A single-engine plane carrying an Ohio businessman and three members of his family on a holiday trip to Washington crashed into the Potomac River yesterday while trying to land at National Airport.

The four were rescued almost immediately by two men in a small sailboat who took them to shore where a helicopter and ambulances carried them to two hospitals. Last night all were reported in stable condition.

The pilot, Harold Swarthout, 46, of Columbus, Ohio, blamed himself for the accident.

"I was just up too high to land on the runway. It was a pilot error, an error in judgement in landing. That's the simplest way to put it," Swarthout said from his hospital bed yesterday. "I was up about 4,000 feet and I should have asked for a missed approach, but I thought I would make it down," he said.

The majestic sights of the city and its monuments became a whirling nightmare as the airplane, a Mooney Mark 12 swooped from the sky toward the Potomac River.

"The plane just stopped flying," Swarthout said."I was trying to make a left turn and it stalled. I couldn't handle it, and it flipped," falling into the river a quarter mile short of the runway.

The accident occurred about 10:46 a.m. as the plane was making a routine visual landing, according to a Federal Aviation Administration spokesman.

The impact of the crash left the four passengers semiconscious. The plane began to sink rapidly. The cockpit filled with cold water within 60 seconds, Swarthout said.

"The water really helped shake them out of it (semiconsciousness)," said Dr. John C. Bucur, chief of neurosurgery at National Orthopedic Hospital in Arlington.

Swarthout said he managed to push his youngest son out onto a wing then retrieve his wife, who was in her seat.

The other son escaped by himself. Holding his wife afloat while he held onto a wing, Swarthout noticed a sailboat moving in for the rescue.

"That was the prettiest sight I ever saw," he said. "I thank God."

The boat that rescued them was a 15-foot fiberglass dinghy, called an Albacore, which was preparing to race in a regatta.

The men aboard, skipper Carl Cheney, 36, of Bowie, and crewman Terry Jarvis, 35, of Alexandria, had just sailed onto the river, Cheney said, when they noticed the plane flying low.

"It seemed to rock a couple times," Cheney said. "It started to climb, but it couldn't make it up, and it stalled and came down sideways. It went into water just 150 yards away from us."

Cheney said the Swarthouts were treading water when the sailboat reached them.

Jarvis said Swarthout was bleeding heavily from facial cuts, his wife seemed in severe pain, and a younger son was shaking and appeared to be in shock. The older son, he said, seemed all right, and helped his parents grab ahold of the boat, while climbing aboard himself.

By the time the family was on board the five-foot side boat, a U.S. Park Police helicopter was flying overhead, Jarvis said. The helicopter pilot motioned for the sailboat to head for a gravel beach just off the end of an airport runway.

When they arrived there. Jarvis said Swarthout and his two sons walked ashore with help from rescue officers while Mrs. Swarthout was carried on a hard stretcher.

"The man said, "Thank you, thank you, thank you'," Jarvis said. "And we just said. 'Glad we could help,' and that was about it.

'Then Carl and I pushed the boat off, and I bailed the water out, and then we went off and raced," Jarvis said.

In addition to Swarthout, vice president for the Captial Manufacturing Company in Columbus, those injured were identified as his wife Clara, 46, and his son Edward 21, and David, 15. A daughter, Diane, did not make the trip, which began in Columbus about 9 a.m. The family was planning a Memorial May weekend visit to Washington.

Doctors said Clara Swarthout was the most seriously injured. She was taken by helicopter to Washington Hosptial Center and treated for a fractured collarbone, broken ribs and bruises. She was listed in stable condition late yesterday.

Swarthout and his sons were taken to National Orthopedic Hospital and treated for lacreation to face and head. They were listed in good condition.

A witness to the crash, Eddie W. Johnson of Suitland, said he was watching the sailboat regatta-just off Hains Points-when the Swarthout plane flew in.

"The plane just seemed to flip over." Johnson said, "and it was down in the water in no time."

Swarthout said about 10 seconds elapsed between the moment he realized something was wrong and the crash.

"Nobody really had time to react. If anybody was saying anything, I wouldn't have been listening," he said.

Doctors said Swarthout's sons suffered from amnesia and could remember only being in the water and later awakening in an ambulance.

Swarthout said he made several mistakes in bringing his plane toward National.

Dropping at a speed of about 500 feet per minute. Swarthout was making a left turn as he descended.

"When I got out over the water. I lost my navigation for a split second. The water and the sky was all confused and I lost my bearing. All I could do was think: level off, and get that nose up."

He didn't.

When asked how it felt to fall from the sky, Swarthout flinched in his bed, "Well, you never think it will happen to you," he began with a forced smile. "Of all the pilot errors I've read about, I've said it won't happen to me. Then when it does, you fell stupid. But I also feel very fortunate."

Swarthout added that he hadn't decided whether he'll fly again.

"I feel like I've been very lucky so far. You learn from experience. Knowledge is accumulative. And if you live long enought, you try not to make the same mistake again."

Swarthout has been a pilot for two years. CAPTION: Picture, Police boat approaches wing tip of light plane that crashed into Potomac on approach to National Airport with Family of four aboard. By C. Jim Moore for The Washington Post