A 21-year-old Washington man who ingested methamphetamine before getting on a plane in Seattle and had what a prosecutor called a "freak out'' on board pleaded guilty Thursday to interfering with crew members and attendants after the California-bound flight was forced to land in Portland.

Douglas B. Smyser is expected to face four months on home detention when sentenced in December.

The judge called the plea agreement negotiated by the prosecution and defense a "very sweet deal'' that appeared too lenient for the crime -- especially after the prosecutor said Smyser has violated conditions of his pretrial release three times by using drugs and walking away from residential treatment.

Assistant U.S. Attorney Paul Maloney urged U.S. District Judge Karin J. Immergut to take Smyser into immediate custody pending his sentencing because he’s worried Smyser could relapse and the deal would then collapse.

“We need to impress upon him the seriousness of this, so he knows there’s going to be consequences. … I think we need to make an impression,’’ Maloney told the court. “I want to see him be successful.’’

Defense lawyer Robert Hamilton argued that Smyser should be allowed to remain in residential treatment before his sentencing. Smyser has struggled with drug abuse since age 16, his lawyer said. Hamilton said he appreciated the prosecutor’s concern and effort to preserve the negotiated outcome but he argued that’s not a justifiable reason to detain someone.

“He’s 21 years old. I would like to keep him out of jail,’’ Hamilton said. “There’s not a lot of great influences in there.’’

The defense lawyer also said his client would have less access to controlled substances at a residential treatment facility than he would in a Multnomah County jail or in federal detention.

Immergut said she found it odd that she was being asked to keep Smyser in custody to preserve his home detention. If the government is concerned Smyser will use drugs while out of custody now, then the possibility he’d relapse and violate his home detention sentence is also great, she said.

The judge let Smyser remain in residential treatment until his Dec. 30 sentencing but added more stringent conditions. He must wear a GPS monitoring bracelet and not possess or use any controlled substances and adhere to regular drug testing.

If Smyser violates any of his conditions, he’ll likely face a stiffer sentence, Immergut warned.

“I think you’re getting a very sweet deal in this case,’’ Immergut told him.

Smyser, according to prosecutors, was headed to drug rehab in Malibu, California, on Feb. 13 when he ate meth before his Compass Air flight from Seattle and became paranoid and suspicious. His behavior caused concerns shortly after he boarded.

While the plane was taxiing, a backpack reported to have been thrown into the aisle by Smyser began to vibrate, FBI agent Damon Bateson wrote in a complaint. An electric razor was inside and had been activated and Smyser later claimed the bag as his.

About 20 to 30 minutes into the flight, Smyser began pacing the aisle, refused to sit down and told a crew member that someone in the back row of the plane had a gun, Bateson wrote. The captain secured the cockpit doors and diverted the plane to Portland because the crew feared Smyser might rush the cockpit, the complaint says.

Upon descent into Portland, Smyser approached the cockpit and had to be restrained by a passenger until the flight landed, the federal agent wrote.

-- Maxine Bernstein

Email at mbernstein@oregonian.com

Follow on Twitter @maxoregonian

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