Event Description
On a flight from Minneapolis to Houston with scheduled intermediate stops, a crew change occurred at Kansas City. The incoming crew noted that the left engine torque meter pressure indication was approximately 90 FSI, compared to a normal reading of 145 PSI. The flight proceeded to Tulsa, arriving at 1244. The left engine torque meter pressure indicator was considered inoperative due to its low reading and was disregarded during the flight.
Following touchdown at Tulsa, after using reverse thrust, the left propeller went to full feathered and the engine ceased. Attempts to restart it while taxiing were unsuccessful. However, on departure from Tulsa, the left engine started without difficulty and the propeller was returned to low pitch using the propeller governor control.
Flight and Incident
Takeoff from Tulsa at 1302 involved 29 passengers (including one infant) and a crew of four: Captain Paul C. Walters, First Officer Forrest A. Hull, and stewardesses Cecelia Littell and Loan Stoltenberg. The aircraft weight was 40,304 pounds, within the allowable 40,500 pounds, and the center of gravity was within limits. Pre-flight checks were satisfactory except for the low left engine torque meter reading. Flaps were set at 24 degrees for takeoff.
During the takeoff roll, the copilot called out VMC at 107 mph, V1 at 121 mph, and V2 at 122 mph. The aircraft became airborne at 124 mph (or slightly higher). Gear was immediately retracted. At an estimated altitude of not over 50 feet and airspeed of 145 mph, the left propeller feathered and then rotated slowly. A severe vibration and momentary left yaw occurred immediately after gear retraction, indicating left engine malfunction. Both engine controls remained at takeoff setting, and a single-engine climb was initiated.
The aircraft climbed to a maximum altitude of approximately 150 feet, during which airspeed decreased to about 124 mph. Doubting the airspeed could be maintained, the captain leveled off and began a shallow left turn to avoid a building. At the start of the turn, at approximately 122 mph, the captain ordered flap retraction from 24 to 12 degrees. The copilot executed the command immediately, stopping at 12 degrees. However, the captain later stated the command was given at 107 mph and further along the flight path. During the left turn, the aircraft steadily lost altitude until it struck a grove of trees about 17 feet above the ground. After striking the trees, it slid on the underside of the fuselage and was destroyed by fire. All passengers and crew evacuated safely.
Probable Cause
The Board determined that the probable cause was the retraction of the flaps from the take-off setting at a critical air speed, following the failure of the left engine torque meter assembly.
Supporting Findings
- The left engine torque meter pressure indication was below normal prior to takeoff.
- The aircraft became airborne at 124 mph or slightly higher.
- Following gear retraction at about 145 mph, the left engine torque meter assembly failed, causing the left propeller to automatically feather. The propeller continued to rotate because the torque meter failure did not affect engine power output (throttle advanced, mixture rich, ADI on, ignition on).
- The aircraft climbed straight ahead to a maximum altitude of about 150 feet, at which airspeed was approximately 124 mph.
- The aircraft began to lose airspeed, and a left turn was initiated to avoid a building.
- At the start of the turn, flaps were retracted from 24 degrees, and indicated airspeed dropped to approximately 107 mph.
- The aircraft continued to lose altitude until impact.
- The flaps were found in the "up" or "near up" position.