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Visual Check. After prying into the records and questioning personnel, the CAB detectives found that both rules had been ignored. The flange had been checked only "visually" (by looking at it) before the cylinder was installed in No. 2 engine of the Convair that crashed as Flight 476. An airline inspector testified that this corner-cutting technique was "handed down" to him by a predecessor. The hand-down proved disastrous. When the flange, slightly bent by the earlier failure of its studs, was drawn tight on the second installation, the stresses set up in the steel must have caused fatigue cracks. The engine ran only six hours before the fire and crash. This conclusion was confirmed by Pratt & Whitney, which strained a brand-new cylinder by stud failure, installed it in an engine, and ran it on a test stand. A similar fatigue crack developed, and the cylinder failed in three hours.
CAB's reconstruction of the Flight 476 crash: the cylinder crack released an explosive mixture of gasoline and air, which was probably ignited by the hot exhaust manifold. The flames passed through the fire wall behind the cylinders, where they should have been stopped, and melted gas and oil lines, which released fresh fuel. The fire, now a roaring blowtorch, burned through the aluminum nacelle skin and heated the front wing spar. It failed, and the wing came off.
Already the CAB men had made one important point. They could not prove what was wrong with the fire wall, but something undoubtedly was. At once American Airlines started overhauling its entire fleet of Convairs, inspecting their fire walls and improving their fire detection and extinguishing devices.
The kind of corner-cutting that the inspector was guilty of is not likely to happen again. New rules now require that a cylinder that has had stud trouble must be mutilated so that it cannot be used again without a trip to the factory for careful rehabilitation. When the report on Flight 476 is circulated through airline bases, inspectors will think twice before cutting corners. But the CAB's detectives will not relax their vigilance. New airplanes have new weaknesses, which must be found and corrected. New accidents, even though fewer in number, will bring new problems for the detectives.