Final report: Extreme turbulence likely caused fatal plane crash during Kruger Rock Fire

The firefighting airplane crash that killed a pilot during the Kruger Rock Fire was most likely caused by extreme turbulence.

The National Transportation Safety Board recently came to its final determination on the Nov. 16, 2021, crash near Estes Park that killed Fort Morgan-based CO Fire Aviation pilot Marc Thor Olson, an experienced military and civilian pilot with an extensive background in flying at night with night-vision goggles. The crash, which is believed to be Colorado's first use of a fixed-wing aircraft at night to fight a fire, happened after sunset, around 6:36 p.m.

The report stated the probable cause of the crash was "the airplane’s encounter with severe to extreme turbulence associated with mountain wave rotor cloud activity, which resulted in loss of airplane control."

Rotor clouds form at or just below the tops of mountains on the leeward, or side opposite of the prevailing (generally west in Colorado) wind. The air movement can cause an eddy effect on the east side of the mountains, including dangerous updrafts and downdrafts, which creates severe turbulence.

Here are key details from the final NTSB report of the fatal flight

• No mechanical anomalies were found that would have precluded normal operation.

• Olson's toxicology report came back negative for all substances tested.

• The pilot was using night vision goggles during the flight.

• The crash site was located on the eastern slope (leeward) of a high mountain range's west winds, which were reportedly gusting to 34 mph near the time of the crash.

• Two witnesses saw the plane roll inverted twice just before the crash. Neither witness saw the crash but one heard it.

More: Firefighter pilot Marc Thor Olson remembered: 'He was like the godfather of the fire pilots'

NTSB report looks into discussions about the weather before the fatal Kruger Rock Fire crash

Justin Whitesell, emergency operations director for the Larimer County Sheriff's Office, said in his witness statement in the report that he talked to CO Fire Aviation's Zach Sullivan and Olson around 3 p.m. on the day of the crash, asking them what they thought about the winds. Whitesell said the two said crosswinds looked good.

"I asked then if they’d be able to fly with the current conditions and they said 'yeah'," Whitesell said in the report. "(Sullivan) said that him and Marc Olson had gone over looking at the fire and looked at the weather, and they were comfortable with everything they were seeing and didn’t have any issues."

According to the report, Olson's first mission fighting the fire that day started in Fort Morgan where CO Fire Aviation is located. He dropped a load of fire retardant on the fire around 4:30 p.m., then landed at the Northern Colorado Regional Airport in Loveland to wait for the wind to subside until he made a second drop.

Whitesell told NTSB investigators he talked to Olson on the phone about an hour before the crash while Olson was at the Loveland airport to make sure he felt conditions were OK to fly, according to the report.

"I specifically said, 'There’s no need to push it if the conditions aren’t right; it’s not a big deal'," Whitesell said in the report. "I said he had to let me know because I’m not a pilot, if the winds were too strong. And (Olson) said between Fort Collins and the fire it was pretty windy but over the actual fire it was pretty calm."

In a Coloradoan interview May 15, Whitesell said it's easy to look back at something with a traumatic ending and wonder about decisions made at the time.

"Knowing the outcome, it's hard to say I wouldn’t look at it any differently," Whitesell said. "But at the same time, I'm not sure what lens I would be looking through. It's one of those things you constantly think about."

A Coloradoan phone call and email to CO Fire Aviation on May 15 asking questions about the NTSB's final determination of the crash and if the company would continue exploring night aerial firefighting went unanswered.

NTSB report: Witness details final minutes of the fatal Kruger Rock Fire airplane crash

Around 6 p.m. Nov. 16, 2021, Olson left the Loveland airport on his fatal Kruger Rock firefighting mission, making a high circle around the fire location, then a second lower circle to view the fire overall and his drop location and assess weather conditions.

Derek Rosenquist, Larimer County Emergency Services division supervisor of the fire at the time of the crash, was the last person to talk to Olson via radio communication.

Rosenquist said while Olson was circling on the fatal flight, he saw a "little bit of a wobble" based on the flood lights the plane had on its wings. He said Olson quickly corrected and mentioned a "little bit of turbulence."

Rosenquist said they discussed how comfortable Olson was making the drop, and the pilot mentioned he would make the drop, then land at the Loveland airport and check conditions around 10:30 p.m. to see if winds subsided.

Rosenquist said Olson in a "perfectly calm" voice let him know he was coming in; asked how the wind was on the ground, which Rosenquist estimated at 15 mph, gusting to 20 mph from the northeast; then flew over top him.

"As I watched him pass, based on the lights on the wing tips, there's a little bit of a wobble," Rosenquist stated in the report. "He started a complete roll to the left, another roll, wing tip over wing tip to the left, and I lost him behind some trees. Then I heard an impact about a second or two later."

Several days after the crash, the Kruger Rock Fire was fully contained after burning 147 acres in steep terrain.

Colorado tribute: Fallen aviation firefighters memorialized near where 4 died in Larimer County

This article originally appeared on Fort Collins Coloradoan: Fatal Kruger Rock Fire plane crash: Report lists turbulence as cause