SANTA BARBARA, CALIF., AUG. 13 -- A small plane passed within 150 feet of the helicopter carrying President Reagan to his ranch northwest of here today, White House officials said.

They said the pilot of the Piper Archer aircraft, a male who was not immediately identified, was detained after a 50- to 60-minute airborne chase by another government helicopter. A passenger in the plane also was detained, United Press International reported.

Col. Michael Glenn, the pilot of Marine One, the president's helicopter, said he viewed the incident as "very serious" and notified the Federal Aviation Administration immediately.

The agency has begun an investigation, according to Russ Tornquist, an FAA spokesman in Los Angeles.

While the incident was officially classified as a "pilot near-miss," White House spokesman Marlin Fitzwater said that the pilot of Marine One did not believe a collision had been imminent. Fitzwater said that the unidentified aircraft was observed by Santa Barbara control tower flying about 100 feet over Reagan's ranch in restricted airspace.

He said that the Santa Barbara control tower radioed Secret Service agents aboard another helicopter, known as Nighthawk Two, which was just landing at the ranch. The agents on Nighthawk Two, piloted by Maj. Don Wherle, in turn alerted Marine One to the presence of the aircraft.

"The marine pilot initiated what he called a gentle climb and right-hand turn. At this point the Marine pilot saw the unknown aircraft. He was convinced he was not on a collision course," Fitzwater said.

White House officials did not release the name of the Piper Archer's pilot. And Fitzwater said he had "absolutely no information" on what the pilot's motive may have been or whether the pilot was deliberately flying over the ranch.

Fitzwater issued a statement late tonight saying that the pilot of the plane had been detained by officials of the Orange County Sheriff's Department and was being interrogated by the Secret Service agents. "The investigation in ongoing," Fitzwater said. "No charges have been brought at this time."

{In Washington, Secret Service spokesman Rich Adams said, "No arrest has been made at this point in time." He said there was "one passenger as well -- I believe it was a male," in the plane, UPI reported.}

The incident occurred about 3:35 p.m. local time four miles southeast of Reagan's mountaintop ranch, known as Rancho del Cielo.

Fitzwater said that neither Reagan nor any other passengers on the president's helicopter saw the plane, and none was aware of the event as it occurred. Among those aboard Marine One with the president and Fitzwater were White House chief of staff Howard H. Baker Jr., national security adviser Frank C. Carlucci and his Reagan's physician, Col. John E. Hutton Jr. Fitzwater said he and the other officials were told of the incident when they returned to Santa Barbara, after dropping off the president at his ranch.

Nancy Reagan was visiting her mother in Arizona at the time of the incident and planned to join her husband later, UPI said.

According to Fitzwater, the pilot said the intruder plane was 200 to 300 feet away from Marine One horizontally and 150 feet below the helicopter at the point where they were closest together.

In FAA parlance, it is a "potentially dangerous" event when two aircraft come within 500 feet of each other and a "dangerous" event when the distance is less than 100 feet.

After Marine One landed at the ranch, Secret Service agents alerted a White House staff helicopter, Nighthawk Three, piloted by Capt. Mike Duva, which followed the airplane to John Wayne Airport in Orange County, more than 100 miles away, Fitzwater said.

Fitzwater said Duva spotted the number N840-A on the tail of the small plane, which was tracked via radar to the Los Angeles basin area by personnel at the Santa Barbara airport. The Piper pilot flew past Santa Barbara without making radio contact with the airport tower, Fitzwater said.

{The apparent near-collision was witnessed and photographed by Rafael Maldonado, 41, a staff photographer for the Santa Barbara News-Press, Associated Press reported.

{"I saw the chopper coming in from the east," Maldonado said. The plane came in from the opposite direction, "moving pretty fast," and flew within several hundred feet of the helicopter, he told AP.

{The photographer said he did not get a picture of the two aircraft in the same frame but took pictures in sequence of the airplane and the helicopter, AP reported.

{The Piper Archer pilot was taken into custody after the plane landed there shortly after 5 p.m., according to Don Adams, a supervisor at John Wayne Airport.

{Kelly Chapman, of Martin Aviation at the airport, said the pilot parked and took a shuttle van to the terminal to pay for his fuel. "He was on the pay phone," she told AP, when "the Orange County sheriff came to the building and asked to speak with him {and} took him over to the main terminal . . . . "}

The FAA on Wednesday issued an emergency order intended to increase air safety in the Los Angeles area. The action followed a near-collision on Tuesday when a small plane passed within 100 feet of an American Airlines jetliner. The small plane was operating on visual flight rules, which require the craft to stay clear of other planes.

In the year ended Aug. 1, the FAA reported 51 near-collisions in the Los Angeles area, which has one of the nation's highest concentrations of small and large aircraft.