Both engines had stopped before the twin-engined aircraft piloted by Rouse Co. President Michael D. Spear crashed in a Boston neighborhood in August, the National Transportation Safety Board said.
The cause of the Aug. 24 crash, which killed Mr. Spear, his wife and one of four daughters, has not been determined, NTSB officials said this week. Safety board officials declined to say whether Mr. Spear accidentally or deliberately shut down the aircraft's right engine, or whether it malfunctioned.
The 49-year-old Mr. Spear, who had a reputation as a well-prepared, methodical and enthusiastic pilot, had left Chatham on Cape Cod, where the family has a summer home, about 6 a.m. on a foggy morning.
About 20 miles from Logan International Airport, he radioed the control tower that the left engine on his Piper Cheyenne II had lost power and was trailing smoke. He told air traffic controllers that he had shut the left engine off, NTSB officials said.
On his first approach to the fog-shrouded airport, Mr. Spear reported that he was coming in too fast. After circling back, he radioed that he couldn't maintain altitude. A short time later, the plane crashed in a residential community about 2 1/2 miles from the airport. Two homes burned, but nobody on the ground was hurt.
Investigators from the safety board sent the propellers and engines back to their manufacturers for disassembly and analysis.
"The tear-down of the left engine and the left propeller substantiated the pilot had shut down the left engine as he had claimed he had," said Drucella Andersen, spokeswoman for the board.
She said that there was little or no "rotational damage" to internal parts of the right engine and that the right propeller was "feathered" -- shifted either manually or automatically to a shut position. That evidence, she said, showed that the right engine was not rotating at impact.
A pilot familiar with the Piper Cheyenne II said the evidence suggested the right engine had quit or been shut down for some distance, since it would take a long time for its internal machinery to stop rotating.
Several pilots said the evidence raised key questions for investigators: whether the right engine failed, whether Mr. Spear shut it down -- and, if so, whether he did that by accident or deliberately.
The failure of both engines in the 4-year-old aircraft was described as highly unlikely by the pilots. And one said he could think of no reason for Mr. Spear to turn off the right engine deliberately.
Another said it would not be unheard of for a pilot faced with an emergency to mistake the controls of one engine for that of the other.