In more normal times, Western Airlines would have its Boeing 727 jet spend an idle night in Seattle before sending it out yesterday morning. But with the nation's 138 DC10s on the ground, the 727 instead was flown to Anchorage, Alaska, overnight and went back to Seattle just in time for its regular flight.

With extra trips like that, retroutings for thousands of passengers, stripped-down schedules and a lot of computer overtime, the nation's airlines are overhauling their lifestyles to take up where the DC10s left off.

"Basically, it's like creating an instant new airline," said Mike Clark of National Airlines.

Yesterday a federal judge extended until June 15 his order grouding the entire domestic fleet of 138 DC10s pending tests for the causes of cracks and other problems in the wing engine mountings. None of the jets will take off, said Federal Aviation Administration chief Langhorne Bond, "until the problem is found, analyzed and cured." No one knows how long that will take.

There will be an exception, however, for "ferry flights," which will get the jets from their scattered locations to testing facilities, a federal judge ruled yesterday. No passengers will be allowed on board, and, under the ruling, experimental flights will be allowed.

Also yesterday the FAA issued formal orders of investigation against McDonnell Douglas, the manufacturer of the DC10, and the eight U.S. airlines that operate DC10s.

The orders give the FAA subpoena power in its search to determine if there is a design problem in the DC10 that led to the engine falling off the left wing in the Chicago crash. The airlines were asked to submit voluminous maintenance records, with emphasis on maintenance difficulties in the DC10 engine mounting assemblies.

Most of the jets were lined up outside inspection stations at major airports yesterday, idling $6 million per day in passenger capacity, the Air Transport Association said. With that muct at stake, the airlines juggled routes and passengers furiously to get as many people as possible into the air some other way.

The hunt was on for idle jets to press into service, but for the most part there weren't any. American Airlines pulled three 727s off its charter service, Continental leased three from Pacific Southwest in an arrangement made before the grounding order, and United Airlines was checking to see which of its 30 charter DC8s it could move.

"We will use them when they're available and we have crews to fly them," said United spokesman Beverly Ishol.

With very few extra planes to be found, existing planes were doing extra duty. National's 727s from Florida to California stopped in Houston to pick up passengers. United put Newark-Chicago and Newark-Los Angeles flights together and came up with one Newark-Chicago-Los Angeles run, freeing a plane for work elsewhere.

Some shorter flights were canceled if other airlines could pick up the slack, inn order to put the airplanes on longer runs. There is a limit to how much of this was possible, however.

"We may be able to get another hour and a half out of the airplane, but there are the crews to consider." said Joe Daley of Continental. Since cockpit crews are not generally interchangeable among planes, roughly 2,000 pilots were idled along with their DC10s, the Air Line Pilots Association estimated.

"Almost all of them are qualified to fly other airplanes but they would have to get current again," said ALPA spokesman Paul Reneau. Each plane requires five crews of three persons to run full time. Meanwhile the Boeing 727 and other jet crews are piling up the hours and no one knows what will happen if they begin to approach the monthly ceilings.

The FAA's ceiling is 100 flight hours per month, but most airline contracts specify a 75-to-80 hour maximum.

Trying to ease the crunch, the Civil Aeronautics Board yesterday authorized airlines to move regular passengers on charter flights and charter passengers on regular flights if necessary, without adjusting the fares.

The stranded charter groups that jammed domestic airports on Wednesday, when the DC10s were grounded indefinitely, had mostly disappeared yesterday. Many foreign airports, however, remained crowded, among them Tokyo's Haneda field and Amsterdam's Schiphol airport.

All the world's airlines appeared to have followed the U.S. lead by grouding their DC10s, including Belgium's Sabena and Venezuela's Viasa, but not without protest. British Caledonian Airways threatened to sue the FAA for "discriminatory" action since the grounding order affected long-range "Series 30" DC10s as well as the "series 10" model on which all of the cracks have been found.

The Series 10 included the model that crashed May 25 in Chicago, killing 275 persons and leading to the current investigation.

Sir Freddie Laker claimed that Laker Airways' DC10s have no cracks, even though they are Series 10, because his maintenance procedures are different and better than those of some U.S. carriers. The FAA action, he said, was "hysterical."

Western, Northwest and World Airways also claimed that their different planes or maintenance procedures made them special cases. They joined several other, carriers in asking to intervene in a suit to ground the plane indefinitely that was brought by the Airline Passengers Association in a Washington, D.C., federal court.

The CAB also waived its rules against discount fare passengers switching airlines at the last moment. Although some passengers still complained they were told they would have to pay full fare if they switched days or flights, the CAB ruling left the airline the option to make the switch without charging extra.

"Any kind of economic restriction that we can remove, we'll do," said Ted Lopatkiewicz of the CAB. "We're economic regulators only; beyond that I don't know what we can do."

The airlines were suffering in a differet manner than they did when United's strike took 24 percent of the industry seats off the books. "Tha was spread systemwide, long and short flights," said Dan Henkin of the Air Transport Association. "This is concentrated on the long-haul routes and is really being felt in those markets."

The DC10s are 41 percent of all the long-distance craft, with 84 Lockheed Trijet £1011s and 112 Boeing 747 jets making up the rest. "It'll become more difficult for those runs the longer they're out because we're coming into the heaviest season," said Henkin. "We really get into it at the beginning of next week."

United Airlines' telephone operators and reservation clerks went to work for their competitors during that walkouts, easing the clerical burden. Not on this one.

"We've had flights leave less than full" because people can't get through on the telephones to make reservations, said a spokesman for Trans World Airlines, which has no DC10s.

The callers instead get a tape recording that says, "We wish to inform you that TWA operates Boeing 727 and Lockheed £1011 wide-body aircraft. We do not operate the DC10." Some travel agents estimated that nine of every 10 callers, before the plane was grounded, asked whether their proposed bookings included the DC10.

Many vacationers appeared to be diverting, their business to ground transport, but most carriers said it was difficult to separate the DC10 impact from that of the gasoline shortage and inflation.

Amtrak, the passenger railroad, was reporting record volume before the Chicago accident and it has not disminished since, a spokesman said. Travel agents reported that people going to more than one place on one trip were having trouble getting to all of them in a short time.

But, so far, two days into the event, the inconvenience had caused more irritation than fury. Passengers seemed so grateful to get to their destinations that they were not yet thinking of legal action, several travel agents reported. Travelers are still holding on to the half-fare coupons being offered by United and American Airlines.

"They can't use them until July 17," said American's Joe Moran, "and let's hope, God help us, that everything is all right by then."