The Salvadoran government has cleared use of a second AC47 aerial gunship following an evaluation of the performance of a similar airplane with rapid-fire machine guns that went into operation in January, U.S. officials said today.

The second aircraft is part of a continuing buildup of the Salvadoran Air Force by the United States.

Although both Salvadoran and U.S. officials have praised the AC47's effectiveness in repelling or forestalling insurgent attacks, there is also some concern among military observers here that increased reliance on air power could blunt the aggressiveness of Salvadoran ground forces.

Liberals in Congress formally asked the State Department to delay deployment of the second AC47 until after an evaluation of whether use of the first one increased the likelihood of civilian casualties.

"The evaluation has been completed," a U.S. official here said. "It's ready to go."

The AC47s are equipped with three .50-caliber machine guns each of which is capable of firing up to 1,000 rounds a minute, although all of the guns normally are not fired at once. An earlier version of the gunship, nicknamed "Puff the Magic Dragon," was widely used in Vietnam.

Salvadoran and U.S. human rights organizations have expressed concern about the plane's heavy firepower, but U.S. officials said that its guns are highly accurate and could therefore cause fewer casualties than less discriminate attacks with bombs or rockets.

They also asserted that the gunship appears to have frightened the guerrillas when it arrived on the scene of several recent battles.

Finally, the gunship's presence was said to have helped to improve morale of troops under attack.

The United States does not want to create a miniature version of the U.S. Air Force in El Salvador, because advanced equipment is too difficult and costly to maintain, military sources said. The main objective still is to train the Army to use small-unit, nighttime ambush attacks against the left-wing guerrillas, they said.

But a decision has been made to equip the Salvadorans with what are called "high payoff" items such as helicopters, small A37 fighter-bombers, night vision devices, the AC47s and four Hughes 500 helicopters with rapid-fire guns that are expected to arrive here soon.