Anxious to avoid upsetting air travelers, NASA is withholding results from an unprecedented national survey of pilots that found safety problems like near collisions and runway interference occur far more frequently than the government previously recognized.

NASA gathered the information under an $8.5 million safety project, through telephone interviews with roughly 24,000 commercial and general aviation pilots over nearly four years. Since ending the interviews at the beginning of 2005 and shutting down the project completely more than one year ago, the space agency has refused to divulge the results publicly.

Just last week, NASA ordered the contractor that conducted the survey to purge all related data from its computers.

The Associated Press learned about the NASA results from one person familiar with the survey who spoke on condition of anonymity because this person was not authorized to discuss them.

A senior NASA official, associate administrator Thomas S. Luedtke, said revealing the findings could damage the public's confidence in airlines and affect airline profits. Luedtke acknowledged that the survey results "present a comprehensive picture of certain aspects of the U.S. commercial aviation industry."

The AP sought to obtain the survey data over 14 months under the U.S. Freedom of Information Act.

"Release of the requested data, which are sensitive and safety-related, could materially affect the public confidence in, and the commercial welfare of, the air carriers and general aviation companies whose pilots participated in the survey," Luedtke wrote in a final denial letter to the AP. NASA also cited pilot confidentiality as a reason, although no airlines were identified in the survey, nor were the identities of pilots, all of whom were promised anonymity.

Among other results, the pilots reported at least twice as many bird strikes, near mid-air collisions and runway incursions as other government monitoring systems show, according to a person familiar with the results who was not authorized to discuss them publicly.

The survey also revealed higher-than-expected numbers of pilots who experienced "in-close approach changes" — potentially dangerous, last-minute instructions to alter landing plans.

Officials at the NASA Ames Research Center in California have said they want to publish their own report on the project by year's end.