The United Airlines DC8 airliner that crashed here yesterday apparently lost all its power just before the crash as it approached Portland International Airport, Federal investigators said today.

Only 10 of the 185 passengers and crew members aboard died in the crash, partly because trees and heavy power lines helped break the momentum of the plummeting jetliner and because no fire or explosion followed the impact, the investigators said.

Witnesses arriving at the scene were stunned to find surviving passengers sitting in the homes of those who lived in the crash area, about three miles south of the runway.

Some 24 hours after the accident, there were many unanswered questions. "We do know they lost power," said Elwood T. Driver, head of a team from the National Transportation Safety Board. "We don't know why."

The jet may have run out of fuel after circling the airport for half an hour trying to correct a landing gear problem or its electrical system may have broken down, he said.

Two "black boxes" which record flight data and cockpit conversation were recovered and could provide new information on the cause of the crash. The recorders arrived in Washington this evening, where investigators will analyze them.

"Incredible" and "miraculous" were the words most used by witnesses and survivors interviewed to describe the relatively small death toll.

Although power lines were ripped from their poles, no one was electrocuted. Although many crashes produce fires with lethal fumes, this did not happen in the Portland crash.

The plane plowed through two houses near the airport before nosediving into a grove of trees. Both houses were unoccupied.

"There was a series of three or four bumps," passenger George Lindeman said. "Just little ones. Then there were bounding jolts and suddenly I was forced into the seat ahead of me. Then it stopped so suddenly everybody was surprised. There was a second or two of panic and then everybody realized we were okay."

The front of the plane, down to row number five, was almost demolished. Most of the dead were seated there. The rest of the aircraft was intact, though there were gaping holes in the fuselage.

Many of the survivors escaped through those holes and many of the 69 injuries occurred as escaping passengers fell from the trees where the plane perched.

"That was the worst," said Deborah Friedrich, 22, "falling through the pine trees. I walked out and the wing just wasn't there."

Survivors unanimously praised the conduct of the crew, who helped prepare pasengers for the crash. "They really briefed us well," said Amy Conner, 17. "They told us what might happen and they braced us for the worst."

A United Airlines official said the pilot had circled the airport for about half an hour while seeking advice from airline maintenance workers about the malfunctioning landing gear.

Then, the pilot, who survived the crash, radioed the Portland control tower that the light indicating the landing gear had lowered properly was failing to light."

"Mayday, Mayday," he shouted over the radio. "The engines are flaming out. We're going down. We're not going to make the airport."