The pilot of the single-engine airplane that Thursday flew within a few hundred feet of the helicopter carrying President Reagan to his ranch near Santa Barbara, Calif., was arrested last night on charges of being AWOL from an Army base in Fort Lewis, Wash., for 11 days.

Ralph William Myers, 32, was turned over to military authorities after being held for several hours in the Orange, Calif., jail.

The Federal Aviation Administration yesterday revoked Myers' pilot's license for seven violations, including "careless and reckless operation of an aircraft" and entering restricted airspace.

Myers told federal investigators he unknowingly strayed into the restricted airspace surrounding the president's Rancho del Cielo after losing a contact lens in the cockpit of the plane. First, Myers said, one lens fell out. Then the other eye grew irritated, so he took that lens out and let his passenger fly the plane, federal investigators said.

After questioning Myers for nearly eight hours, Secret Service agents released him without learning he had been AWOL.

White House officials traveling with Reagan played down the importance of the incident. Presidential spokesman Marlin Fitzwater, asked whether additional security measures are needed, joked, "You mean like a 6,000-foot steel wire fence, or what do you have in mind?" {Details, Page A9.}

White House chief of staff Howard H. Baker Jr. said the president "did not seem particularly upset" when told of the incident after the helicopter landed. Baker added, "I suppose there's a fair amount of danger when a plane comes that close to a helicopter."

At its closest point, the small plane, a Piper Archer PA28-181, was within 200 to 300 feet horizontally and 150 feet vertically of Marine One, the presidential helicopter. The chopper pilot, Col. Michael Glenn, filed a near-collision report with the FAA.

Fitzwater said that Glenn did not believe the two aircraft were on a collision course at any time, and the Secret Service maintained that there was no security lapse.

"As soon as the plane got close to the restricted area, we knew about it. We knew that the plane was in the air," said Secret Service spokesman Bill Corbett. "We took action to make sure that the president was not in any danger. If we had gone any steps further, people would be saying the opposite: 'You went too far.' "

Myers and his passenger, tentatively identified as Harlan Lee Jones of Lake Oswego, Ore., were apprehended by Secret Service agents after they landed at John Wayne Airport in Orange County, Calif. FAA officials said they did not know whether Jones had a pilot's license.

The Secret Service said it did not plan to file criminal charges against Myers or his passenger, but U.S. Attorney Robert Bonner of Los Angeles said the case is under investigation. In revoking Myers' pilot's license, the FAA cited Myers for "careless and reckless operation of an aircraft" and intruding into restricted airspace.

Aircraft are prohibited from flying through a 3 1/2-square-mile zone surrounding the ranch. Restrictions for the zone prohibit airplanes flying over the ranch from descending lower than 4,000 feet above sea level. When Reagan is at the ranch, the upper limit of the zone is extended to 5,000 feet above sea level.

After interviewing Myers and reviewing air traffic control data, White House officials and federal aviation investigators were able to reconstruct the incident this way:

Myers rented the Piper Monday from Vancouver Aviation, a flying service at the Vancouver, Wash., airfield on the edge of the Columbia River. He did not provide any details of his trip when he rented the plane, according to Marianne Fulman, chief pilot for the firm. But Myers intended to return the plane Friday, she said.

The Piper radioed a flight service station south of Portland about 10:15 a.m. Tuesday asking for weather information between Eugene, Ore., and San Francisco, FAA officials in Seattle said yesterday. About 12 minutes later, the Piper radioed again, requesting weather information for the South Lake Tahoe, Calif., area.

By Thursday afternoon, Myers was in Lompoc, Calif., 800 miles south of Vancouver. In midafternoon, he took off from Lompoc and climbed to 3,000 feet and headed southwest -- a course that took him toward Rancho del Cielo.

Meanwhile, Marine One and two other helicopters, Nighthawk Two and Nighthawk Three, departed from Point Mugu Naval Air Station south of Santa Barbara and headed northwest toward the ranch.

As it neared the ranch, the small, white plane dipped, at times to just 150 feet above a 2,000-foot mountain ridge near the ranch. It was during these moments, FAA officials said, that the search for the contact lenses may have been going on. Other officials labled Myers' explanation as "cockamamie" and suggested that Myers may have been on a sight-seeing trip to get a look at the president's house.

About 3:35 p.m., air traffic controllers at Santa Barbara Airport alerted Nighthawk Two, which was about to land at the ranch, that "unknown traffic" 1 1/2 miles away was headed toward Marine One. Nighthawk Two, carrying Secret Service agents, spotted the plane about 100 feet above the ranch and radioed Marine One, Fitzwater said.

As the Piper approached Marine One, the chopper pilot made a slight turn to the right and stopped his descent until the Piper passed underneath, FAA officials said. The FAA did not disclose the Piper's altitude.

Fred Farrar, an FAA spokesman, said it would be impossible to order a private plane out of the airspace if the plane's radio was turned off. He said air traffic controllers were unable to determine to which frequency Myers had tuned his radio or whether he even had it turned on.

Pilots flying under visual flight rules north of Santa Barbara are under no requirement to have a radio turned on until they approach busy airspace.

Aboard Marine One were the president, Baker, national security adviser Fank C. Carlucci; Fitzwater; James F. Kuhn, the president's personal assistant; White House photographer Bill Fitz-Patrick and the president's physician, Col. John E. Hutton Jr.

First Lady Nancy Reagan flew to the ranch separately to join her husband for the start of their 25-day vacation.

After the encounter, the Piper continued along the mountain ridge and headed toward the Santa Barbara Airport, dropping at times within 100 feet of the ground, federal officials said.

Nighthawk Three, a Marine helicopter waiting on the ground at Santa Barbara, chased the Piper 50 to 60 minutes as it flew south. Nighthawk Three's pilot, Capt. Mike Duva, got close enough to read identification numbers on the Piper's tail.

When Nighthawk Three ran low on fuel and abandoned the chase, air traffic controllers at Los Angeles International Airport picked up the trail and tracked the Piper on radar until it landed at the John Wayne Airport in Orange County, about 120 miles southwest of Reagan's ranch.

Myers and his passenger were questioned at the airport in a room with newspapers taped over the window and then taken to an undisclosed location until Secret Service agents let them go at 1:25 a.m.

A spokesman at Fort Lewis said Myers is a medic with the 3rd battalion, 47th infantry, 9th Infantry Division. He was reported absent without leave Aug. 3, according to George Polich, the fort's public affairs officer.

Polich said military police were in the process of upgrading Myers' status from AWOL to deserter, something usually done only after 31 days of absence, in order to list him on the national crime information network. The Secret Service said yesterday Myers was released because his name did not turn up on the crime information network computer.

Myers served with the Nevada Air National Guard as an aircraft maintenance technician from 1980 to 1983, according to Army records. From 1980 until going on active duty, he also worked for the Reno Flying Service and as a pilot for Harlan Jones Contractors in White Salmon, Wash.

In 1983, he switched to the Army Reserve and was training as a medical specialist at Fort Sam Houston, Tex. In June 1986, Myers transferred to Fort Lewis, about 120 miles from his parents' home in Bingen, Wash., on the Columbia River.

"He's a young, good-looking guy full of vim, vigor and vitality," said Fred Heany, a neighbor of Myers' parents.

Heany added that Myers' father told him the soldier had worked to get his air transport rating in hopes of becoming a commercial airline pilot. Staff writers Jay Mathews in Los Angeles and David S. Hilzenrath in Washington contributed to this report.