Federal investigators are blaming pilot error for two fatal Hawai'i air crashes last year that killed four people.

A National Transportation Safety Board investigation into the June 17 crash of an Island Hoppers tour plane on the southwest slope of Mauna Loa found that pilot Katsuhiro Takahashi, 40, deviated from his planned flight route and failed to switch from visual to instrument flight as he headed into mountainous terrain amid cloudy conditions.

The single-engine, four-seat Cessna 172M slammed into the volcano at the 4,500-foot elevation, where a dense rainforest of ferns and trees kept the wreckage hidden for five days.

Also killed in the crash were visitors Nobuhiro Suzuki, 53, and his wife, Masako Suzuki, 56, of Chiba, Japan.

The company yesterday declined to comment on the findings.

The plane left Kona International Airport about 10:20 a.m. on a 2 1/2-hour around-the-island tour. The pilot was flying under visual flight rules and had filed a flight plan to operate at an altitude of 2,000 feet, the NTSB report said.

The tour plane flew north to 'Upolu Point, then south toward Hilo, and was observed by another Island Hoppers airplane about 11:30 a.m. at Kalapana, where lava was flowing into the ocean. The plane was reported missing when it did not return to Kona at its scheduled time.

A witness on the ground about a mile from the crash site told investigators he heard the "deep rumble" of the tour plane's engine at about noon before it suddenly stopped. The witness said the weather was foggy, misting and rainy at the time.

The NTSB report said there was no evidence of any mechanical problems with the plane before impact.

A separate NTSB report on the Jan. 14, 2008, crash of an Alpine Air cargo plane into the ocean southwest of Lihu'e Airport found that pilot Paul Akita, 37, likely experienced "spatial disorientation" during the night flight partly caused by competing cockpit duties that included monitoring an Aloha Airlines Boeing 737 jet flying ahead of him.

The Hawker Beechcraft 1900C departed Honolulu International Airport around 4:45 a.m. carrying 4,186 pounds of mail and was scheduled to arrive at Lihu'e at 5:15 a.m.

The pilot had routine contact with air traffic controllers, and at 5:03 a.m. was instructed to follow the Aloha jet and was cleared for a visual approach to Lihu'e Airport, the report said. A few seconds later, Akita replied, "OK, we'll follow him in visual approach and, ah, we're switching (unreadable). So long."

Radar tracking of the flight path indicated the pilot altered his flight course to the west, most likely for spacing from the jet, and descended into the water as he began a turn back toward the airport, the NTSB said. At the last radar contact, the plane was 6.5 miles from its destination, flying at an altitude of 100 feet. A plane approaching the Kaua'i airport at that point typically would be at 1,500 feet.

The pilot didn't issue a distress call and there was no indication of any problems, the report said.

Most of the wreckage sank in 4,800 feet of water and was not recovered, so examination and testing of the engine, altimeter and other systems could not be performed. The NTSB report noted that although the skies were clear enough for visual flight, no natural horizon and few external visual references were available during the pilot's approach to Kaua'i, contributing to his disorientation.

"This increased the importance of monitoring flight instruments to maintain awareness of the airplane attitude and altitude. The pilot's tasks during the approach, however, included maintaining visual separation from the airplane ahead and lining up with the destination runway," the report said. "These tasks required visual attention outside the cockpit. These competing tasks probably created shifting visual frames of reference, left the pilot vulnerable to common visual and vestibular illusions, and reduced his awareness of the airplane's attitude, altitude and trajectory."

Vestibular relates to the sensory system responsible for maintaining equilibrium.

The Advertiser was unable to contact Alpine Air officials to comment.