The Case of Flight 476

At 11:36 on the morning of last Aug. 4, American Airlines Flight 476, a Convair 240 with two 18-cylinder engines, took off from Springfield, Mo. headed for St. Louis. Twenty-one minutes later, at 12:17, the pilot called on the radio: "Does anybody read 476?" American's radio at Springfield acknowledged, but got no reply. The ground station in St. Louis and two other American airliners heard Flight 476 reporting a fire in No. 2 engine. Three minutes later one of the airliners heard: "Springfield, are you reading 476? We have a bad engine fire." That was the last message from Flight 476. Two minutes later (12:22) the stricken airplane, trailing flames and smoke, was seen heading for the Army field at Fort Leonard Wood. Just short of Runway 14 the right wing came off, and the airplane crashed in heavy timber, killing all passengers (27) and crew (three).

The scattered fragments had hardly cooled when Civil Aeronautics Board investigators arrived to inspect the wreckage. Last week the CAB was circulating its report on Flight 476: an example of the meticulous detective work that it does to find the culprits, men or machines, responsible for every major airline crash. Since Jan. 1, 1954, it has analyzed 401 accidents to U.S. airplanes. Its sleuths have marked only one of them "cause undetermined."

Guilty Cylinder. At the scene of the crash of Flight 476. the CAB men searched out every scrap of wreckage. Then all parts that might be concerned with the accident were taken to American Airlines' Overhaul and Supply Depot at Tulsa, where the No. 2 (righthand) engine, with its adjacent landing gear and wing structure was assembled in flight position. By this time the CAB detectives had a good notion where the trouble started, but they came to no decision until masses of evidence had been accumulated.

Suspicion pointed early to No. 12 cylinder, whose barrel had broken clear through. The CAB men sent the cylinder parts to the National Bureau of Standards, where laboratory examination showed several small fatigue cracks that had joined to form a single large crack one-third of the way around the cylinder. The steel itself proved sound, with no microscopic abnormalities that might have caused the cracks. So there must be other culprits.

The CAB detectives turned next to the cylinder's records, which must be kept meticulously by every airline. Cylinder No. 12 had first been installed in the No. 18 position in another engine. After 1,052 hours of operation, eight of its hold-down studs had failed. The damage had been found on a routine inspection; the cylinder had been removed and sent to American's base at Tulsa.

Now suspicion pointed straight at the airline's inspectors at Tulsa. American Airlines' own rules require that any cylinder found with more than two studs broken must be scrapped or sent back to the manufacturer (in this case Pratt & Whitney), because such a failure indicates that the base may be warped. Another rule requires that the base flanges of all cylinders going through overhaul must be inspected for flatness with delicate instruments.

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.

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