Required maintenance procedures for commercial aircraft are regularly ignored by U.S. airlines, and the Federal Aviation Administration has done little to prevent such safety violations, an official of the International Association of Machinists and Aerospace Workers Union told a congressional subcommittee today.

"Our members on the job each and every day see violations of their carrier's maintenance manual procedures and are asked by [supervisors] to circumvent these procedures so that [airlines] schedules can be met," IAM McCulloch told the House subcommittee on government activities and transportation. And, McCulloch charged, mechanics have been "totally frustrated in their attempts to report" the violations to the FAA.

McCulloch's assertions came in the wake of revelations earlier this month that improper maintenance may have been the prime cause of the May 25 crash of a DC10 that lost an engine during takeoff in Chicago, killing 273 people.

Today, the subcommittee, which has been investigating airline safety for nearly a year, heard of another violation of FAA-required maintenance procedures that allegedly caused a plane to lose part of its engine assembly in mid-flight.

McCulloch testified that several years ago two mechanics for Continental Airlines urged that a plane be grounded because bolts securing a "thruster reverser" to the aircraft's engine had been fastened "upside down."

A Continental maintenance supervisor was said to have okayed the plane for flight despite the mechanics' objections, McCulloch claimed, the reverser later "came apart" and pieces of it fell off the plane upon landing.

McCulloch's complaints were echoed by James D. Sparling, safety and standards director for the Aircraft Mechanics Fraternal Association, who told the subcommittee that "inadequate" number of FAA maintenance inspectors had led to a situation where " industry abuses can flourish in the vacuum created by the FAA."

The subcommittee was presented with statistics showing that the FAA inspector force has been decreasing in size by 3 to 4 percent a year; that the number of quality control inspectors was down by 30 percent over a five-year period, and that the number of aircraft mechanics employed by airlines fell nearly 15 percent over the last decade.

The number of aircraft in service, meanwhile, has risen from 2,226 in 1968 to 2,413 today, an "all time high," according to McCulloch.

Sparling said the FAA had turned a deaf ear to complaints by mechanics about safety violations, and that official agency statistics severely under-report both the number of major repairs conducted on airplanes and industry. He called for the FAA to publish a regular "consumer index" of safety violations, rating the performance of individual airlines.

FAA Deputy Administrator Quentin S.C. Taylor admitted in his testimony that the agency's "reporting of major repairs appears in need of refinement," and that the investigation into May's DC10 crash revealed "instances of unacceptable maintenance practices."

The FAA will move to upgrade its reporting system and take "a long hard look at the way maintenance is regulated and performed," Taylor promised.