Increases in jet fuel expenses completely offset the improvement in passenger revenues the nine publicly traded U.S. passenger airlines achieved in the first nine months of 2018, reported Airlines for America (A4A) v-p and chief economist John Heimlich on Wednesday.

Although the total revenues generated by the nine carriers—Alaska, Allegiant, American, Delta, Hawaiian, JetBlue, Southwest, Spirit, and United—in the nine months increased by $8.668 billion, those airlines’ total costs for the period grew $12.767 billion. While their total operating revenues increased 7 percent over the comparable 2017 figure, the airlines’ operating costs jumped 12 percent, meaning that for every $1 in additional operating cost the airlines had to absorb they managed to recover 68 cents in extra operating revenue, Heimlich said.

A 34 percent overall increase in the carriers’ jet fuel costs—90 percent of it resulting from higher fuel prices—accounted for 57 percent of the entire operating-expenses hike, according to Heimlich. The benchmark Brent Crude oil spot price climbed to more than $80 per barrel in October, its highest level since October 2014, when the benchmark price was tumbling in the wake of its run-up to about $112 per barrel in June of that year.

The airlines’ operating-cost imbalance produced a $4.1 billion year-over-year drop in their combined operating profit for the nine months to $10.8 billion, reducing their operating margin to 8.2 percent from 12.1 percent in the equivalent 2017 period. While the nine carriers’ passenger revenues in the first nine months of 2018 grew approximately $7 billion, for a 6.5 percent increase, their jet fuel costs also increased by about $7 billion. Labor represented the next fastest-growing cost area during the period.

Heimlich noted that about 75 percent of the airlines’ passenger revenue growth during the first three quarters of 2018 came from a 4.8 percent passenger-traffic increase in terms of revenue passenger miles flown. The remainder came from higher fares and fees or passengers upgrading their bookings to higher fare levels.

The nine carriers’ combined passenger traffic grew 5.6 percent for the first three quarters of 2018, on a 4.4 percent seat-capacity increase. The growth gives “every reason to believe that” the total number of passengers carried by U.S. airlines this year will reach nearly 900 million after totaling 849 million in 2017, according to Heimlich.

Despite the current mismatch between publicly traded U.S. passenger carriers’ operating revenue growth and operating cost growth, inflation-adjusted round-trip domestic U.S. airfares—excluding taxes—have reached their lowest level since 2015, according to A4A. Domestic round-trip fares including ancillary fees in the first half of 2018 averaged $360, compared with $380 in the first six months of 2017. Including taxes and fees, inflation-adjusted domestic round-trip fares are down 11.1 percent on the 2010 level and are lower than at any time since the low point of the last major recession in 2009, the association said.