Boeing 377 accident at Near Carolina, Brazil, 29 Apr 1952

Accident narrative

At approximately 0740 Greenwich Mean Time on April 29, 1952, a Pan American World Airways Boeing 377, operating as Flight 202, crashed in dense Brazilian jungle about 887 nautical miles north-northwest of Rio de Janeiro. The aircraft was completely demolished. All 50 persons aboard, consisting of 41 passengers and a crew of nine, lost their lives.

### The flight

Flight 202 originated in Buenos Aires, destined for New York, with scheduled stops at Montevideo, Rio de Janeiro, and Port of Spain. Following a routine crew change at Rio de Janeiro, the flight departed at 0217. The new crew consisted of Captain Albert Grossarth, First Officer L. A. Penn, Jr., Navigator John T. Powell, Flight Engineer Paul L. Stilphen, Radio Operator Leroy R. Holtzclaw, and four cabin attendants.

The aircraft was cleared for an off-airways direct route from Rio to Port of Spain. The flight plan called for cruising altitudes of 12,500 feet to the Barreiras check point, 14,500 feet to Santarem, and 18,500 feet to Port of Spain. The take-off weight was approximately 3,550 pounds below the maximum allowable.

After taxiing out, the crew ran up the engines and advised at 0230 that they were returning to the ramp for maintenance. Shortly thereafter, however, the aircraft taxied to the end of the runway, waited approximately 10 minutes, and took off at 0243.

### The sequence of events

The flight made routine position reports as it proceeded north, stating it was flying under Visual Flight Rules (VFR) conditions, off airways. At 0616, a message was relayed from the flight reporting that it was abeam Barreiras at 0615, flying at 14,500 feet under VFR conditions, and estimating abeam Carolina at 0745. This was the last known message from the aircraft.

When Flight 202 failed to report at Carolina or arrive at Port of Spain, a search was initiated. On May 1, the wreckage was spotted from the air by a United States Air Force search unit. The location was approximately on course, 36 nautical miles south-southeast of the Carolina check point.

### What the investigation found

Reaching the remote crash site required a complex expedition involving amphibious aircraft, helicopters, and ground parties cutting trails through dense, burning jungle. The hostile environment and the onset of the rainy season severely limited the time investigators could spend at the scene.

The wreckage was scattered over an irregularly shaped area approximately 4,000 feet long. The distribution indicated that the aircraft had disintegrated in flight over a very short interval of time. The main wreckage site contained the forward fuselage, the right wing, the Nos. 3 and 4 engines, and the landing gear. The outboard portion of the left wing, along with the No. 1 engine, was found 2,300 feet away. The tail assembly was located 2,500 feet from the left wing.

Examination of the left wing structure showed that it had separated in flight due to a tension failure on the lower chords, pointing to an upward failure of the wing due to excessive aerodynamic loads. The left horizontal stabilizer had also failed in flight from an excessive upward load, and the left elevator had separated into several pieces.

The Nos. 1, 3, and 4 engines were recovered and examined. Investigators found no evidence of in-flight fire, structural failure, or propeller malfunctioning in these three powerplants.

Despite extensive air and ground searches, the No. 2 engine and its propeller were never found. However, the No. 2 engine mount remained attached to the right wing and fuselage wreckage. Examination of the mount revealed failures typical of a shear failure of the bolts and torn lugs, indicating the engine separated from the aircraft due to the application of forces beyond its design limits.

Investigators found a film of engine oil on widely scattered parts of the aircraft, including the vertical fin, rudder, left flap, and left stabilizer. The Board noted this indicated an abnormally large quantity of oil was discharged from the No. 2 nacelle area before these parts separated from the airplane, likely from severed lines when the engine tore away.

The wing flap actuating screws were found in positions corresponding to an extension of approximately three to four degrees. The Board noted this was not normal for cruising flight and suggested the probability of an attempt by the crew to stop tail buffeting caused by disturbed airflow over the No. 2 nacelle after the engine separated.

The Board reviewed the service history of the Hamilton Standard propellers installed on the Boeing 377. Testing and service experience had revealed that the blades were subject to fatigue failures originating from comparatively minor damage or internal voids. The Board noted previous instances where a blade failure on this aircraft type resulted in destructive unbalance that tore the engine from the wing.

In analyzing the sequence of disintegration, it appeared to the Board that the emergency originated in either the No. 2 propeller or engine, causing it to tear away. The Board believed it probable that severe buffeting, set up by the missing engine, caused a partial failure of the left stabilizer. The oscillation of the stabilizer may have snapped the elevators upward, causing a high down-load on the tail and a great increase in lift on the wing, leading to the upward failure of the left wing and the subsequent downward separation of the tail assembly.

The Board ruled out several other potential causes for the disintegration. Weather analysis indicated no storms were occurring and the air was generally smooth. Investigators found no evidence of an explosion, sabotage, fire in flight, airframe fatigue failure, or malfunctioning of the autopilot or rudder boost systems.

### Probable cause

The Board determined that the probable cause of this accident was the separation of the No. 2 engine and propeller from the aircraft due to highly unbalanced forces, followed by uncontrollability and disintegration of the aircraft for reasons undetermined.