A final report from the National Transportation Safety Board said a fuel pump failed which caused a helicopter to lose altitude, hit a treeline and catch fire in Pigeon Forge in 2016.
All five people who were on the helicopter died.
The report blames abnormal wear on an internal part of the fuel pump, possibly accelerated by an incorrectly sized part called a "spacer." That part may have allowed grease to escape the fuel pump which led to the increased wear.
"Something like that is usually hard to catch until you do have a failure," Randy Safewright, an aviation crash investigator said. Safewright did not investigation this incident but reviewed the report for 10News.
He said it appears to him that, as the engine failed, the pilot pulled on the aircraft controls so hard he broke them.
"It looks like he was fighting, trying his best to keep the aircraft flying."
As the engine failed, Safewright said the rotor was still spinning, but there was no power.
"It's kinda like putting your car in neutral. You're going down the road, you've got good speed, you put your car in neutral, you're still going to be going," he said.
An emergency landing under those circumstances is one of the most difficult and the helicopter wasn't very high to begin with.
April 2016: Sightseeing helicopter crashes in Pigeon Forge
"He could see he was sinking, may not make that ridge," Safewright said. "I know if I was put in that same position, I would try my best to squeak as much altitude out of that helicopter as possible."
The chopper hit a tree on the ridge and the 300 pound of fuel onboard exploded.
"Helicopter fuel is something that it's glorified kerosene," Safewright said.
The helicopter was not equipped with a crash resistant fuel system designed to prevent a fire. The aircraft was certified before it was legally required.
Autopsies found at least two of the 5 on board could have lived if it weren't for the fire.
"It had to have been a catastrophic, terrifying ride," Safewright said.
10News asked Smoky Mountain Helicopters, which operated the aircraft, for comment. A manager reached by phone said he had not seen the final report and declined to comment.