Goaltender Wayne Stephenson walked onto the runway at Baltimore-Washington International Airport at 1 a.m. today, looked at the Shorts 330 commuter plane that was carrying the Washington Capitals here and said, "I think I have to draw the line on this one."

The combined pleas of General Manager Max McNab, Coach Gary Green and trainer Gump Embro failed to lure Stephenson aboard. He did arrive later, on a commercial flight, and doubtless wishes he hadn't bothered. In fact, the whole team might as well have stayed home.

The Pittsburgh Penguins thrashed the Capitals, 4-0, and wrapped up a playoff berth for the fourth time in the last five years.Meanwhile, although clinging to 16th place, the Capitals were losing the power to rule their own destiny.

Should Edmonton win two of its last three games, Hartford capture one of its last four and Detroit extract seven points from its last four, the Capitals would be bystanders even if they won their last three.

"What bothers me most is that we blew the game the way we did," said the coach, Green. "Logically, guys had to be tired, playing Saturday night and then today, but that's no ex- excuse. Lots of teams have to play tough games, then come back the next day in lots bigger games. We had plenty of opportunities. We just didn't capitalize."

The big reason Washington was blanked, for the fifth time this season, was the Penguins' 22-year-old goaltender, Greg Millen. In recording his second shutout of the year, Millen stopped 28 shots, including breakaways by Mike Gartner and Bob Sirois.

The key play, with Washington down, 2-0, saw Bengt Gustafsson fake Millen completely out of the net, then set up Rolf Edberg in front. Pittsburgh defenseman Ron Stackhouse filled the void, blocking Edberg's backhander, and Gustafsson's rebound hit a goal post.

"With Rolf's line that time, I don't know how the puck didn't go in," Green said.

Edberg lost control of the puck on another breakaway.Millen foiled Sirois from point-blank range, then somehow blocked Paul Mulvey's deflection of a Gartner slap shot.

Pittsburgh hit a few posts, too, but the Penguins converted enough opportunities to keep the pressure off Millen.

Rod Schutt got the only goal Millen needed early in the second period, blocking a Robert Picard slap shot, regaining possession after Edberg poked the puck a few feet away and beating Stephenson with a low wrist shot on the glove side.

Rick Kehoe, flying down the right wing on a power play, took a pass from George Ferguson, who had just stepped out of the penalty box, and beat Stephenson at close range to make it 2-0.

A 45-foot blast by Bob Stewart and a shorthanded breakaway by Ferguson completed the scoring in the final period. Ferguson's score was only the third shorthanded goal yielded by Washington this season.

There was a moment of worry for Capital watchers in the third period when Mulvey, tripped by Mario Faubert, crashed against the boards and was slow to get up.

"My elbow and shoulder got banged when I was crushed into the board pretty hard," said Mulvey, his left foot planted in a bucket of ice after it was later hit by a Pittsburgh shot. "I went to get up and I couldn't, then Gumpy was there. I'm O.K. now." Nevertheless, Mulvey is scheduled for X-rays Monday.

Stephenson was all right, too, and he hardly could be faulted for a game in which he made 30 saves and his teammates failed to score. Although he was playing his 19th straight game and second in 21 hours, he declined to complain of weariness.

"It's no problem," Stephenson said. "A goaltender is not different than anybody else."

Of his air phobia, Stephenson said, "Flying's not bad, but certain planes bother me. I knew I'd get here okay. I was with the equipment. If I didn't make it, there wouldn't have been a game anyway."

The Penguins' fans were not ecstatic about the playoff clinching and only 9,638 turned out to welcome the team back from two weeks on the road.

A sign proclaimed "Bob Johnson for Coach" and Coach Johnny Wilson was the target of dissenters during the relative quiet of the first period. Very little happened and Green said the poor condition of the ice forced adjustments.

"We're a skating club and the ice was absolutely terrible," Green said. "We had to shoot it in, because we couldn't handle it. It was full of cracks. But both teams had to play on it."

For a final note on the flight situation, linesmen John D'Amico and Joe Dame tried to fly in from Washington today, but were turned back by engine trouble. They eventually got here on a chartered six-passenger plane. Stephenson declined their offer to return on it.

He also skipped the second half of the weekend "Shorts" circuit, choosing to take a commercial flight home.