1955 MacArthur Airport United Air Lines crash

On April 4, 1955, a Douglas DC-6 named Mainliner Idaho operated by United Air Lines crashed shortly after taking off from Long Island MacArthur Airport, in Ronkonkoma, New York, United States.

The flight was operated for the purpose of maintaining the currency of the instrument rating of two of the airline's pilots. Shortly after takeoff and only seconds after climbing through 150 ft, the plane began banking to the right. It continued to roll through 90 degrees; the nose then dropped suddenly and moments later it struck the ground. All three members of the flight crew were killed upon impact.

An investigation found a simulated engine failure procedure was being conducted, which involved a member of the crew pulling back the throttle lever for engine No. 4 prior to taking off. Investigators found that if the throttle lever was pulled back too far, it would cause the propeller to reverse—a feature designed to slow the aircraft upon landing. Once the landing gear was raised, the crew would have to raise a metal flag in the cockpit to bring the propeller blades back into the correct position, since a safety device prevented electric power from operating the rotating mechanism at the roots of the blades unless the aircraft was on the ground or the flag was manually raised. The Civil Aeronautics Board (CAB) concluded one of the flight crew applied full power to No. 4 engine, thinking this would bring the aircraft out of the increasing bank. Because the blades were reversed and the flag was not raised, that increased the reverse thrust from No. 4 engine, causing the DC-6 to spiral out of control. Since the plane was so close to the ground, the suddenness of the bank and dive meant the flight crew had no chance to recover the aircraft before impact.

In the aftermath of the accident, the Civil Aeronautics Administration (CAA) issued an Airworthiness Directive ordering all DC-6 and DC-6B aircraft to be fitted with a manual device which could prevent the inadvertent reversal of the propeller blades. United Air Lines also stated they had begun installing reverse thrust indicator lights in the cockpits of their DC-6 aircraft, which would warn pilots when a propeller had reversed.

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 Air Lines 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.

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. One aviation author wrote of the crash,