The federal investigation into what caused a small plane to lose power and ditch in the ocean about two miles off Koko Head on Friday evening will be hampered by a missing piece of essential evidence:

The plane.

The 1968 single-engine, two-seat Cessna 150 sank in deep waters after it was ditched by the 29-year-old pilot, a South Carolina man who, along with his wife, 31, was rescued by fire department crews with an assist from the Coast Guard.

The couple, who identified themselves to the Visitor Aloha Society of Hawaii as Michael and Christina Allen of Beaufort, S.C., were not injured and both declined medical attention.

Yesterday, the Federal Aviation Administration said the investigation will necessarily be an exercise in gathering records and taking statements.

"Without the wreckage there's only so much we can do," FAA spokesman Ian Gregor said from California.

"And it's going to be up the insurer of the aircraft to decide whether to raise the wreckage. So assuming that we don't get access to it, we're pretty much limited to talking to the pilot, the passenger, the witnesses, and reviewing air-traffic control tapes to see if we can glean anything from them."

The Allens yesterday could not be reached for an interview and had declined an offer of help from the Visitor Aloha Society of Hawaii.

"When I told him about the Visitor Aloha Society, he asked if we were a scam," said Jessica Lani Rich, president of VASH, a nonprofit organization funded by the Hawai'i Tourism Authority that offers free assistance to travelers in distress.

"He's the only person who's ever said anything like that," Rich said. "I had contacted Roy's (restaurant) in Hawai'i Kai, and they were very generous in offering the couple a dinner. I offered to have them come to my office, or I would go to them — anything we could do to support them.

"But Mr. Allen declined all assistance. He sounded very stressed out, because he said he was being investigated by the FAA because of the airplane accident."

Rich said Allen told her the couple was scheduled to leave Hawai'i today.

In Virginia, National Transportation Safety Board spokesman Ted Lopatkiewicz said the NTSB is not sending anyone to Honolulu to investigate the crash, in part because the aircraft probably can't be examined and because no one was killed.

Lopatkiewicz said FAA personnel in Honolulu will conduct interviews, gather the aircraft's background information and maintenance history, and submit the results to an NTSB investigator in Seattle, who will file a probable-cause report based on those findings.

"We will determine a cause, but we don't determine fault or liability or anything like that. That's not part of the NTSB's mandate."

George Hanzawa of George's Aviation Services, which rented out the Cessna to Michael Allen, yesterday said it is likely the plane ran out of fuel.

Hanzawa said it was full of fuel after Allen — who Hanzawa said has a commercial pilot's certificate — completed a "check-out flight" and took off from the Honolulu airport at 7 a.m. Friday for an air tour of the Islands.

"He was gone all day," Hanzawa said. "He didn't put gas in. ... We don't know exactly where he went — the investigators will figure all that out — but we know he cruised around, did some sightseeing and eventually landed at Maui. He spent the day on Maui, and then he came back. ... He determined he had enough fuel to get back." Hanzawa said the plane has a 24-gallon fuel capacity.

Hanzawa said any number of things could have caused a fuel shortage, from a loose gas cap or a leak of some sort to running the engine too "rich."

Hanzawa disputed the notion that the Allens were "lucky" to have survived. It was not luck, he said, but it was because Allen brought the plane down exactly by the book after he had radioed in an in-flight emergency at 5:53 p.m.

"He did everything exactly right," Hanzawa said.

Reach Will Hoover at whoover@honoluluadvertiser.com.