Investigation Summary
The investigation centered on the No. 12 cylinder of the right-hand Pratt & Whitney R-2800 engine. This specific cylinder had previously been removed from another engine after eight of its hold-down studs failed, a condition known to cause flange warping. American Airlines' maintenance protocols required that any cylinder with more than two broken studs be scrapped or returned to the manufacturer. However, an inspector at the Tulsa overhaul base performed only a visual check, ignoring the requirement for precision instrument measurement of the flange. The warped cylinder was reinstalled on a different engine, where the uneven stress led to rapid fatigue cracking. After only six hours of operation, the cylinder barrel failed completely, releasing a fuel-air mixture and oil that ignited against the hot exhaust manifold. The fire breached the diaphragm seal and entered the nacelle, where it severed fuel and hydraulic lines. Although the crew feathered the propeller and discharged CO2 extinguishers, the fire remained fed by a fuel shutoff valve that stayed open, likely due to fire damage to its electrical wiring. The intense heat eventually caused the structural failure of the right wing's front spar.
Final Conclusions
The Civil Aeronautics Board (CAB) determined the probable cause was the installation of an unairworthy cylinder, the failure of which resulted in an uncontrollable fire and subsequent loss of a wing in flight. The board found that maintenance inspectors had countermanded official overhaul manuals with verbal instructions that bypassed critical safety checks. Additionally, the intensity of the fire was exacerbated by the failure of the right main tank fuel shutoff valve to close, as the fire had destroyed the electrical circuits required to operate it.
Video Analysis
The Case of American Airlines Flight 476 — A detailed look at the 1955 crash of a Convair 240 caused by maintenance shortcuts and a catastrophic engine fire.
If playback is blocked, the owner has disabled embedding — use the link above.
Airframe & Maintenance
Crew Experience
Captain
Age 45, employed by American Airlines since 1942.
First Officer
Age 35, employed by American Airlines since 1944.
Systems & Failure Modes
No. 12 Cylinder
A steel engine component that failed due to fatigue cracks caused by a warped mounting flange.
Firewall Shutoff Valve
An electrically operated valve designed to cut off fuel to the engine; it failed to close because the fire destroyed its wiring.
Front Wing Spar
The primary structural member of the wing, which failed in tension after being weakened by extreme heat.
Interesting Facts
- 01The No. 12 cylinder had logged only six hours of flight time since its improper overhaul and reinstallation.
- 02The cylinder had previously suffered eight broken hold-down studs in a different engine, which should have mandated its destruction.
- 03The fire was so intense it melted the aluminum nacelle skin and the lower rail of the front wing spar.
- 04The right main tank fuel shutoff valve was found in the open position because fire had burned away its electrical connector.
- 05Army engineers had to bulldoze a road through dense forest to allow heavy rescue equipment to reach the crash site.
Safety Actions & Advisories
Cylinder Mutilation Policy
American Airlines implemented a rule requiring that any cylinder with faulty studs be physically mutilated to prevent it from being accidentally returned to service.
Fire Seal Inspection Overhaul
The airline began renewing diaphragm fire seals at every overhaul rather than 'on condition' and introduced chalk tests to ensure proper mating between the diaphragm and cowl.
Location
APPROXIMATE POSITION · AIRPORT LEVEL · Coordinates are the estimate published in the official investigation report. The ring shows the accuracy the record supports, not the point of impact.