The Army grounded its fleet of new AH64 Apache helicopter gunships yesterday, calling the action a precaution after an instructor at Fort Rucker, Ala., was killed in a crash last week.

The service said the directive applied to all 264 Apaches now in its inventory and would remain in effect until an investigation is completed into an Aug. 21 crash, which also seriously injured a student pilot.

"This action is a precautionary measure taken as the result of the crash of an Apache for what appears to be a mechanical failure in the tail rotor swash plate assembly," the Army said in a statement.

The Aug. 21 crash was the third major accident involving AH64s this year, but the first to result in a fatality. The other two accidents also occurred at Fort Rucker, on Aug. 10 and July 9.

Earlier this summer, the Army ordered all Apaches temporarily grounded for modifications to an engine wiring harness. Those modifications took only about four hours per helicopter.

The Apache is replacing the Cobra as the Army's primary attack helicopter. Designed to knock out enemy tanks, the Apache is a twin-engine copter loaded with Hellfire and Hydra 7 missiles and a 30mm cannon.

It is built by McDonnell Douglas Helicopters, formerly Hughes Helicopters, and costs about $11.5 million. The Army plans to buy 573 of them.

The Apache fleet was grounded twice last year for relatively minor problems.

On Jan. 30, 1986, the choppers were grounded following the discovery of cracks in some rotor blades. It was determined the cracks were caused by an improperly designed maintenance tool.

On March 12, 1986, the fleet was grounded after the Army decided to replace certain bolts used to hold the flight-control assembly together. The service determined it had erred in specifying the use of a hardened steel for the bolts.

The Army said the grounding order would also apply to 36 Apaches being shipped to West Germany for the annual North Atlantic Treaty Organization Reforger exercises.

The Army identified the pilot killed in last week's accident as Chief Warrant Officer 4th Class Ronald Rivera, 36, of Brentwood, N.Y.