The grounding, ordered on Thursday, was the second for the Apache fleet in a little over a year. In November 1999, the fleet was grounded to replace their tail rotor bearings or to replace transmission parts.
There are 742 Apaches in service worldwide. The aircraft are made by Boeing.
The latest problem was discovered this week during a preflight inspection of an Apache at Fort Bragg, N.C., according to Dan O'Boyle, spokesman for the Army's aviation program executive office at Redstone Arsenal, Ala. He said there was no connection between this problem and the one that grounded the fleet in 1999.
O'Boyle said it was too early to estimate how long the fleet will be out of action this time.
Apaches are the Army's best attack helicopters and were used extensively in the 1991 Gulf War against Iraq. Flown with a crew of two, they are armed with as many as 16 laser-guided Hellfire missiles designed to knock out tanks. In addition, they carry 70-mm rockets and a 30-mm cannon that can fire 625 rounds per minute. The Apache program has cost over $14 billion.
The latest problem found at Fort Bragg was a faulty tail rotor swashplate assembly. That is a component on the rear of the Apache that manipulates the tail rotor in accordance with the pilot's foot pedal movements to direct the helicopter right or left. A similar system on the main rotor moves the copter vertically.
Failure of a swashplate assembly on the tail rotor or main rotor could cause the Apache to crash.
The Army is investigating what caused the fault in the Apache at Fort Bragg. In the meantime, all Apache serial numbers are being inspected to see if they are among ones identified as having potentially faulty swashplates.
Questions about the Apache's safety have not been limited to the Nov. 1999 grounding. In February, CBS News reported that an Apache component, the backup control system or BUCS, was linked to 10 to 15 crashes of the chopper.
Nor is the Apache the only military aircraft about which safety questions have been raised.
In May, CBS News reported that an investigation into a possibly faulty gear in the Chinook helicopter had led to the Justice Department filing a $400 million lawsuit against Boeing.
The Marines grounded its entire fleet of Harrier jump jets in July the 27th such grounding in the past eight years.
Earlier this week, the Marine Corps grounded all eight of its MV-22 Osprey aircraft after one crashed in North Carolina, killing four Marines. The Osprey is a hybrid that takes off and lands like a helicopter and cruises like an airplan. The cause of the crash is under investigation.