Jet fuel is now over double what it cost a year ago: Fuel is an airline's single largest operating expense, and it's the one cost they can't fully control. The gap between jet fuel and crude, the jet fuel crack spread, has spiked toward $70 a barrel, more than double the post-Covid average and far above the $14 pre-Covid norm. For decades, jet fuel tracked crude closely, so airlines could manage fuel risk through oil. That link has snapped. Years of shrinking refining capacity, plus tighter rules on middle-distillate fuels since 2020, have left less slack in the system and more exposure to shocks. And what many don't realize is that diesel drives the whole stream. Diesel just hit an all-time record $6.27 a gallon, dragging its refining cousin jet fuel up with it, and crude is back above $100/bbl on the escalating attacks in the Middle East. Airlines' fuel bill is set to jump nearly +40% this year, and industry profits are on track to roughly halve. Airlines are under pressure.