Southwest has shipped jet fuel from Texas to California amid a supply crunch

How volatile will fuel markets be this year?

Southwest Airlines The company chartered a ship this spring to send jet fuel from Texas to California, where prices are much higher and concerns about supply have grown, Chief Financial Officer Tom Doxey told CNBC. This was a first for Dallas Airlines.

“It brought about a week’s supply to the West Coast at a time when supplies were very constrained… when it was most vulnerable,” Doxey said.

The ship, which left Houston and passed through the Panama Canal, arrived May 28 in Los Angeles with about 12.6 million gallons on board, Southwest said. For context, Southwest used 564 million gallons of jet fuel last quarter.

The West Coast relies more on imports than other parts of the country. Jet fuel prices have risen and become volatile since the United States and Israel struck Iran in February.

Southwest said Thursday that fuel expenses rose about $900 million in the second quarter from a year ago.

For the shipment to California, the airline said it used a waiver of the Jones Act, a 1920 law that requires shipments between U.S. ports to be transported on a U.S. ship. President Donald Trump waived that requirement in March as fuel prices rose in the weeks after the start of the Iran war and subsequent shipping difficulties erupted in the Strait of Hormuz, a major channel.

Concerns about supplies have increased as countries restrict their exports this year for fear of running out of fuel. A Southwest spokesperson said those concerns have since subsided.

Jet fuel is airlines’ largest expense after labor. Prices fell in the late spring and early summer but rose again with renewed tensions with Iran this month.

last week, United AirlinesThe airline, which flies more internationally than any other U.S. carrier, said it used the latest available fuel prices for its quarterly estimates because prices have been so volatile.

In its July 15 report, it said jet fuel increased $575 million, or $1.12 in adjusted earnings, for the third quarter alone.

US airlines have abandoned fuel hedges, which helped them lock in costs through futures contracts, over the past decade or so, when the US was overwhelmed with supply, capping prices.

This time, airlines have scaled back their plans to increase capacity, which is also helping to boost prices. Airline executives said this month that demand remains strong despite higher prices, which they say is likely to continue.