By Andrea Shalal-Esa
WASHINGTON (Reuters) - Two minor cracks have been found in a bulkhead of a Lockheed Martin Corp
The cracks were discovered during durability testing in late August and occurred after about 9,400 hours, which equates to more than 17 years of operational flying, according to the company and the Defense Department's F-35 program office.
The B-model planes, which can take off from shorter runways and land like helicopters, are designed to last about 8,000 hours.
"Because of the high hours accumulated on this test article, this discovery does not affect current F-35B flying operations," Lockheed spokeswoman Laura Siebert said.
The cracks are also not expected to affect the U.S. Marine Corps' plans to start operational use of the planes from mid-2015, according to Pentagon and Lockheed officials.
Lockheed said the issue could be corrected with a design change that would add less than 2 pounds (0.9 kg) to the weight of the plane.
Officials said the cracks were less severe than a similar bulkhead problem discovered three years ago, and the B-model had 300 pounds (136 kg) of extra weight margin.
"The initial estimates indicate that the fix should not be too time-consuming or costly," said Kyra Hawn, spokeswoman for the Pentagon's F-35 program office.
"We take each discovery in test seriously, however this type of discovery is not unusual (relative to historical development) with air frames of this size, weight and complexity of design," she added.
Hawn said the discovery also had no impact on the F-35 models being built for the Air Force or the Navy.
Air Force Lieutenant General Chris Bogdan, who runs the program for the Pentagon, has discussed the issue with the Marine Corps, Britain and Italy, the main customers for the B-model of the F-35.
News of the cracks came after Bogdan and Lockheed officials highlighted progress on the $392 billion F-35 program, the Pentagon's costliest arms program, at the annual Air Force Association conference last month.
Hawn said Bogdan did not discuss the cracks during the conference because they were found on the Marine Corps' version of the plane, not the Air Force version.
Lockheed said there were about 50 F-35Bs with the same bulkhead. Modifications would be made and incorporated beginning with the eighth batch of low-rate production planes.
(Reporting by Andrea Shalal-Esa; Editing by Xavier Briand)