Engine failure may have caused a Durham doctor to fatally ditch his antique plane in the ocean near Portland Head Light in Cape Elizabeth last year.

Just before noon on June 24, 2012, Louis Hanson was pulled from the water by paddle boaters and taken to nearby Fort Williams by a Sea Tow vessel, a Coast Guard spokesman said the next day. By the time Hanson reached rescuers on shore, he was unresponsive. He never regained consciousness.

Pieces of the No. 3 piston skirt in Hanson’s 1946 Stinson Voyager had entered the engine — including the camshaft gear — which affected the magneto timing, according to the National Transportation Safety Board’s factual report released last Wednesday.

Changes to the magneto timing can sometimes cause engine detonation, Bob Gretz, one of the NTSB investigators working on the case, said Tuesday.

However, “there was no evidence of detonation here,” Gretz said, declining to say what caused the engine to lose power.

But, he said, it wasn’t good that several pieces of metal had entered the engine.

The report said the FAA inspector investigating the retrieved wreckage of the 66-year-old four-seater, was able to rotate the propeller by hand and confirm crankshaft, camshaft and valve train continuity to the rear accessory section.

Manufactured in 1946, Hanson’s plane was equipped with a 150-horsepower engine that was installed new in 1947, according to the report. A subsequent partial disassemble of the engine revealed that the No. 3 piston skirt had fractured.

“During a telephone interview, the mechanic who overhauled the pistons in 2007, stated that he was not surprised that one failed, as they were 60 to 70 years old,” the report states.

The mechanic added that as part of the overhaul procedure, he balanced the six pistons by ensuring they were the same weight.

The FAA investigator said the No. 3 piston was forwarded to the NTSB Materials Laboratory in Washington, D.C., for further examination. The metallurgical examination revealed two fracture surfaces on the piston skirt, which was smeared with mechanical contact.

Eric Weiss, an NTSB spokesman in Washington, D.C., said Tuesday that the piston skirt is the portion of the piston that extends the lowest.

“It is tasked with keeping the piston from rocking excessively in the cylinder,” Weiss said, reading from a definition.

He said the piston skirt is typically machined with small grooves to aid in holding and transporting oil to the cylinder walls to provide proper lubrication.

In a combustion engine, the pistons are sealed inside of the cylinder walls by the piston rings, Weiss said. The rings are making contact with the cylinder wall while the piston rides up and down, centered by the rings in the cylinder wall.

At the top and bottom of each stroke, as the piston is changing direction, the piston rocks, Weiss said. It is then that the piston skirt makes contact with the cylinder wall, setting the piston straight once again to continue its journey, he said.

Hanson’s journey on June 24 started at about 10:30 a.m. when he flew the plane from Twitchell’s Airport in Turner.

According to the FAA, the Voyager was in radio and radar contact with Portland International Jetport in Portland as Hanson flew it along the coast between 500 and 1,000 feet above the ocean from south to north.

He radioed that he was flying low for sightseeing and photography. At 11:54 a.m., Hanson radioed that the airplane had experienced a total loss of engine power at about 500 feet up, the factual report states.

Hanson “indicated that he was not going to be able to glide the airplane far enough to reach the shore,” the report states.

The last radar target was recorded at 11:55:20, indicating an altitude of 200 feet above the ocean.

According to an FAA inspector, the airplane hit water about 100 yards from the shore and sank to a depth of about 70 feet.

Hanson exited the plane and donned a life vest dropped from a nearby helicopter, but couldn’t inflate it, the report states. A boater subsequently retrieved the pilot and noted that he was unresponsive.

According to the factual report, the FAA inspector said a sheen of fuel was noted on the ocean surface when the airplane was recovered.

Once the airplane was recovered, the inspector was able to confirm flight control continuity, the report states.

Weiss said the factual report is just that. It lays out the facts.

“The fact-gathering portion looks at man, machine and the environment,” he said.

Probable cause — why NTSB investigators believe the crash occurred — is the next and final report. It will be released in mid-May, NTSB investigator Gretz said.

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