An off-duty pilot accused of trying to shut off the engines of an Alaska Airlines jet on a weekend flight over the Pacific Northwest was due to make his first court appearance on Tuesday in Portland, Oregon, where he faced 83 counts of attempted murder.

Joseph David Emerson, 44, was arrested on Sunday in Portland, where the twin-engine aircraft landed safely after it was diverted during what the airline called a "credible security threat" on a flight from Everett, Washington, to San Francisco.

Emerson was off-duty at the time and riding as a validated "jump seat" passenger in the cockpit of the plane - a routine mode of transport for commercial airline pilots hitching flights back home or to their next assignment, according to the Federal Aviation Administration (FAA).

He was booked into jail on 83 counts of attempted murder, 83 counts of recklessly endangering another person and one count of endangering an aircraft, according to a spokesperson for the local prosecutor in Portland.

Formal charges were expected to be filed on Tuesday in Multnomah County court in Portland, where the suspect was due to appear for an afternoon arraignment and enter a plea, the spokesperson added.

Alaska Airlines Flight 2059 was operated by Alaska Air Group's regional subsidiary Horizon Air, the carrier said.

The FAA told U.S. airlines in a notice seen by Reuters that the off-duty pilot had sought to disable the engines of the Embraer 175 by attempting to activate the engines' fire-suppression system while the plane was at cruise altitude.

"The crew was able to subdue the suspect and [he] was removed from the flight deck," the FAA notice said. The engines were never disabled, Alaska said.

An FAA pilot database showed Emerson listed as a certified pilot who received a medical clearance last month. Aviators are expected to self-report any mental health conditions, two U.S. pilots told Reuters.

The Air Line Pilots Association (ALPA), the world's largest pilots union, which represents aviators at Alaska, said in a statement that the airline "profession in North America is one of the most highly vetted and scrutinized careers."

Adam Silverthorne, president of California-based NRI Flying Club, said the incident was incongruous with the kindhearted, methodical family man he knew Emerson to be several years ago when Emerson was a club member and provided flight instruction. A club newsletter mentioned Emerson was at NRI in 2016.

The FBI in Portland said there was "no continuing threat related to this incident" to the traveling public.

The FAA told airlines in a separate notice on Monday the incident "is not connected in any way, shape or form to current world events" but said it is "always good practice to maintain vigilance."

(Reporting by David Shepardson and Allison Lampert; Additional reporting by Steve Gorman; Editing by Chizu Nomiyama, Jonathan Oatis and Jamie Freed)