Accident at Chicago, Illinois, 16 Sept 1951 (N-74689)

Accident narrative

On September 16, 1951, a Peninsular Air Transport C-46D (N-74689) crash-landed shortly after takeoff from Midway Airport in Chicago, Illinois. Operating as Flight 108, the aircraft was extensively damaged. Of the 49 passengers and four crew members aboard, thirty-six passengers and all four crew members were injured, while 13 passengers were unhurt. There were no fatalities.

### The flight

The day before the accident, the aircraft flew from Miami, Florida, to Chicago, with stops in St. Petersburg, Florida, and Covington, Kentucky. Following takeoff from Covington, the left engine began misfiring. The crew corrected the issue by using a full-rich mixture, but when the mixture was leaned at cruising altitude, the engine started cutting out. The pilot found that the roughness returned when operating solely on the left engine's right magneto. The flight continued uneventfully to Chicago with the mixture in full-rich and the ignition on "Both."

At Chicago, the aircraft was taken to the Butler Company, a CAA-approved repair station. Maintenance personnel found that the rear spark plug of the left engine's No. 12 cylinder had blown out and was lying in the engine cowling with badly burned threads. The threads of the spark plug bushing had been partially burned away. Because it was a Sunday and a replacement bushing was not readily available, the service manager and the captain agreed to install a new spark plug in the old bushing, screwing it in more firmly than usual. After the engine was run up several times at partial power settings to their satisfaction, the service manager approved the work, noting that a new bushing should be installed soon.

The flight was planned over the reverse route with the same crew. The Weight and Balance Manifest showed a takeoff weight of 45,000 pounds, the maximum allowable for the aircraft. However, the manifest incorrectly listed the fuel load. The investigation later determined the aircraft was actually carrying about 1,860 pounds over its maximum authorized gross weight.

Cleared for Runway 31L, the aircraft became airborne at a speed of about 100 to 105 miles per hour. Simultaneously, the left engine started misfiring and backfiring. The captain placed the mixture in full-rich and briefly turned off the left engine's right magneto, but the engine lost all power. The copilot raised the landing gear and radioed the tower that they were returning with an engine out. The left propeller was feathered, and the aircraft began a slight turn to the left.

The aircraft climbed slowly on its right engine to an estimated altitude of 150 to 200 feet. The captain stated he believed the right engine was also losing power. The airspeed sometimes dropped as low as 100 miles per hour. Losing altitude and turning 90 degrees left of the takeoff direction, the captain aligned the aircraft with an open stretch of land. At about 75 feet, he cut power to the right engine, nosed down, and flared for a crash landing with the gear and flaps retracted. The aircraft landed exceptionally hard, breaking both engines completely free before sliding to a stop. The crew evacuated the passengers in an orderly manner.

### What the investigation found

The crash landing occurred in an open, level area about one and three-quarters miles west of the airport boundary. There was no indication of structural failure or control malfunction prior to the crash.

Examination of the left engine revealed that the newly installed rear spark plug in the No. 12 cylinder had been subjected to extreme heat. A significant portion of its shell and threads had burned away, and bushing material had fused to the plug. Two of its four shell electrodes had melted. The cylinder's intake valve and piston showed signs of abnormally high temperatures, and the intake pipe showed conclusive evidence of backfiring and torching into the induction system.

The Board concluded that because the mating surface of the bushing had been burned away, there was insufficient metallic contact to allow the new spark plug to cool. During the high power settings of takeoff, the plug overheated, acting as a point of heat concentration that ignited fuel as the intake valve opened. This caused blowback into the induction system and a complete power loss.

Investigators also found defects in the ignition systems of both engines. The right magneto of the left engine had a badly burned and partially missing rotor contact, which the Board noted could have caused some power loss, though it did not contribute to the No. 12 cylinder failure. The right magneto of the right engine also had a broken rotor contact and evidence of abnormal heat from arcing. The Board stated it was possible this condition caused a small power loss in the right engine, aligning with the captain's belief. Subsequent bench tests showed the right magnetos from both engines functioned abnormally at takeoff speeds.

The Board criticized the captain for taking off in an overloaded aircraft and for allowing it to leave the ground at an airspeed too low to maintain single-engine flight. The CAA Airplane Flight Manual specified a break-ground speed of 105 mph at the maximum gross weight of 45,000 pounds. The Board also criticized the service manager for approving the use of the eroded bushing. Following the accident, the Civil Aeronautics Administration filed violation charges against the captain and the carrier, and suspended the service manager's certificate for three months.

### Probable cause

The Board determined that the probable cause of this accident was "the poor technique used by the pilot in taking off at too low an air speed to maintain single engine flight, followed by a critical loss of power from the left engine, and subsequently a partial loss of power from the right engine, conditions which were aggravated by the effects of the overloaded aircraft."