The grounded DC10 cleared two important legal hurdles yesterday only to suffer another serious reverse when federal inspectors discovered cracks in another section of the engine support pylons on three planes.

Legal barriers to renewed passengers services by the jumbo jet were removed by the US. Court of Appeals for the District and by a National Transportation Safety Board administrative law judge, William E. Fowler Jr.

The appellate court overturned District Court Judge Aubrey E. Robinson's June ruling requiring the Federal Aviation Administration of give 24 hours notice before lifting its grounding order. Fowler dismissed the FAA's grounding order, but permitted two days for appeal to the full board.

Another unfavourable development yesterday was the release of an FAA report on i'C10 maintenance practices by eight airlines showing that mechanics were ill-trained and handled equiptment carelessly. The engine pylon on the American Airlines DC10 that crashed in Chicago May 25 and killed 273 people was handled roughly by the line's mechanics, the report showed.

The DC10 in Chicago crashed after the left wing's engine and pylon fell off just as American Airlines flight 191 was lifting off the runway. Vital controls and warning systems were made inoperable, and the plane rolled to the left and crashed.

Subsequent insepctions of the support pylons on all 138 U.S.-operated DC10s found 91 with problems of one kind or other, some serious. The most serious were cracks in the aft bulkhead of the pylon (a 10-inch crack had crashed airplane). Those cracks were blamed on maintenance procedures used by American and Continental airline. All U.S. DC10s were grounded June 6 and have stayed on the ground since.

Last Friday the FAA approved a new detailed inspection on DC10 pylons in anticipation of returning the planes to service. By yesterday about 30 planes has been checked with FAA inspectors standing by, and on three of them cracks were found in the center spar on the plyon.

When the first crack in this new area was discovered, on a United Airlines DC10, senior FAA officials didn't think it was critical.However, when center spar cracks were found on two Trans International DC10s, FAA officals decided that "we have to understand those before the plane can be returned to service," spokeman Fred Farrar said.

Pylon inspections of the entire U.S. DC10 fleet will probably be completed by the end of this week, according to FAA officials, and they will have a better idea of the center spar problem.

McDonnell Douglas, manufacturer of the DC10, said it would have no comment on either the new cracks or the maintenance report.

A central conclusion of that report was that "mcDonnell Douglas should re-evaluate the design of the entire pylon assembly to minimize design factors which are resulting in sensitive and/or critical maintenance and inspection procedures."

The report also found that McDonnell Douglas gave inconsistent instructions to the various airlines using the DC10 and that it knew one maintenance procedure could damage the aft plyon bulkhead.

That procedure involved removing and replacing engine and pylon as a single unit, rather than separating them. The huge assembly was maneuvered into place by a fork lift at American and Continental, inspectors found.

It wasn't always easy, even though it halved the time needed to remove and replace engine and pylon, according to the report. Ralph W. Osborn, an American Arlines mechanic who was working on another component, told investigators of a time his colleague where having trouble re-installing a pylon:

Q: Did you hear anything that caught your attention?

A: Well, I did hear some pretty heavy pounding. . .

Q: What kind of noise was it?

A: Hammer noise.

Another American Airlines employe, forklift operator Raymond Lattanzia Jr., said that the engine and pylon sometimes shifted on the lift. Sometimes, he said, the forklift ran out of gasoline while supporting the plyon. That meant a reduction in holding ability that had to be restored once the lift was fuelded.

That happened when he and others were working on the plane that subsequently crashed in Chicago, Lattanzia said.

Because of the weight of pylon and engine together and a forklift that Lattanzia said "jerked," investigators have theorized that the aft pylon bulkhead could easily have cracked against its fitting in the wing.

The procedure of removing and replacing engine and pylon as a unit has been outlawed by the FAA since the Chicago crash.

The maintenance report also said that American Airline employes working on 40 of 46 pylons that were removed had never been trained in the forklift.

The report provides substance for many findings that have already trickled out of the DC10 investigation, and many of its recommendations have already been adopted by the FAA.

Ultimately, however, the report must be interpreted as critical of the FAA itself, which is responsible for overseeing airline maintenance. Federal Air Regulations, the report found, do not adequately define what the airlines can do by themselves and what they must seek approval to do. FAA Administrator Langhorne M. Bond has promised changes here.

American Airlines had no comment yesterday. Continental Airlines, which was the first to damage a pylon with a forklift and report the problem to McDonnell Douglas, said yesterday it no longer uses the unit removal procedure.

The Continental dadamage was deemed minor, and therefore never was passed on to the FAA.