USAir announced yesterday that it plans to spend $347 million on new Boeing aircraft, including 10 Boeing 737-300s if the firm produces the model. b

USAir's order is the first for ithe 737-300, a derivative of Boeing's popular twin-engine 737, and considered ideal for relatively short-haul, high-density, high-frenquency routes. Similar to the 737-200, the new 737-300 will be quieter and more fuel efficient.

USAir has also placed orders for 15 Boeing 737-200s, to be delivered over two years beginning in late 1982. Assuming production proceeds, delivery of the 737-300s will begin in late 1984. In addition, USAir took options on five 737-200s and 10 of the 737-300s valued at an additional $224 million for the 15 planes.

A Boeing spokesman said yesterday that the company must have orders from two or more other airlines before it would go ahead with production. "But we're very confident that we're going to get those orders," he said. "There's an excellent market for the plane." Among the airlines considering the plane are Southwest Airlines, Frontier Airlines, Air Canada and Lufthansa.

Since the plane is so similar to the 737, it will be built at Boeing's Renton, Wash. production facility on the same line with the current airplane, cutting capital costs significantly.

EDWIN I. Colodny, chairman of the board and president of the Washington-based airline, said yesterday that USAir spent two years studying the aircraft market before deciding on the advanced Boeing model. "The quiet, fuel-efficient 737-300 comes closer than any other aircraft to meeting our specifications for a short-to medium-range jet," he said.

The new plane will differ visibly from the standard 737 models. It will be almost nine feet longer, seat between 17 and 21 more pasengers -- USAir's would seat 138 -- and will carry larger engines mounted forward of the wings on struts, instead of tucked up directly under the wing. Boeing plans to use engines manufactured by General Electric and Snecma, a French firm.

The new 737-300 also has six feet of span added to the horizontal tail and small wing-tip extensions. Besides the additional seating capacity allowed by the longer fuselage, the plane will also have 22 percent more capacity for cargo, according to Boeing.

USAir said its 737-200s will have JT8D-15A engines manufacturered by Pratt and Whitney, a division of United Technologies. CAPTION: Pictures 1 and 2, USAir Chairman Edwin I. Colodny and drawing of a 737 with the airline's markings: a $346 million decision reached after a two-study of the aircraft market. The Washington Post