Most of the nation’s air fleet remains grounded, and transportation officials say they do not know when regular, full-scale service will resume. This much is known: When air travel does resume, it will be under much heavier security than has ever existed before. “It’s quite apparent that we need to revamp and move forward on security issues,” said Rep. John Sweeney, R-N.Y., a member of the House Appropriations transportation subcommittee. The few planes that did take to the skies on Wednesday did so with provisos. Transportation Secretary Norman Y. Mineta announced Wednesday that planes that had been diverted on Tuesday could take off and land at their originally scheduled locations. Some planes began leaving later Wednesday, carrying only those passengers who had begun the journey. Other planes could leave as soon as their destination airports had finished improving security procedures. In addition to permitting stranded passengers to get to their original destinations, Mineta said airlines could also move empty planes from airport to airport to prepare for normal operations.