Airlines last year experienced the steepest drop in international passenger traffic in the history of modern aviation, paving the way for more losses and belt tightening in the months ahead, an industry trade organization said Wednesday.

Traffic declined 3.5% in 2009, with the average plane flying 75.6% full, the International Air Transport Association reported.

“In terms of demand, 2009 goes into the history books as the worst year the industry has ever seen,” the association CEO, Giovanni Bisignani, said in a statement.

On the heels of a global recession that has winnowed the number of travelers, industry analysts have hoped airlines will begin to turn the corner this year if more fliers, particularly lucrative business fliers, return to the air.

But while passenger traffic inched up toward the end of 2009, it wasn’t enough to overcome the steep losses the association says erased more than two years of growth.

“The worst is behind us, but it is not time to celebrate,” Bisignani said. “Adjusting to 2.5 … years of lost growth means that airlines face another Spartan year focused on matching capacity carefully to demand and controlling costs.”