The Army yesterday said it has grounded its fleet of AH64 Apache helicopters after discovering cracks in 14 main rotor blades. The move is the latest in a series of controversies plaguing the military's newest generation of attack heli- copters.

The Army also has temporarily stopped accepting any more Apaches from the manufacturer, McDonnell Douglas Helicopter Co., until completion of an investigation into the cracks, a spokesman said last night.

Touted as the world's most lethal attack helicopter, the $13 million Apache was designed to provide the Army with close combat support against enemy tanks. It is armed with laser-guided Hellfire missiles, and has the capability to operate at night and in foul weather.

The 68 Apaches grounded at Fort Rucker, Ala., and Fort Eustis, Va., are the initial deliveries of a fleet of 675 helicopters that the Army plans to buy for $9.1 bil- lion.

Maj. Phil Soucy, an Army spokesman, said a hairline crack was found earlier this month in the main rotor blade of an Apache during testing. An investigation of the full fleet turned up similar cracks in 13 other blades, he said.

Each Apache has four blades, 22 feet long and made of metals and composite material. They are designed to last at least 4,500 flight hours and survive hits from enemy gunfire and contact with tree limbs. Soucy said the blade first found to be cracked had 330 hours of flight time.

Soucy stressed that there have been no accidents caused by the cracked blades, but the Army decided to ground the entire fleet as a "precautionary measure."

"A comprehensive investigation has begun to determine if there is any possible design flaw in this blade, or if there are other factors which contributed to the crack," according to an Army statement.

Efforts to reach McDonnell Douglas last night for comment were unsuccessful.

The Army has portrayed the Apache as "the most advanced attack helicopter in the free world" and initially hoped to buy 1,206 of them, 612 for the active forces, 396 for reserve units and and 198 for training. The defense secretary's office has refused to approve that large a buy, however.

For several years after the Apache program was started in 1971, it appered Congress would cancel the program because of growing costs, which have now climbed to $13.3 million apiece. Other critics said the gunship duplicated the A10 tank-killing airplane.

But the Army needs the Apache to offset the Warsaw Pact advantage in tanks and other armored vehicles, Brig. Gen. August M. Cianciolo of the Army's weapons branch told a House Appropriations panel last year.

The Apache was built from the beginning as an attack helicopter -- in contrast to the UH1 gunships used in Vietnam that were little more than troop carriers with machine guns poking out the side doors. The Cobra gunship sent to Vietnam late in the war was modified for attack missions, but lacked the firepower and sophisticated gear of the Apache.

The navigation and night-vision devices giving the Apache the ability to fight in darkness and bad weather were cited by critics who said the helicopter was overloaded with such gear, driving up the price.

The Army would like to replace its Cobra helicopters with Apaches, but will continue with the mix of the two until technical and cost issues are resolved.