Federal Aviation Administration chief Langhorne M. Bond told Congress yesterday that new cracks have been found on five of the 46 DC10s that have been reinspected and that the jumbo jet will remain on the ground until he has a "comprehensible" explanation.

The new cracks are in a center support section of the pylon that holds the engines to the wings. Before Sunday, all previous DC10 pylon problems had been found in other areas. Bond authorized new inspections late last week preparatory to lifting the grounding order and it is those inspections - far more comprehensive than any others - that are turning up the cracks.

So far, the new cracks have been found on one United Airlines plane, two Trans International Airlines planes and two Continental Airlines planes. There are 138 U.S.-operated DC10s.

The current DC10 problem began May 25 when American Airlines flight 191 crashed in Chicago, killing 273 people. FAA reports and National Transportation Safety Board examinations have suggested that improper maintenance procedures by American Airlines caused a 10-inch crack in the pylon that ultimately resulted in the crash. The plane crashed after the engine and pylon fell off the left wing on takeoff.

American vice president Donald Lloyd-Jones said in an interview yesterday that the airplane that crashed was manufactured with three shims in the pylon area that were not called for in specifications. Shims are small strips of metal used to close gaps.

He said the presence of the shims "could contribute" to the 10-inch crack. "We cannot say that we did not cause the crack, nor can we find any evidence that we did," Lloyd-Jones said.

Lloyd-Jones is to testify today before the Senate Commerce aviation subcommittee, chaired by Sen. Howard W. Cannon (D-Nev.)).

Both Bond and safety board chairman James B. King, appearing before Cannon's panel yesterday, discounted the importance of the shims in the crashed airplane.

Bond said that "if American Airlines has replicated exactly the maintenance procedure, including leaving for lunch and hammering on the clevis . . . If that was replicated, then maybe the [shim] case has merit."

An FAA report has found that American Airlines supported the engine and pylon of the crashed plane on a forklift and that the forklift sagged while it ran out of gas or during a lunch break. The FAA also has taken testimony to the effect that American Airlines mechanics had to hammer on part of the pylon-wing attachment, including a section known as the clevis.

Paul Soderlind, a retired chief pilot for Northwest Airlines who is highly regarded for his expertise in the cockpit, told Cannon that if all warning systems had been functioning on American flight 191 it "would have been a very positive factor" and given the pilot a chance to save the plane. Two warning systems were cut off when the engine separated from the wing.

Soderlind is one of several experts who has "flown" a sophisticated simulator duplicating the condition of the plane that crashed.

In New York yesterday, Pan American Airways said it had discovered cracks in the engine mount of one of its Boeing 747s and grounded the plane immediately at Kennedy International Airport. But the airline said the cracks could not have affected the plane's safety.

Robert Fulton, an FAA spokesman, said officials would comment later. Another FAA spokesman said there is possibility that all 747s would be grounded.