An 86-year-old Seattle man and his 84-year-old brother on a flying trip around the country in their single-engine plane are missing near Yellowstone National Park.

The search for the pair has been hampered by deep snow and poor weather over the weekend, according to authorities.

Ward Zimmerman, a retired Boeing engineer, and his brother, Robert Zimmerman, of Huntsville, Al., had been flying for several weeks on the pleasure trip.

Both were experienced pilots, said Ward Zimmerman’s son, Jim Zimmerman, of Renton.

The National Weather Service has posted a “hazardous weather outlook” for the area where the plane disappeared.

The son, who has been in contact with the search operation for the 1963 Mooney M20 airplane, said, “They have encountered 90 inches of snow.”

Zimmerman said the two brothers kept in touch with various family members, who had not heard from them since Tuesday.

On Thursday, they began making calls to authorities.

The Park County Sheriff’s Office in Cody, Wyo., said the plane was last seen Tuesday departing Yellowstone Regional Airport.

The brothers were headed from Cody to Twin Falls, Idaho, and from there to Seattle. Radar last detected the plane about seven miles outside the Yellowstone east entrance in northwest Wyoming, said the sheriff’s office.

Because of the size of the aircraft, the two brothers were not required to file a formal flight plan.

And, said the sheriff, family members said it was not unusual for the brothers to divert from their intended travel plans.

The sheriff said that its search team had not been able to detect the plane’s Emergency Location Transmitter, which is designed to activate automatically in the event of a forced or “hard” landing.

However, it cautioned that older-model aircraft sometimes don’t have such a device, or they carry an outdated model that is difficult to track.

As for concerns about the age of the two pilots, William Webber, 90, of Spokane, state representative of the United Flying Octogenarians (UFO), said that older pilots know their limitations.

To join the group, which has 86 members in this state, and 1,500 worldwide (although mostly in the United States), one has “to be a pilot in command when 80 or more,” Webber said.

This week, he said, he’s flying his Piper PA-32 Cherokee Six from Spokane to Paine Field in Everett.

The state group is having a state get-together Wednesday.

He said about older pilots, “When their coordination goes bad, they are the first ones who say, ‘I’m ringing the bell. Hey, I’m all through.’ ”

In any case, said Webber, there’s also the mandated FAA medical tests for pilots.

You get to a certain age, he said about the tests, “that’s what gets most of us.”

Erik Lacitis: 206-464-2237 or elacitis@seattletimes.com Twitter @ErikLacitis