Senior administration officials assured a House subcommittee yesterday that the interim air traffic control system is functioning safely, but many of their contentions were challenged by some skeptical congressmen and the head of a private aviation safety group.

During 2 1/2 hours of testimony before a Government Operations subcomittee, Transportation Secretary Drew Lewis and Federal Aviation Administration chief J. Lynn Helms said that all controllers working in traffic facilities are fully qualified and are not overworked.

Reports of near mid-air collisions have declined from 60 in August last year to 30 this year, Helms said in a statement. Air traffic is being held to levels that the reduced work force can handle safely, he said.

"I am more interested in the safety of this system than anybody else in this room," Helms said.

The FAA has fired or begun dismissal proceedings against about 11,500 controllers who went on strike on Aug. 3. About 9,000 supervisors, military controllers and nonstriking civilians are operating the air traffic system while replacements for the strikers are trained.

Helms said that scheduled flight operations probably would hold steady at 75 to 80 percent of normal levels through most of 1982. Recovery to prestrike levels would not come until the first half of 1984, he predicted, as new controllers come on the job.

Rep. Ted Weiss (D-N.Y.) lectured Helms because different safety groups have released widely divergent numbers concerning near collisions. There is no formal system to ensure that all incident reports reach FAA investigators, Helms said. Weiss suggested one be formed.

Weiss told Helms he had been planning to fly to New York. But "on the basis of your testimony, I'm getting even more nervous than I was before." That remark drew applause from supporters of the controllers' union in the audience.

Rep. Tom Lantos (D-Calif.) accused Helms and Lewis of arrogance in stating that the system was functioning so smoothly. "If you would be a little less self-assured, if you would just show a touch of anxiety and not even of humility, we would be far more reassured," he said.

"It's self-evident," Lantos continued, "that when seasoned people leave any organization. . . there is an enormous negative impact on the quality of performance, and I would have expected you to understand that."

Lewis responded: "I'm not sitting here self-assured . . . . I'm sitting here with a problem and trying to the best I can to overcome that problem."

In other testimony, William Reynard, head of aviation safety reporting for the National Aeronautics and Space Administration, said the situation appeared to be routine, but noted that reports of near mid-air collisions had dropped. Between Aug. 3 and 28, his statement said, his office received reports of nine near mid-air collisions. In August 1980, the number was 45.

Reynard expressed concern over the drops, noting that reporting to NASA is entirely voluntary. For uncertain reasons, he said, a far lower proportion of the reports is now coming from controllers. He compared his job now to "looking at the system with one eye closed."

In the afternoon, the subcommittee heard testimony from John Galipault, president of the Aviation Safety Institute, a privately funded group based in Worthington, Ohio.

Galipault's prepared testimony said the institute "had been besieged with hazard reports since Aug. 3," receiving about 350 compared with 180 to 200 in a normal period.

So far his group has confirmed 40 near-collisions since the strike began, most of them in its first five days, and 60 less serious incidents of planes coming too close. Conditions improved after the first week, he said.

These confirmed numbers are between 50 and 100 percent higher than normal, he said. But he cautioned that much of the increase could be due to growing familiarity with his institute.

Galipault was questioned harshly by Rep. Robert Walker (R-Pa.) about funding that his group has received from the controllers' union, the Professional Air Traffic Controllers Organization (PATCO). Galipault responded that PATCO support had consisted of a $1,000 grant three years ago and minor donations totaling perhaps $400.