Allegheny Airlines yesterday said it would institute fare reductions ranging from 20 to 50 percent below current fares on proposed nonstop flights between Pittsburgh and the West.

If it gets the desired routes, it will aslo extend the discounts to flights carrying passengers to Pittsburgh from connecting Eastern cities such as Washington, the Washington based carrier said yesterday.

Allegheny is currently in competition with American Airlines, Northwest Airlines, and United Airlines in seeking nonstop authority between Pittsburgh, on the one hand, and Denver, Los Angeles and San Francisco on the other. Trans World Airlines is now the only carrier authorized to serve those routes nonstop.

If awarded the new routes. Allegheny President Edwin I. Colondny said the carrier would buy two of Airbus Industrie's A300B wide-body jets, now being used exclusively in the United States by Eastern Airlines, on the two California-bound routes.

Allegheny's top priority for some time has been to get some routes to some sunnier spots and "away from the problems of the Northeast," colodny noted in a meeting with reporters yesterday.Awards of the proposed routes would enable Allegheny to make a major transition away from a local service carrier to trunk status, he suggested.

Allegheny said its new fare plan, called "US Air Service," would price standard day coach fares at 20 per cent below normal industry coach class fare levels; off-peak coach fares would be priced at 30 percent below current fares.

In addition, Allegheny would have a "Super Fare" with discounts of 50 per cent applicable to certain capacity and time limitations. On the California runs, Allegheny proposed a "premium" class, priced at about 25 percent below other airlines' firstclass fare levels, which would offer more amentities and more spacious seating than regular coach class.

As an example of its proposed fares, Allegheny noted that the current coach fare from Pittsburgh to San Francisco is $198 one-way; its regular day coach fare would be $158, while its "Super Fare" would be $99, its "Super Fare" to Denver would be $69, compared to the regular coach fare of $129.

Colodny said yesterday that Allegheny had decided on the foreign bult Airbus because it was "a very great machine" but that it wouldn't go forward with it unless Eastern, or another U.S. carrier, also decided to purchase it.

In a relaed development. Allegheny yesterday reported record earnings for 1977 of $16.4 million ($2.62 per share.), compared with $13.8 million ($2.19) for 1976. Without a retroactive mail pay increase of more than $7 million, the net earnings for the year were $9.4 million, the highest in the company's history. Total operating revenues for 1977 totaled $492.7 million, compared with $439.1 million in 1976.

Fourth-quarter net earnings in 1977 were $7.1 million, compared with last quarter profits in 1976 $1.1 million, excluding the gain of $7.7 million exchange of debt.

In response to a question. Colony said the matter of paying out dividents is "under review."