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CENTERVILLE — Witnesses said an experimental light plane made a “backfiring” sound before it nosedived into a field, ultimately killing the two men aboard, according to a preliminary investigative report.

The National Transportation Safety Board document, obtained Tuesday by the Standard-Examiner, reported investigators’ initial findings into the June 25 crash that killed Andre Kostrzewa, 72, of Salt Lake City, and Jason Sorensen, 47, of Layton.

Kostrzewa, the experimental Sonex Light Sport kit plane’s owner, was dead at the scene and Sorensen, a recently retired Davis County sheriff’s deputy/paramedic and a LifeFlight air rescue crew member, died several days later in a hospital.

A man washing his car about 3 miles north-northwest of the crash site told investigators he watched a small, white, low-winged plane flying from east to west.

“He heard a ‘backfiring’ sound, which he said did not sound normal,” the report said.

A second witness said he was driving south on a nearby highway when he saw the Sonex flying northbound not more than 100 feet above ground.

“Shortly thereafter, he observed the airplane in a hard banking turn to the right,” the report said. “It then nosedived into the ground where it burst into flames.”

The report said the plane hit the field with its right wing before it came to rest upright on a fence about 30 yards north of the initial impact.

The engine separated from the rest of the plane, and the cabin and cockpit section was consumed by fire, the report said.

A final report will not be issued for several months, said Kevin Harvey, acting airworthiness frontline manager at the Federal Aviation Administration’s Salt Lake City office.

The plane had taken off from the Bountiful airport, the NTSB report said.

The document listed Kostrzewa as the pilot-owner and Sorensen as the pilot-in-command and operator. On the day of the crash, local police identified Kostrzewa as the pilot.

But Harvey said the NTSB document’s piloting designations are just as preliminary as the rest of the report.

The Sonex’s flight controls are accessible by both occupants, according to company documentation of the plane.

“Most aircraft have a pilot and co-pilot,” Harvey said. “It doesn’t mean the pilot was responsible for the flight. Until there’s an autopsy report from the NTSB, there’s no way to tell who was at the controls at the time.”

He said such determinations “are not something that is done lightly.”

Kostrzewa’s private pilot certificate was revoked in 1994 and suspended in 2014, according to an FAA pilot certification record obtained by the Standard-Examiner.

Details of those actions were not disclosed by the FAA, per U.S. Privacy Act restrictions.

Sorensen held a commercial pilot’s license, FAA data shows.

The Sonex crash was the second of two mishaps in as many days involving homemade planes in Northern Utah.

On June 24, a Capella XLS plane crashed shortly after takeoff in the Eden-Liberty area, but the pilot was not seriously injured, according to NTSB and Weber County Sheriff’s Office reports.

The plane clipped a fence and crashed into a shed, the sheriff’s office said.

Homemade planes, assembled from manufactured kits, have become increasingly popular, but FAA statistics show they carry a significant crash rate.

“Amateur-built and other experimental aircraft were involved in almost 25% of U.S. fatal general aviation accidents over the past five years and account for an estimated 5% of total general aviation fleet hours,” a 2018 FAA report said.

Loss of control remains the leading cause of fatal crashes involving amateur-built craft, the report said.

An FAA program is designed to help reduce the high rate of experimental plane crashes during flight testing.

Qualified co-pilots and powerplant testing are emphasized in the program.

The Experimental Aircraft Association, based in Oshkosh, Wisconsin, says crash rates of experimental planes and general aviation craft are roughly equivalent when the flight test period is not considered.

“A lot of what you see in the stats is borne out by what happens in the early test flight hours, the first 10 hours,” said EAA spokesperson Dick Knapinski.

“Regardless of home-built or factory-built, 70 to 80% of accidents are pilot error,” he said. “As things continue to get safer, it comes down to the human factor inside the cockpit.”

Robert Katz, a Dallas-based pilot who has been flying corporate planes for 39 years, said he follows crashes as an avocation.

“I’m interested in learning from the mistakes of others,” he said. “I have no problem concluding that every one of these incidents is preventable.”

From the time of the Wright Brothers to modern day, “every conceivable disaster” has been experienced, he contends, and pilots must learn from past errors. Because when they get in trouble, sometimes others are affected, “such as a plane clobbering a house,” he said.

You can reach reporter Mark Shenefelt at mshenefelt@standard.net or 801 625-4224. Follow him on Twitter at @mshenefelt.

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