The Federal Aviation Administration announced a major decision yesterday on the type of air safety equipment that will be used in the future to minimize the possibility of mid-air collisions.

FAA Administrator J. Lynn Helms told the Aero Club yesterday that the new system will allow a private pilot to be warned that another aircraft is dangerously close before it can be seen and without any need for ground contact. The system to be used in the airlines' 2,200 aircraft will do even more, providing important new collision-avoidance capability, he said.

"If we're asking for a 100-percent-effective collision-avoidance service, the only one I know of is ground all the airplanes but one," Helms told reporters following his speech. A "possibility" of a collision will remain even with a new system, "but the probability will be drastically reduced," he said. "There's no question that it will be a great step forward for aviation safety."

The system Helms decided on involves two different but compatible elements, one for use in the smaller planes flown by private pilots and the other for the scheduled airlines and business aircraft users:

The less extensive "threat alert" element for the private aircraft pilots will provide, at a minimum, a visual or aural alarm alerting the pilot that he or she is close to another airplane carrying the same system, Helms said. This system will cost an estimated $2,500. For a little more money, Helms said, the system also can advise the pilot of whether the other aircraft is higher or lower in altitude or provide a display showing the "o'clock position" and range of "threat" aircraft with the more advanced equipment.

More capable than anything used so far, the system for the major aircraft users will provide Collision-avoidance capability in very high-density traffic situations such as Chicago and New York. Estimated to cost between $45,000 and $50,000, it will be able to see a smaller airplane, locate it by bearing and altitude, then send a direct signal to the smaller plane advising its pilot of the larger plane's position and altitude. The system can have a link to the ground but not be dependent on it, Helms said.

An aeronautical engineer and former military test and research pilot who has logged more thant 10,000 miles, Helms said he personally flew in a small airplane last Friday to test the system in six airborne experiments with the FAA's 727. He said they "firmly proved" that it worked.

The 727 system picked up the small airplane before visual contact was made, was able to maneuver around and, in addition, sent a signal to the small airplane that advised the pilot which way to go to avoid a collision, he said. These tests included exact head-on flying but were done in perfect weather so that the pilots could see one another if the equipment hadn't worked, an FAA spokesman explained.

Under the FAA's plan, minimum technical standards for the element to be used in the small aircraft will be drawn up and published by the FAA, and manufacturers whose products are certified can compete for business. The FAA said private business firms have indicated that the least complex system could be in volume production in 36 months, the more complex system in 48 months.

Helms acknowledged that there has been a continuing debate over the "best" system of the future, but insisted that the time for study was over. "I believe we've researched it long enough," he said. "The question is what is the best thing in the shortest period of time. . . . Let's start giving protection now instead of waiting for the perfect system which we may never get."

There are about 100,000 airplanes in regular daily use, Helms said, with that number expected to double in the next nine years.

Helms also hinted at a willingness to take on the private aircraft owners, whose powerful congressional lobby have allowed them generally to stave off government efforts to put new safety equipment on their planes or impose new safety requirements. For instance, FAA proposals designed to reduce the chances of aerial collisions made in the aftermath of a 1978 collision over San Diego which killed 144 were dropped a year later after a storm of opposition.

While Helms said he doesn't want to have to make use of the equipment mandatory for private pilots flying in the high-density areas, he suggested that such a decision might be made without "their voluntary participation" in the program.