A valve that supplies gas from the plane's left tank to its left engine was stuck so both of the aircraft's engines may have drawn all of the fuel from the right tank, 
NTSB senior air safety investigator Tim Monville said yesterday.
Flight 9399 was en route from Key West to Southwest Florida International Airport in Fort Myers when it reported the loss of power in its engines and was diverted to Naples Municipal Airport on Florida's west coast, Federal Aviation Administration spokeswoman Kathleen Bergen said.
The plane glided to a safe landing. There were six people and the pilot on board the Cessna 402C, Bergen said. Nobody was injured and there was no other damage to the plane, she said.
Cape Air flies the nine-passenger Cessna 402C planes out of several locations around the country, including between Cape Cod, the Islands and Boston. The company's planes fly daily between Key West and Fort Myers in Florida. They also have flights out of Guam and nearby Pacific islands.
Cape Air mechanics and investigators replicated the valve problem and continued to investigate the incident over the weekend, Monville said.
"We proved repeatedly that the left tank was not providing fuel to the left engine," he said.
Although there were 12 gallons of gas in the right tank, Cape Air personnel suspected the gas had transferred from the right tank overnight after the plane landed, Monville said. There were 275 pounds of gas in the left tank, he said.
Aircraft fuel is measured by both volume and weight.
After the valve was lubricated both engines started and ran normally, Monville said. The plane was flown to Fort Myers and the NTSB asked that the suspect parts be removed and preserved, he said.
"It reflects very well on the training and the experience level of the pilot," Cape Air CEO Dan Wolf said yesterday.
The pilot would have been able to bypass the stuck valve to draw fuel from the full tank but seeing the nearby airport decided to land the plane quickly and troubleshoot on the ground, Wolf said.
With more than 25 years flying Cessnas the pilot, who Wolf declined to name, did the "smart and prudent thing," Wolf said.
The pilot told investigators that he had noticed a decrease in fuel in the right engine but believed it was a problem with the instruments that read fuel levels, Monville said.
A preliminary report should be complete within the next week and a final report within the next six months, the investigator said.