Investigation Summary
The accident occurred during a proficiency check flight intended to maintain the instrument ratings of two United pilots. The investigation by the Civil Aeronautics Board (CAB) focused on why the aircraft entered an uncontrollable right-hand roll almost immediately after breaking ground. Examination of the wreckage revealed that while engines 1, 2, and 3 were at a standard takeoff pitch of 34 degrees, the No. 4 propeller was found at a pitch of minus 8 degrees, indicating it was in reverse. On the DC-6, a throttle must be moved past a gate to enter the reverse range, a feature normally used for braking after touchdown. A safety switch on the landing gear strut prevents the propellers from reversing unless the aircraft's weight is on the wheels. However, if the throttles are moved to reverse while on the ground, the propellers will stay in reverse even after the aircraft becomes airborne unless a manual override (a metal flag in the cockpit) is activated to provide electric power to the pitch-changing mechanism. The CAB concluded that the check captain likely retarded the No. 4 throttle too far during the takeoff roll to simulate an engine failure. Once airborne, the asymmetric reverse thrust caused a rapid roll that the crew could not recover from at such low altitude.
Final Conclusions
The Civil Aeronautics Board released its final report on October 4, 1955. The Board determined the probable cause was the unintentional movement of the No. 4 throttle into the reverse range just before the aircraft became airborne. With the other three engines at high power, the resulting asymmetric thrust made the aircraft uncontrollable at low altitude. The investigation noted that the suddenness of the roll left the crew with insufficient time to analyze the situation and manually override the reverse pitch mechanism.
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
Crew Experience
Check Captain
Stanley C. Hoyt, age 45
Systems & Failure Modes
Hamilton Standard 43E60-317 Propellers
Variable-pitch propellers capable of reversing thrust to assist in braking during landing.
Propeller Reversing Safety System
A system including a landing gear strut switch and a cockpit 'flag' override designed to prevent accidental in-flight reversal.
Interesting Facts
- 01The aircraft was named 'Mainliner Idaho'.
- 02The flight was a training mission for instrument rating proficiency.
- 03The No. 4 propeller was found at a pitch of minus 8 degrees (reverse).
- 04The aircraft was significantly below its maximum permissible takeoff weight at 61,000 pounds.
- 05The aircraft's center of gravity was within the prescribed limits.
- 06All three crew members were killed instantly upon impact.
- 07Weather at the time was clear with strong southwest winds of 20 knots, gusting to 30 knots.
- 08The aircraft serial number was 43001.
Safety Actions & Advisories
CAA Airworthiness Directive
The Civil Aeronautics Administration ordered all DC-6 and DC-6B aircraft to be fitted with a manual device to prevent inadvertent propeller reversal.
Reverse Thrust Indicator Lights
United Air Lines began installing indicator lights in DC-6 cockpits to provide pilots with a clear warning if a propeller entered the reverse range.