Many airline pilots are concerned that safety has deteriorated since the air traffic controllers' strike began two weeks ago, according to internal memos from the Air Line Pilots Association.

A memo dated Aug. 11 reports a "general deterioration in the ATC air traffic control system," and adds that increases in private airplane traffic are producing "a definite safety hazard."

The same memo reports fatigue among controllers and says two pilots "were concerned enough about the deterioration of the system not to fly."

The air traffic control committee of the pilots' association prepares these memos daily for John J. O'Donnell, president of the organization, which includes 33,000 airline pilots. Copies of some of the memos were obtained by The Washington Post.

An Aug. 10 letter to O'Donnell from Tom Sheppard, chairman of the air traffic control committee, concludes "that at least three definite trends are arising which indicate a decreasing level of safety in the present modified ATC system."

Sheppard identified those trends as:

Fatigue and improper qualifications impairing the work of controllers.

An increase above normal levels of military and private aircraft flying below altitudes of 18,000 feet.

The closing by the Federal Aviation Administration of some airport control towers without reducing air traffic there.

The letter recommended that there be no increases in total air traffic until the system is proved safe, that military flying be reduced and that the FAA restore control towers at some airports.

Congestion in lower-level air traffic below 18,000 feet concerns airline pilots because they must pass through the lower zones when taking off and landing.

In those lower levels, two small planes collided yesterday and crashed above San Jose, Calif. One person was killed and two were reported injured in the planes, but no one was injured in the business district near where the planes fell. The planes were on visual flight rules, which means they do not regularly communicate with controllers and are supposed to stay apart by sight.

In another incident yesterday at the Fort Lauderdale-Hollywood International Airport in Florida, a Pan American World Airways Boeing 727 skidded off the runway during a rainstorm. Minor injuries were reported in the accident. A Pan Am spokesman said the jet's main landing gear collapsed during acceleration for takeoff, indicating the incident was not linked with the controllers' strike.

A People Express Boeing 737 also was forced to turn sharply to avoid hitting a small plane Sunday night above northwest New Jersey. An FAA spokesman said both planes were outside the area covered by a tower and were operating by sight, so that no controller involvement was apparent.

The Air Line Pilots Association has said publicly that there is no safety problem, and spokesmen repeated those statements yesterday. Spokesmen stressed that the memos about safety were intended only for private circulation and would have been prepared more carefully if the authors had known they would be publicized.

"Obviously, the level of safety we enjoyed before the strike is not the same today," said an association offical who asked not to be identified. "But the level of safety has not degenerated to the point where we consider it unsafe."

The officials said the information in the memos was taken from reports from pilots, airline managers, the FAA and the striking Professional Air Traffic Controllers Organization. A report on safety will be presented today to the executive board of the pilots' association, they said.

An FAA spokesman said yesterday that "safety has not been impaired in any way. We have reduced the amount of traffic to match the reduced staff. We believe the system is perfectly safe."

Most controllers will work 40 to 46 hours this week, down from 60 in the first week of the strike, and fatigue has not been a problem, the FAA spokesman said. Restrictions on instrument flying above 18,000 feet were lifted this weekend, so there will be fewer planes crowded in lower altitudes, he said.

The FAA spokesman said the tower closings did not constitute a hazard because new safety restrictions were in place at those airports, for example rules governing whether planes could fly in during low visibility. Of the nation's 600 airports with airline service, only 450 have towers, the spokesman said, adding that the FAA has closed only 25 since the strike began Aug. 3.

Air Force spokesman Sherry Stetson-Mannix said routine military training flights were being curtailed but that the reduction was "fairly minimal." Many of the military flights were in special areas outside normal air traffic lanes and none threatened safety, she said