Severe overloading caused a plane to plummet from the sky after takeoff from Merrill Field in Anchorage in June 2010, according to a probable-cause report from the National Transportation Safety Board released Thursday.
"Two witnesses said that just before it took off the airplane was loaded so heavily that its tires looked almost flat," the probable-cause report said.
Police at the accident scene identified the father and pilot as Preston Cavner, 34. The crash and resulting fire killed Cavner's 4-year-old son, Miles, and burned Cavner and three others inside, the NTSB said.
Cavner's wife, Stacie Cavner, 32, son Hudson, 2, and 16-year-old Texas resident Rachel Ziempak survived with critical injuries.
The Cessna 206 was bound for Port Alsworth and packed with construction materials, groceries, luggage, plants and other items for a family lodge, according to the NTSB.
Investigators estimated Cavner had overloaded the plane by at least 658 pounds. The amount of cargo impeded rescuers trying to get to victims inside the plane, the report says.
That weight estimate is a conservative number, said Jim LaBelle, chief of the NTSB's Alaska region.
"In 26 years of aviation accident investigation, both as an investigator and as a manager, I have never seen an aircraft involved in an accident that was that much overweight," LaBelle said.
The NTSB also faulted Cavner for using the plane's flaps outside of recommended limits, which caused increased drag, and for allowing the two younger children to ride on other passengers' laps, a violation of federal air regulations that require kids to be strapped in.
On the day of the accident, the plane took off and climbed to an altitude of about 150 feet. Witnesses said the engine was loud and the plane, its nose at a high angle, was struggling to climb, according to the report.
The crash occurred about 5:05 p.m., police said. The plane fell yards away from the busy intersection at Sixth Avenue and Ingra Street.
Brian Caton, a former manager of the nearby Ingra House hotel, recalled Thursday witnessing the plane crash and subsequent rescue.
"It hit the top of our building, spun around, and crashed into the building across the street," Caton said. "It was pretty mangled."
A small fire broke out under the crumpled plane, and 15 to 20 people ran toward the scene, some with fire extinguishers, Caton said. They worked to get the door open and pulled out the four survivors just before the plane exploded into flames, he said.
The plane had hit a small, empty building located on a defunct used-car lot. The building also caught fire.
Firefighters had the blaze under control by about 5:30 p.m, police said.