National Transportation Safety Board Chairman James King told Congress yesterday there is "no doubt in our minds" that the engine support pylon on the American Airlines DC10 that crashed in Chicago May 25 "was hit," and thus cracked, during routine maintenance.

The crack is believed important in reconstructing the sequence of events that led to the crash and 273 deaths in the nation's worst air disaster. Furthermore, the crack apparently resulted from a shortcut in the recommended maintenance procedures without specific FAA approval.

The procedure in question involved the method of removing the engine and pylon from the wing. According to evidence and documents turned up by the safety board in its investigation, DC10 manufacturer McDonnell Douglas recommended to airlines that they remove first the engine, then the support pylon when maintenance on the pylon was required.

The pylon should be replaced first, then the engine added, under the recommended procudure. However, board investigators discovered at the American Airlines maintenance base in Tulsa that the engine and pylon were being removed and replaced as one unit, a procedure that saved time.

As the giant unit - weighing almost 14 tons - was replaced onto the wing with a forklift, the board found it was easy to strike the aft pylon bulkhead flange against the wing support, thus inducing a crack. At least four American airlines DC10s have been found with such cracks, King testified yesterday.

King stressed during his testimony yesterday that it is not yet clear if the crack in the aft bulkhead was a factor in the engine and pylon separating from the wing. Further, he said, "We are not completely persuaded that the loss of the engine and pylon itself should be enough [to cause the crash] . . ."

King said that, although no physical evidence had been found to support the conclusion, the board's investigators believe that fuel lines and vital hydraulic lines were severed when the engine and pylon came off the wing. There is growing evidence that hydraulic control of the leading wing edge surfaces, called slats, was lost on the left wing - thus inducing a roll to the left that was beyond the pilot's ability to control.

Rep. John L. Burton (D-Calif.), chairman of the House Government Operations transportation subcommittee, pressed both King and Bond hard on the question of whether the DC10's three hydaulic systems are truly independent and redundant.

Both safety board and Federal Aviation Administration experts said that there was no interconnection of hydraulic fluid among the systems on the plane, but that there were mechanical interconnections.

Hydraulic pumps in several locations on the aircraft were designed to serve more than one of the three systems.

Board witnesses also said they felt the FAA should have required scientific techniques when it ordered a round of DC10 engine pylon inspections after the Chicago crash. Instead, the FAA approved visual inspections.

Furthermore, the board said, sealant around the suspected areas of cracks should have been removed before the inspections were conducted, and the FAA order did not require that. Bond said yesterday that, in retrospect, the words "remove sealant" should have been included in the order.

Burton made much of a memorandum from FAA's Tulsa office pointing out that the shortcut in engine-pylon mounting "cam result in substantial loads and jiggling." That memo was dated June 1, but Bond did not order new inspections to check possible damage due to maintenance until June 3. CAPTION: Picture, King: "We are not completely persuaded that loss of the engine and pylon should be enough" to cause the crash. By James K. W. Atherton-The Washington Post