Each green tag notes a wiring "squawk" found by inspectors. Photo: Spaceflight Now

The biggest chore has been inspecting more than 100 miles of electrical wiring running throughout the shuttle, while adding protecting to shield against future damage and chaffing.

Wiring became a serious concern at NASA when Columbia suffered an electrical short five seconds after liftoff in July, knocking out two of its engine control computers. That left the astronauts one computer failure away from aborting the launch and attempting a dangerous emergency landing.

A frayed wire arced to a screw head beneath the payload bay causing the problem. The incident caused the shuttle program to be grounded last fall for a fleet-wide inspection of wiring aboard Endeavour, Discovery and Atlantis at Kennedy Space Center.

Boeing is making the checks of Columbia's wiring and so far has found 2,000 "squawks." These areas are tagged when inspectors find nicks, cracking, exposed conductors or other concerns.

"We haven't found anything significant that would compromise safety," Hoffman said.

The damaged wire and the blackened screw is found aboard Columbia. Photo: NASA

The exposed wiring in Columbia's payload bay that caused the short in July was uncovered at KSC before the shuttle was shipped to the West Coast.

To date, about three-quarters of the wiring checks have been completed on Columbia. Hoffman noted the inspections at Palmdale are more in-depth than done in Florida given the access his team has available.

"Any place we can get to wiring, we are inspecting."

The shuttle has about 230 miles of wiring and Boeing is checking about half it in areas that can been reached, like inside the crew module, forward fuselage, payload bay, mid-fuselage, aft compartment, vertical tail stabilizer and wings.

The wiring not being examined is located in places "not practical to go," says Hoffman, like beneath the crew module, which would require the compartment to be detached from the rest of the shuttle.

Extra protection is being given to the wiring too, adding shielding against possible contact with screws, rivets or other sharp edges that could pose a problem during the vibrations of launch or impact during ground processing work.

Hoffman says this shuttle maintenance effort is a bigger project than the others undertaken over the past several years.

"This one is easily 20 times greater just because of the sensitivity to wiring."