Wreckage examination

The Civil Aeronautics Board (CAB), charged with investigating the accident, examined the wreckage at Long Island MacArthur Airport. Reports from witnesses of the crash indicated the aircraft appeared to have made a normal takeoff and began climbing normally. But, moments later it began banking sharply to the right. Investigators examined the four charred engines and concluded all were producing power at the point of impact.^[3][5] They could not conclusively determine the amount of power being produced, but stated there was no evidence found in the wreckage that suggested the engines might have suffered an operational failure.^[5][6]

They were also able to determine all of the flight control surfaces, including the elevators, ailerons and rudder, were functioning properly at the point of the crash, and there were no faults in the flight control system. The flaps were extended to between 15° and 20°, the standard setting for take-off. The propeller blades of No. 4 engine— on the far right side of the aircraft—were reversed—minus 8°, while the blades of Nos. 1, 2 and 3 engines were at 34° positive pitch (also standard for take-off).^[5][6]

Reversed thrust

The propellers of a DC-6 are designed to provide reverse thrust after the aircraft touches down. The pilot then retards the throttle levers to a point below idle speed and that directs the electric mechanisms in the propeller hub to rotate the blades to a position in which they will provide reverse thrust.^[6] Should the pilot need to perform a go-around, he moves the thrust levers forward to a positive position again and that will produce forward thrust, enabling the pilot to execute a go-around maneuver.^[6]

The Douglas Aircraft Company designed a system that would prevent the accidental reversal of propeller blades in-flight. During development of the DC-6, the company installed a system that cut electrical power to the mechanisms which rotated the blades while the airplane was in the air. When there was enough weight on the landing gear (which would only be the case when the aircraft was on the ground), a switch which supplied electrical power to the mechanisms was closed—meaning that when the aircraft touched down the blades could be reversed and thus the airplane could be slowed. When the switch was closed, a red flag would swing into view in the cockpit of the aircraft, warning the crew that the blades could be reversed. Should the switch fail to close upon landing, the flag could be raised manually and electrical power to the mechanisms would be restored. When the aircraft took off, electrical power would be cut to the mechanisms so that the propeller blades could not be inadvertently reversed, and the red flag swung out of sight. Reverse thrust warning lamps, which would have warned the crew if the propellers were reversed, were not fitted on Mainliner Idaho.^[6]

Flight tests

The CAB carried out flight tests using a DC-6. They found that if the propellers were reversed prior to take-off they would not, if the flag was not raised, be rotated automatically again in the air to produce forward thrust if full power was applied. Tests performed by United Airlines showed that, if the propellers of just one engine were reversed and full power was applied to all four engines, then the aircraft would spiral into a dive. If METO (maximum except take-off) power was applied to Nos. 1, 2 and 3 engines, and full reverse thrust was applied to engine No. 4, then the aircraft would become uncontrollable.^[5][7]

If full left aileron was applied, the aircraft could be recovered for a short period of time, but a violent turn to the right would continue, and the competing forces would cause the aircraft to stall, and violently roll and pitch down. Flight tests, investigators said, accurately reproduced what happened to Mainliner Idaho during the accident sequence. The tests performed by United and by the investigators showed that if, after the aircraft became airborne, full power was applied to an engine whose propellers were reversed, the propellers would produce not positive thrust, but increased reverse thrust.^[5][7] One aviation author wrote of the crash,

"The flight tests showed conclusively that, at take-off configuration, a DC-6 becomes uncontrollable with an outboard engine at full power with its propeller in reverse pitch. Control is lost so quickly that there is little the crew can do at low altitude. In the case of this accident, it was doubtful if there would have been time for forward thrust to be restored before control was lost."

— Macarthur Job, Air Disaster Vol. 4, 2001^[7]

Conclusions

While the wreckage was being examined, investigators found that all four engines were producing thrust at the time of impact. There were only two ways that the propeller could be reversed during the take-off sequence. Investigators ruled out electrical malfunction since, after detailed examination of the engine hub, there was no evidence found of this happening.^[5][8] Therefore, it was concluded that the only way the propeller could have been reversed was through an unintentional crew action. Although there was no formal evidence that a simulated engine failure was being performed, statements submitted by witnesses suggested that it was likely this was the case. United Airlines procedure calls for No. 4 engine to be shut down in a simulated engine failure—the same engine which was found at the crash site with its propellers reversed.^[5][8]

The investigation concluded the accident sequence began when the check pilot, while the aircraft was on the ground, retarded the throttle lever for No. 4 engine past the idle position, and therefore reversed the propellers of that engine. Once the airplane took off and started banking to the right, it would have been a natural reaction for one of the flight crew to increase power to No. 4 engine, thinking that by doing so the engine would start producing positive thrust and the aircraft could be recovered. However, since the metal flag was not raised, there was no electrical power to the rotating mechanisms—and increasing power to No. 4 engine would only have created more reverse thrust.^[5][8]

The final accident report concluded there wasn't sufficient time for the crew to react, since the dive began suddenly while the plane was so close to the ground. "Control will be lost so quickly that there is little, if anything, that the pilot can do if it occurs at low altitude," the report stated. "He must recognize what is occurring, analyze it, and take action to unreverse in a very limited amount of time. It is doubtful that unreversing could have been accomplished in this instance before control was lost."^[5]

On October 4, 1955, the CAB released the final accident report, which concluded the reversal of the propellers and subsequent increase in power of the No. 4 engine had caused the accident.^[5]

"The Board determines that the probable cause of this accident was unintentional movement of No. 4 throttle into the reverse range just before breaking ground, with the other three engines operating at high power output, which resulted in the aircraft very quickly becoming uncontrollable once airborne."

— Civil Aeronautics Board, Accident Investigation Report; United Air Lines, Inc., MacArthur Field, Islip, New York, 1955^[5]

Technological advances

Following the accident, the Civil Aeronautics Administration (CAA) issued an Airworthiness Directive ordering all DC-6 and DC-6B aircraft to be fitted with a sequence gate latch, known as a Martin bar. The device is a metal bar which a crew would manually swing in front of the thrust levers over the idle line, physically preventing the thrust levers from being retarded into the reverse position. According to the CAB report, a United Airlines engineer told investigators the Martin bar should make propeller reversal "a more reliable and safer device [than the system fitted to Mainliner Idaho] ... with its numerous switches, relays, and automatic operation."^[5]

United Airlines issued a statement saying it had begun installing the device on its fleet of DC-6 and DC-6B aircraft one week before the accident, having used it successfully in service on their fleet of Douglas DC-7 aircraft. A Martin bar had not yet been fitted on Mainliner Idaho.^[7] United Airlines also said a program had begun to install reverse thrust indicator lights on all their DC-6 and DC-6B aircraft. The signals, fitted in the cockpit of the aircraft, would have warned the flight crew that the thrust lever had been pulled back too far, and the propellers had been reversed.^[5]

Similar accidents

Since the crash, there have been several other accidents involving reverse thrust. A Douglas DC-8 operating United Airlines Flight 859 crashed in 1961 when the first officer attempted to reverse all four engines during the landing roll. The left engines remained in forward thrust, while the right engines went into reverse, causing the aircraft to veer rapidly to the right and collide with airport construction vehicles, killing 17 of the 122 people aboard and 1 person on the ground.^[9] Japan Airlines Flight 350, a DC-8, crashed in 1982 short of the runway in Tokyo, after the mentally ill captain attempted suicide during the final approach phase of the flight, by putting the inboard engines into reverse thrust. Of the 174 people aboard, 24 died.^{[10][11][12][13]} In 1991, Lauda Air Flight 004, operated by a Boeing 767, crashed after the left engine thrust reverser deployed in-flight for reasons that could not be determined.^[14] The crash of a TAM Airlines Fokker 100 in 1996 was attributed to the deployment of the thrust reverser on No. 2 engine. The aircraft rolled to the right and crashed in a populated area of São Paulo, Brazil.^[15]