Federal Aviation Administration chief Langhorne M. Bonb told Congress yesterday he is willing to permit side-by-side tests of competing British and American electronic systems that provide precision guidance for landing airplanes.

Bond also said that all technical information the U.S. has developed in a $83 million, 10-year search for such a landing guidance system is available to the British.

Both of those points represent a change in the U.S. position, according to Alexander Gordon-Cumming, the British aviation attache here. "It's a welcome change, too" Gordon-Cumming said in a telephone interview.

Landing systems devised by the British and the U.S. are competing for international recognition as the world standard and a final decision is scheduled in April by the International Civil Aviation Organization.

Both the British and American developments fall under the general heading of "microwave landing systems" or MLS, as they are called in the aviation world.

A fully developed MLS is supposed to improve dramatically the guidance information a pilot receives as he comes in for a landing and thus improve aviation safety. Almost three-fourths of all aviation accidents happen during approach and landing.

MLS also would provide a theoretical capability of increasing the number of planes that could be handled at a given airport in a given period of time and would improve the capability for fully automatic "hands-off" landings in bad weather.

There has been a viogorous debate for several years about whether the British or the American system is better. Neither side will gain an industrial advantage when the decision is made, both sides agree, because both U.S. and British manufactureers are ready and able to build either kind of system.

The American system is based on the use of time measurements to locate the plane; the British system depends on measuring changes in radio frequencies.

The Federal Aviation Administration decided after some years of testing to go with the time principle and stopped development of the radio frequency system, although the British continued on that track. Under an international agreement, data developed from both testing programs was supposed to be mutually available.

Bond was testifying yesterday at hearing of the subcommittee on transportation, aviation and weather of the House Science and Technology Committee.

Rep. John W. Wydler (R-N.Y.) introduced an affidavit from a British engineer in which the engineer detailed delays and obstacles he said he had received at the hands of an FAA subcontractor working on MLS.

Bond was asked several times if he really meant it when he said the British could have all data.Finally, in exasperation, he replied: "I mean all. There is no word of art or attempt at deception in my statement."

He also said that, although he questioned the technical usefulness of side-by-side tests, he was willing to proceed with them if the British were interested.

Rep. James F. Lloyd (D-Calif.), said "the major point is not so much whether your system is right or wrong - it's whether every measure of fairness has been met."

Comparative testing of the two systems has, so far, been primarily computer simulation, according to testimony yesterday.

After that testing, Bond said yesterday, "we probably felt - on marginal grounds - that (the American system) was superior . . . We have yet to find any flaws."

Gordon-Cumming, in the telephone interview, said that "if we agreed completely with that then we would opt immediately for the (American) system. Our concern is solely that the world get the best system."