LAKE BUENA VISTA, Fl. -- NASA's efforts to address foam debris issues with its shuttle fuel tanks has proven an object lesson of how even the most diligent spaceflight risk management efforts can fail, the agency's shuttle chief said Wednesday.

Wayne Hale, NASA's shuttle program manager, told about 400 risk experts, engineers and managers that the loss of a one-pound chunk of foam from a protuberance air load (PAL) ramp during the Discovery orbiter's STS-114 launch was not among the top risks expected for the mission before flight.

"Our mistake was that we thought we understood the mechanism for foam loss," Hale said here during NASA's Risk Management Conference 2005, adding that the PAL foam fell well clear of Discovery. "We were lucky."

The foam loss was reminiscent of a similar problem that doomed the space shuttle Columbia and its crew, and marred Discovery's otherwise successful flight to the International Space Station (ISS).

Engineers may now decide to remove PAL ramps altogether from an external tank to fuel NASA's next shuttle launch, Discovery's STS-121 flight, set to fly no earlier than May 2006. A meeting to discuss the matter could be held next week, Hale told SPACE.com.

STS-114 pilot James Kelly said that while external tank foam debris was critical issue before the flight, he and his crewmates were more interested in ensuring their spacecraft's integrity.

"The most critical thing for us as a crew was knowing the health of our vehicle," Kelly said during the conference. "If you don't know there's something to fix, it doesn't matter how well you can fix it."

NASA's STS-114 flight carried a 50-foot (15-meter) boom extension for the orbiter's robotic arm. Tipped with laser sensors and a camera, the boom allowed Kelly and his fellow Discovery astronauts to scan the orbiter's heat shield in flight and send the data down to engineers who later cleared the shuttle for reentry.

Kelly said the STS-114 crew did receive a good lesson in determining risk trade-offs while preparing for Discovery's flight.

The lack of a backup system to ensure that latches for stowage lockers remained closed during launch required the use of tape to safe their doors, he said, adding that the STS-114 crew also traded a series of exchanges with structural analysts to develop contingency plans for securing a 600-pound (272-kilogram) ISS gyroscope in Discovery's payload bay for return to Earth.

Kelly said STS-114 mission managers did overrule Discovery's crew when it came to adding a pair of heat shield repair tools on the orbiter's launch manifest. Two cure-in-place-ablative-applicators (CIPAA), backpack-mounted tools designed to squirt a pink goo-like material into damage tiles, rode aboard Discovery even though the repair method itself was not ready for orbital testing.

"Our crew never expected to have it onboard...[but] people did not want to send us on orbit without something to save us, it was a very emotional issue," Kelly said, adding that the important thing was that Discovery's astronaut crew voiced their opinions. "Our concerns were noted and I think that was a success story."