IS AIR SAFETY just as good today as it was be fore President Reagan fired 11,400 striking controllers 21 months ago? That's what administration officials would have you believe. But there is evidence still that any return to the higher traffic levels of pre-strike days should be made with considerable caution. This is the essence of the second major post-strike study of the National Transportation Safety Board. The Federal Aviation Administration should take careful note of it before stepping up traffic any more than it has.

This does not--repeat, not--mean that flying is unsafe today. The conclusion of each NTSB study is that flying is still basically safe, and the record so far backs that up. "Based strictly on the absence since the strike of a significant number of accidents" attributed to air traffic control factors, says the safety board, the controller system "has been operated safely." But the latest report adds that the board has "identified several specific safety areas of concern" indicating that "the margin of safety is less than the safety board believes to be desirable."

One of the more troubling findings is that the FAA has yet to come up with an effective way to monitor and curb excessive workloads, fatigue and stress among controllers. In some instances, the board found, supervisors are still directly handling traffic, which means they are not supervising. There also has been "incomplete reporting" of errors by controllers and pilots with the result that sometimes planes fly too close to each other.

That's hardly grounds for automatically proceeding with planned increases in air traffic. More controllers are needed, as are improvements in the reporting of mistakes. Currently reporting is hampered by a fear of disciplinary action against whistle-blowers.

There have been, and surely will continue to be, pressures from airlines to increase flights to pre- strike levels before the year is out. At the same time, the safety board has no authority to require any new regulations, and the FAA is not required to adopt any of the board's recommendations. The important decision will rest with Transportation Secretary Elizabeth Dole, whose response should be based on more than just an acceptance of general reassurances from FAA that all is well so long as planes aren't crashing into each other.