Boeing's super 747s may become flying white elephants of the '80s

The largest airplane in the world to fly scheduled commercial service is something of a flying white elephant. So far, there are only 22 firm orders for Boeing's latest jumbo 747-300. These are from foreign airlines with long international routes which can afford (with or without government subsidies) the $99 million price tag for each behemoth jet.

In the United States, few 747s are still flown on domestic routes, because ''with the increased competition under deregulation, it's very difficult for any airline, except under special peak scheduling conditions, to get enough passengers,'' says Joseph Murphy of Avmark Inc., an aviation consulting firm in Arlington, Va.

''When you have six or eight airlines competing in a market, for anybody to corral enough (passengers) to make a 747 pay its way is becoming less and less possible,'' Mr. Murphy says. Five US carriers - Delta, World Airways, Eastern, National, and Continental - have sold off their 747s and stayed with fuel-efficient mid-size aircraft.

And those US airline companies with a dozen or more 747 models in their fleets already simply can't afford the expensive new model. ''You might say the US carriers are having a little trouble with money,'' says a Boeing spokesman in Seattle, Wash.

Trans World Airlines, for example, ''is watching its pennies very carefully, '' says Avmark's Murphy, and the new 747 ''is the last thing in the world Pan American (with 45 747s) would consider right now,'' he says. Spokesmen for both companies agree.

But both Swissair and France's Union de Transportes Aeriens already have the new 747-300 aircraft in service. (The total cost for Swissair's order of five planes: $550 million.) Singapore Airlines (eight on order), Royal Dutch Airlines , Japan Air Lines, and South African Airways are waiting in the wings for theirs.

The basic difference between the $22 million 747 that was introduced in 1970 and the almost $100 million 747-300 is 23 feet added to the characteristic ''bubble'' on the upper part of its fuselage. The lengthened upper passenger deck can now accommodate up to 91 passengers.

A ''combi'' arrangement also allows for passengers in the forward section and cargo in the aft section of the main deck.

The new 747-300 also has more fuel-efficient Pratt & Whitney engines that burn less fuel than the original, despite the added weight, says a Boeing spokesman.

Avmark's Murphy says the only competition for the new jumbo (which he says is probably ''the safest airplane in the sky'') may be from smaller, long-range airplanes still on the drawing boards, such as the Airbus TA-11.

When it comes to the 500-plus passenger planes, ''Boeing owns the market,'' such as it is, he says. There is even a plan for the late 1980s to stretch the 747's ''bubble'' the entire length of the aircraft.