Air Force officials grounded 30 C-130 Hercules airplanes and restricted 60 others around the force Thursday due to stress cracks in the aircrafts’ center wing box structure, where the wings meets the frame, officials said.

Three of the restricted aircraft are at Yokota Air Base, Japan.

The move came on the advice of the C-130 System Program Office at Robins Air Force Base, Ga., the body responsible for managing the Air Force C-130 fleet.

Inspectors have discovered stress cracks in the box structure during inspections since 2001, officials said. Using a formula of flight hours compounded by certain maneuvers, the System Program Office determined that certain aircraft must be restricted or grounded.

The restricted aircraft still are able to fly, but with shorter hours or reduced loads, officials said.

“It doesn’t mean they’re unsafe. The aircraft are still safe to fly,” said Capt. David Faggard, a Pacific Air Forces spokesman at Hickam Air Force Base, Hawaii.

One more Yokota C-130 may be restricted in three to four months, when it reaches a certain number of computed flight hours, Faggard said.

A fourth PACAF aircraft at Elmendorf Air Force Base, Alaska, also may be affected in six to nine months, Faggard said.

The problem is not new for the C-130s. About 20 years ago, most of the planes had their wings replaced to extend the life of the aircraft, said Col. Mark Schissler, commander of the 374th Airlift Wing, which operates about 10 C-130s.

The more difficult the flying operations, including flying low, carrying heavy loads and operating in turbulence, the more stress on the wing box.

“It’s the most critical part of keeping the plane together. That is the part of the airplane where we’re seeing cracks develop,” Schissler said. “It’s not a surprise. We know those aircraft are aging and we use them hard.”

The affected planes will remain restricted until the Air Force decides what to do next: either repair them — a potentially extensive process — or retire them.

In the meantime, flight engineers of the affected aircraft will be required to follow nine restrictions designed to reduce the stress on wings, including limiting cargo and fuel loads, reducing air speed or maneuvers and adjusting configurations.

“We’ll still be able to meet the mission,” Schissler said. “Scheduling will be much more challenging.”

All of the grounded aircraft are model E Hercules, the same flown by the 36th Airlift Squadron under the 374th Airlift Wing at Yokota.

The 60 restricted aircraft include E models and other variants.

The 374th is the only airlift wing in the Far East and supports all Department of Defense agencies. The wing and the 36th returned late last month from an arduous mission to South Asia in support of tsunami relief. Yokota’s C-130 crews logged nearly 2,700 hours of flight and hauled 4.7 million pounds of humanitarian aid and equipment, Schissler said.

On Friday, the New York Times reported that Air Force Chief of Staff Gen. John Jumper told the Senate Armed Services Committee that some of the oldest C-130s, mostly working in Iraq, are developing cracks in their wings from heavy loads and long hours.

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