Spirit Airlines (SAVE) revealed a 151% jump in fourth-quarter profit as the discount carrier reported much higher sales and improved load factor.

However, investors seemed disappointed by the results, sending its shares lower Thursday as Spirit revealed a 21% increase in operating expenses, 29% higher fuel costs per gallon and sales that missed expectations. 

The carrier reported net income of $24 million, or 33 cents a share, compared with a year-earlier $9.5 million, or 36 cents. Analysts in a Thomson Reuters poll were looking for a 32-cent profit.

Revenue for the three-month period was $273.9 million, up 26.7% from $57.7 million a year ago, missing the Street’s view of $275 million. Spirit attributed the gains to higher passenger volumes coupled with both stronger ticket and non-ticket revenues.

Total revenue per available seat mile climbed 21% to 11.89 cents, while load factor increased 1.1 points to 85.4%. Average ticket sales per passenger flight segment were up 8.9% to $78.

“Our ultra low cost structure, combined with our customer-empowering optional pricing structure, allows us to offer the low fares that customers have come to expect while delivering strong margins for our stockholders,” said Spirit CEO Ben Baldanza.

Major U.S. airlines have been struggling with higher jet fuel costs, forcing several to cut capacity in 2011. Spirit purchased lighter, more fuel-efficient aircraft from Airbus during the quarter in an effort to soften expenses.

It expects the 30 A320 and 45 A320NEO aircraft to be delivered from 2016 through 2021. 

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