UPDATE 4-California wildfire more than doubles on sixth day

* Thousands of homes remain under evacuation

* Fire crews retreat from key mountain summit

* Flames trap people who defied orders to leave (Recasts, adds threatened power lines, details)

By Steve Gorman

LOS ANGELES, Aug 31 (Reuters) - A massive wildfire roaring through mountains north of Los Angeles forced some firefighters to retreat on Monday as it menaced foothill homes and Mount Wilson, a broadcasting hub and site of an historic observatory.

The so-called Station Fire more than doubled in size as it burned out of control for a sixth day, charring 105,000 acres (42,500 hectares), up from 42,000 acres (17,000 hectares) late on Sunday, and sending up towering plumes of smoke that fouled the air for miles (km) around.

Two firefighters died on Sunday when they were overrun by flames in the Angeles National Forest and rugged San Gabriel Mountains. Nearly 2,600 firefighting personnel, some from as far away as Montana and Wyoming, were battling the blaze.

Fire crews fought to protect the slopes around the 5,700-foot (1,740-metre) peak of Mount Wilson, home to 50 buildings plus a world-famous array of telescopes and a critical cluster of transmission towers for broadcasters.

After dousing the area in fire retardant and laboring to clear brush away from structures on the site, they fell back to avoid the flames expected to engulf the area.

"They've done everything they can do and it's unsafe for them to be there when the fire hits," Los Angeles County Fire Captain Mark Whaling said.

Elsewhere in the forest, 65 firefighters retreated from a wall of flames advancing on their positions, he said.

PEOPLE TRAPPED

Rescue teams were standing by to save five people who became trapped in the forest after they disregarded evacuation orders, authorities said.

Eighteen houses were destroyed on Sunday and at least three structures were reported lost early on Monday, said Scott Visyak, spokesman for the California Department of Forestry and Fire Protection.

The fire threat eased in some foothill communities of suburban Los Angeles that had been menaced over the weekend. But other neighborhoods were now at risk, including 200 to 300 homes on the southwest flank of the blaze just inside Los Angeles city limits.

Residents there were ordered to evacuate, city fire department Captain Steve Ruda said.