The FAA quickly removed a new requirement easing crew oxygen use in Part 121 operations upon learning that it apparently used inaccurate data to justify the rule. The rule would have changed the flight-level requirement at which the flying pilot must use his oxygen mask if the other pilot leaves the cockpit, from above FL250 to above FL350. Shortly after the rule was published, the NTSB alerted the FAA that it had “relied on time-of-useful consciousness data that did not represent actual pilot performance under realistic decompression conditions.”