spare parts. Chalk's Ocean Airways had to resort to acquiring several un-airworthy Mallards to cannibalize for spares. In addition, the sort of aircraft used for Chalk's operations, a passenger-carrying flying boat, is of a type no longer manufactured by aircraft companies, so the option of replacing the aging Mallard fleet with newer designs was not available. At the time of the accident, the accident aircraft was 58 years old, had accumulated more than 31,000 total flight hours, and had completed over 40,000 take-off/landing cycles. When certified in 1944, the Mallard design was required to satisfy a static strength analysis. However, no fatigue requirement was yet in force, as no satisfactory fatigue analysis method had been developed at that time. The Mallard therefore had not been designed with any 'safe life' figure, unlike most civil transport aircraft today, which have designed fatigue lives of around 65,000-70,000 hrs, or twenty years. In addition, no authorized repair manual for the aircraft type had been issued by the manufacturer, responsibility for authorizing repair techniques having been acquired by an outside company after Grumman discontinued support for the type. The accident aircraft had been acquired by Chalk's in 1980 and had been upgraded to a G-73T Turbo Mallard in July 1981 when its original Pratt & Whitney Wasp H piston engines had been replaced with Pratt & Whitney Canada PT6 turboprops. Pilots' concerns As a result of a number of incidents involving Chalk's aircraft, concern had arisen among Chalk's pilots about the state of repair of the aircraft. Pilots had experienced a number of engine failures, and in one incident an elevator control cable snapped in flight, although fortunately the pilot was able to land that aircraft safely. This