The Dash 7, a new Canadian airplane that can take off and land on short runways and is designed especially for brief trips, made its debut at Washington National Airport yesterday.

De Havilland Aircraft of Canada, the manufacturer, is hoping the 50seat plane will fit nicely -- and profitably -- between the small aircraft now being flown by commuter lines and the large planes used by major airlines.

The "demonstrator model," as it was called by aviation officials, was brought to Washington by de Havilland and Ransome Airlines, an Allegheny commuter, which hopes to put the plane in service on its Washington-Philadelphia route this fall.

Besides its short take-off and landing capability, which allows it to use the runways built for smaller planes, the Dash 7 will carry sophisticated navigational gear enabling it to operate outside the airways that conventional planes are using, the manufacturer said.

Small conventional planes flown by commuter lines now operate in the same airways en route and use the same landing and holding patterns as the larger jets, de Havilland and Ransome officials explained. Because of the Dash 7's advanced technology, it can be separated from that flow and can take advantage of the shorter runways of existing airports without interfering with other operations.

Ransome Airlines President J. Dawson Ransome said using the plane can help meet expanding passenger demand and reduce delays without constructing new runways.