The pilot and passenger were reported unhurt and the plane did not hit any vehicles.
FAA spokesman Allen Kenitzer said in an email that the pilot of the Cessna 182 declared an emergency because of engine failure.
The plane suffered minor damage to its landing gear.
The Washington State Patrol identified the pilot as 63-year-old John Townsley of Spokane. The patrol said the plane was flying from Richland to Spokane when it lost power.
A State Patrol dispatcher in Yakima said one southbound highway lane was blocked late Friday night but traffic was getting by. He said the plan was to tow the plane several miles to a wider spot on the highway shoulder until it could be removed.
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