THE LATEST interim report by the Sept. 11 commission describes a litany of errors and miscommunications by aviation and air defense authorities during the attacks. Under normal circumstances, these failures would be simply unforgivable, yet the tone of the report, if not commissioners' questions at a public hearing yesterday, is relatively understanding. So little about the attacks actually corresponded to situations these officials had ever envisioned that they ended up, as the report's title aptly puts it, "Improvising a Homeland Defense." Blaming them for the inadequacies of the improvisation smacks of perfect hindsight. That said, it is critical that officials have a more flexible and comprehensive strategy for heading off catastrophe from the skies. Such errors would not be forgivable a second time.

The 29-page report details just how muddled communications were both within the Federal Aviation Administration and among the FAA, the military and the White House. Different FAA offices did not know that others were tracking hijacked planes. The military was not notified of the hijackings promptly. Given the timing of the notification, it never had a chance of actually shooting any airliners down, yet Vice President Cheney at one point believed that two planes had been intercepted. While Mr. Cheney and President Bush authorized incoming jets to be shot down, they did so after the last had crashed, and pilots in the air never learned of the order. Communications were so poor that at one point, the military spent time chasing a plane that, it turned out, had already crashed into the World Trade Center.

The U.S. air defense system was designed to deal with Soviet bombers -- and was greatly ramped down after the end of the Cold War. While protocols did specify how the military and the FAA were supposed to interact in the event of a hijacking, the assumption was that it would be obvious which plane had been hijacked and that the hijacking would be a traditional one. Nobody had contemplated coordinated, multiple hijackings in which the hijackers themselves would be able to fly the planes and their ambition would be to use them not for extortion but as missiles. In that context, it hardly seems surprising that systems functioned less than optimally.

The events of Sept. 11 demonstrated that defense systems need to be adaptable and speedy if they are to be effective against terrorists, who are constantly envisioning new and creative modes of attack. Officials testified yesterday that their systems are much improved. But it is not enough to be able today to prevent a recurrence. An attack exploiting the same vulnerabilities that al Qaeda took advantage of on Sept. 11 probably will not happen again, after all. Rather, future attacks will seek to flummox U.S. defenses in ways just as surprising as airplane-weapons were then. The key question is whether American defenses will be flexible enough to respond quickly and appropriately -- a task for which flawless communications and clear lines of authority are obviously key. Testimony before the commission yesterday suggests that more work needs to be done in the field of aviation. Rigorously assessing the adequacy of the changes since Sept. 11, not just the response that day, will be an important contribution for the commission to make as it puts together its final report.