OAKLAND -- A power outage affecting one terminal forced Oakland International Airport to run on its backup generators and some passengers to board planes the old-fashioned way Thursday morning, but flights in and out of the airport were not interrupted, a spokeswoman said.

The outage, which started around 10 a.m., kept the airport operating on its generators until around 12:25 p.m., when power was restored, airport spokeswoman Kim Domerofski said. The cause of the outage was not known immediately, but PG&E spokeswoman Tamar Sarkissian said it was not related to the agency.

PG&E supplies power to the airport, Domerofski said.

A terminal that serves all flights except Southwest Airlines went dark momentarily until the generators went into operation, she said.

The Southwest terminal was not affected, she said.

Some flights out of the affected terminal boarded passengers by having them walk onto the tarmac and up the stairs of the plane, because the boarding bridges normally used were unable to operate, she said.

Flights were departing and arriving on time despite the outage, Domerofski said.

Contact Rick Hurd at 925-945-4789 and follow him at Twitter.com/3rdERH.