The Federal Aviation Administration (FAA) in Washington tonight ordered the third grounding in a week of some DC10 jumbo jets after the discovery that a shortcut in recommended maintenance procedures may have been a factor in the May 25 Chicago air crash that killed 274 people.

The shortened procedure, regularly used before today by American Airlines at its fleet maintenance base here, was used on the ill-fated DC10 during maintenance work last March. It was used on at least four DC10s during inspections of the planes last week, the FAA said.

The FAA grounding order directs that at least two of those planes, owned by domestic airlines, be reinspected to check if last week's inspection caused any new problems. The other two are foreign-owned and therefore not subject to FAA orders.

The potential exists that a structural problem in the DC10 engine mounting assembly could actually have been initiated in those four instances during last week's inspections of the DC10 fleet, FAA sources here and in Washington said tonight.

The procedural shortcut involves the method of removing and reattaching the engine and engine support assembly, called a pylon, to the wing. The assembly has become central to the Chicago crash investigation because both engine and pylon fell off American Airlines 191 just as it was taking off at O'Hare International Airport.

FAA officials said there was no legal requirement that the shortcut in the procedure be approved by FAA inspectors stationed here.

Only the two U.S.-registered DC10s appeared to be affected by the reinspection order, FAA officials said late tonight. Although FAA officials said several airlines used the American procedure, other airlines have reported to the FAA that engine mountings were inspected after remounting and showed no structural fatigue problems.

Investigators have known since the day after the Chicago crash that there were signs of structural fatigue or failure in the three points where the pylon attached to the wing. Those three points are called the forward bulkhead, the thrust link assembly and the aft bulkhead. A bolt was broken in the thrust link assembly and captured early attention in the crash investigation.

However, a major crack was evident in the aft bulkhead as well, and investigators have been trying to determine from the beginning what broke first and thus led to the breaking of other parts.

Late today the National Transportation Safety Board (NTSB), which is investigating the crash, told the FAA that the aft bulkhead crack was "caused by physical impact that likely occurred when the pylon was installed during previous maintenance work." The board added "preliminary evidence [in the Chicago crash] indicates that the forward flange of the aft bulkhead had failed completely."

Furthermore, the board said, similar cracks on forward flanges of aft bulkheads were found on two other American Airlines DC10s inspected last week.

The cracks apparently were caused, safety board and FAA officials said today, when the engine and pylon assembly were lifted together into the fittings that hold the pylon on the wing. The entire assembly weighs about 11,000 pounds and moving it requires the use of a forklift. The forklift operator, the board said, had "limited control" in an area with very fine tolerances.

The metal bulkhead apparently struck the fitting in the wing and was cracked as it was being reinstalled, sources said.

McDonnell Douglas, builder of the plane, recommended that the pylon and engine be dismounted separately and mounted separately. The pylon, which weighs only about 2,500 pounds, is easier to manage and less subject to cracking than it would be with an engine attached.

The airplane in the Chicago crash had its engine and pylon removed here in March so that the monoball - a key connecting bearing in the assembly - could be replaced as McDonnell Douglas had recommended. The other two American Airlines planes discovered with cracks had the same service performed here on Dec. 7 and March 17, the safety board said.

American Airlines spokesman Art Jackson said here tonight that, "we thought that [the shortened procedure] was a safe way to do it. That we weren't compromising anything. We're going to use the recommended procedures now. If it was improper procedure, why was it approved by the FAA?"

George House, head of the FAA's inspecction office that oversees American's maintenance here, said in an interview today that there was no requirement that FAA inspectors approve the modifications American Airlines made in the McDonnell Douglas recommended procedure.

"American Airlines is authorized to design their own repairs and their own modifications," House said.