CHICAGO (AP) _ Many people trying to overcome their fear of flying are suffering reversals as a result of the Northwest Airlines crash and a recent series of problems involving U.S. aircraft, some counselors say.

''Our clinicians report more people experiencing a setback in their treatment because of the latest events,'' Susan Kanaan said in a telephone interview Tuesday. She is executive director of the Phobia Society of America.

The Aug. 16 crash of Northwest Flight 255 at Detroit Metropolitan Airport killed 156 people.

''Something this dramatic always drops the scale back from progress,'' said Ms. Kanaan, whose 6,000-member organization of researchers and pyschologists based in Rockville, Md., provides information on the treatment of all kinds of phobias.

There also have been several near-collisions, mechanical failures and other problems involving U.S. carriers during the past few weeks.

Counselors said such incidents apparently also have increased the number of people seeking help.

''Our secretaries report more inquiries lately, and we get several hundred inquiries each week,'' Ms. Kanaan said.

T.W. Cummings, a retired Pan Am pilot who wrote ''Freedom from the Fear of Flying'' and offers tapes and seminars for fearful fliers, also has noticed a change.

''More people have been interested in my book and tapes since the problems began,'' Cummings said from his home in Coral Gables, Fla. ''And people in my last seminar asked me some questions about the crash in Detroit.''

But psychologist Burt Siegel, who heads the Institute for Stress Control in suburban Hinsdale, said he had noticed little change as a result of recent incidents.

Siegel also said he has not had any increase in patients since the Northwest crash.

A former Siegel patient, Helen Gaynor, said recent accidents ''obviously makes one wonder, but the statistics are there. It's safer to fly than to drive your car, and I just drove home from downtown in the rain and I can vouch for that.''

Mrs. Gaynor, 48, a suburban Riverside resident who took her first flight in May after ''eight or nine sessions'' with Siegel, is studying to become a travel agent.

Cummings also emphasized the safety of flying.

''Over the last five years, the yearly average for deaths from flying U.S. (scheduled airline) airplanes is less than 100. The daily average is 123 on the nation's highways,'' he said.

''It's front-page news if one jet engine malfunctions, but all jets fly safely with only one engine operating,'' Cummings said.

Deepak Patel of Contrail Travel Inc. in Chicago said Tuesday he has noticed no drop in airline business.

''Some people have asked not to fly Northwest, especially to Detroit,'' he said. ''But this is normal after a crash like that. This always happens. In two weeks, things are back to normal. They forget about the crash and start using the airline again.''