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A Seattle pilot and his brother, both in their 80s, whose single-engine plane crashed last week near Yellowstone National Park, are presumed dead, said the Park County Sheriff’s Office in Cody, Wyo.

The Sheriff’s Office said the Wyoming Army National Guard on Monday evening flew a Black Hawk helicopter to the wreckage discovered earlier in the day, intending to lower a rescuer.

But, said Martin Knapp, commander of the county’s search-and-rescue operation, the avalanche risk was too high.

“I simply could not, in good conscience, risk any more lives,” he said. “I realize it’s hard for the family of the brothers, but I have been in contact with them, and they were in complete agreement with our decision.”

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After crashing, the plane had settled in a steep ravine on a 60-degree slope with a large cornice of snow above it, the Sheriff’s Office said.

A photo taken from above the crashed plane shows heavy front-end damage and a wing that has been torn off. The tail is bent up. There appear to be no footprints near the site.

Sheriff Scott Steward said he believes the brothers died in the crash.

He said, “Given the damage to the aircraft as well as the extremely harsh environmental conditions in the area, there’s no other conclusion we can make.”

Ward Zimmerman, 86, a retired Boeing engineer from Seattle, and his brother, Robert Zimmerman, 84, a retired nuclear physics professor from the University of Brazil at São Paulo, had been flying around the country for several weeks on a pleasure trip.

Initial reports had Robert Zimmerman as being from Huntsville, Ala., but that was because his plane — the one that crashed — was registered there, said a niece, Ruth Brandal, of Everett.

She said both brothers were experienced pilots and well-respected in their professions. Brandal said her dad took her on her first airplane ride when she 3 or 4.

After initial stories about Ward Zimmerman ran, a former co-worker at Boeing recounted how Zimmerman was responsible for the electronic engine control that’s now in all jet engines.

Brandal told how her uncle lectured around the world on nuclear physics, most recently in Turkey.

When the brothers got together, she said, “The conversations we had over the dinner table were just phenomenal.”

Brandal said both men were careful pilots.

“They’d analyze everything down to every last detail,” she said.

On May 6, even though the snowy, rainy weather was bad enough that the man in charge where they refueled said he wouldn’t have done it, the two brothers took off around noon.

Joel Simmons, director of operations at Choice Aviation in Cody, said the two brothers felt that a departure at around noon gave them “a little window” in between “some big weather systems.”

Rescuers who went on foot into the backcountry looking for the plane reported trekking through 5 feet of snow, “extreme mountainous terrain, in winds of up to 50 miles an hour,” the Sheriff’s Office said.

Brandal said that the family was used to the two brothers making plans at the spur of the moment. But they also kept in regular contact with the family, she said.

By Thursday, when no one had heard from the brothers, Brandal was one of the family members who began calling authorities.

Eventually the family pieced together that the brothers were to next fly to Portland to visit a granddaughter of Ward Zimmerman’s.

“He absolutely loves lemon pie. She had made it, and it was waiting for him,” Brandal said. “When he didn’t show up, she knew it was serious.”

She said her dad had asked his children how they felt about the flying trip he was planning with his brother.

It was OK with the kids, Brandal said.

She said she told him that if he died flying, “I always imagine you died smiling, having this adventure.”

Erik Lacitis: 206-464-2237 or Twitter @ErikLacitis

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