Douglas DC-3C accident at Charlottesville, Virginia, 2 Oct 1957 (N-3947A)

Accident narrative

On October 2, 1957, at approximately 1211, the main landing gear of a Piedmont Airlines Douglas DC-3C collapsed as the aircraft was being taxied to the ramp at Charlottesville, Virginia. The aircraft, leased from the U.S. Navy Department and registered N 3947A, sustained substantial damage. The crew and 14 passengers deplaned immediately, and no one was injured.

### The flight

Flight 85 departed Washington, D.C., at 1130, destined for Knoxville, Tennessee, with several scheduled intermediate stops including Charlottesville. The gross takeoff weight was 23,845 pounds, well under the maximum allowable of 25,346 pounds, and the load was correctly distributed.

The crew consisted of Captain Howard Coe Kelly, who had 12,110 flying hours including 6,282 in the DC-3, and Copilot Fred Kozak, who had 2,095 flying hours with 1,871 in the DC-3.

### Sequence of events

The flight to Charlottesville was made in VFR weather conditions and was routine. The copilot flew the aircraft from the right seat while the captain performed the duties of copilot. Approaching Charlottesville, the crew determined the wind was southerly and elected to land on runway 2L. The copilot made a normal landing and rollout.

After the landing roll, the aircraft was turned around on the runway and taxied approximately 300 feet to a taxiway. A 90-degree right turn was made, and the aircraft taxied an additional 531 feet. As the aircraft approached the ramp, the captain took over the controls. At this time, both main landing gears collapsed. No fire occurred.

The pilots stated that they had followed the usual checklist in preparation for landing. The landing gear was lowered, and each pilot visually checked that the gear on his side had extended. Hydraulic pressure was allowed to build to 750 pounds per square inch. The mechanical latch on the cockpit floor was then engaged, and the gear selector handle was returned to NEUTRAL. Both pilots stated that the green "gear safe" light indicated the gear was down and locked, and that the warning horn did not sound when the throttles were closed on the flareout. They noted no indication of an unsafe gear condition during the landing, rollout, or taxiing, but stated that the warning horn and red warning light came on simultaneously with the collapse at the ramp. Both pilots testified that the landing gear controls had not been moved after the gear was lowered and locked, nor after it collapsed.

### What the investigation found

The crew secured the cockpit immediately following the accident. Investigators found the landing gear selector valve handle in the NEUTRAL position and the mechanical latch locked in the POSITIVE LOCK position. The hydraulic system pressure gauge indicated 750 pounds pressure, while the landing gear pressure indicated zero. When the battery master switch was turned on, the landing gear warning light indicated a red or unsafe condition, and the warning horn properly sounded when the throttles were retarded.

The DC-3 main landing gear utilizes a spring-loaded safety latch in each nacelle that engages a hook slot at the lower end of each actuating strut piston to lock the gear in the safe landing position. These latches are controlled by a single handle on the cockpit floor, which has three positions: UP LATCH, SPRING LOCK, and POSITIVE LOCK. When placed in POSITIVE LOCK, a cable-actuated piston exerts 100 pounds per square inch of pressure on the latches to prevent them from being withdrawn from the hook slots.

Both main landing gears were found in the fully retracted position. The left gear was undamaged, and its mechanical safety latch was in the full down position. On the right gear, the inboard side of the wheel axle had broken through the nacelle structure and was forced upwards four inches beyond its normal travel. Its mechanical safety latch was also full down and undamaged. However, the right gear's actuating cylinder hydraulic down line was ruptured, and trapped hydraulic fluid was found in the nacelle and on the pavement directly below.

When the aircraft was raised, the mechanical safety latches were found to be properly rigged. When the latch control was placed in SPRING LOCK and the gear allowed to fall, the left gear locked down with slight hand pressure. The right gear fell to the full down position, but the mechanical safety latch would not enter the strut end hook slot. Investigators determined that excessive pressure built up as a result of the partial collapse of the gear, rupturing the hydraulic down line. The weight of the aircraft forced the gear up beyond its normal travel, and the resulting severe twisting moment slightly bent the actuating strut end hook, preventing the latch from entering. Once this hook was replaced, the right gear locked down normally.

The gear was cycled a number of times and operated normally, with all warning system indications functioning properly. No mechanical defect or irregularity was found that could have interfered with the gear's operation.

### Analysis

The Board noted that for both gears to collapse as they did, both safety latches would have had to jam in the UP LATCH position. The Board found this extremely unlikely, noting no evidence on either latch mechanism that could have caused such a double failure. Furthermore, if one or both latches had hung up, it would have required considerably greater than normal force to latch the cockpit control, the red warning light would have come on, and the warning horn would have sounded.

The Board determined that the hydraulic down line burst as a result of excessive pressure. This line could not have failed before the gear was extended for landing, as the main system hydraulic fluid would have been lost. The fluid found on the ramp indicated the line burst after the aircraft reached that point. For the line to rupture in this manner, the landing gear selector handle had to be in NEUTRAL with the latches in the UP LATCH position; otherwise, excessive pressure from a partial collapse would have been relieved through pressure relief valves.

The Board concluded that for the accident to have occurred in the manner indicated by the crew, a number of simultaneous malfunctions would have had to occur and then correct themselves: both safety latches would have had to hang up and then seat properly; the cockpit latch control would have had to misrig itself and then correct itself; the three microswitches operating the warning system would have had to fail and reverse their normal functions to give a green light instead of a red one, then correct themselves; and the warning horn would have had to be inoperative and then correct itself. In the absence of evidence to substantiate such malfunctions, the Board concluded no mechanical failure contributed to the accident.

### Probable Cause

The Board determined that the probable cause of this accident was "that the cockpit landing gear safety latch lever was accidentally or inadvertently moved to the UP LATCH position while the aircraft was being taxied to the ramp."