DALLAS (AP) — Delta Air Lines reported that fourth-quarter profit slipped 8 percent, but the results still beat expectations as average prices for passengers continued to creep higher.

The airline also raised its forecast of 2018 earnings, citing the recent tax law that includes a lower corporate rate.

The shares rose in trading ahead of Thursday’s opening bell.

Airlines are benefiting from strong travel demand that is resulting in many full flights and higher prices. At Delta, average fares and fees rose for the third straight quarter, reversing a two-year slide caused by fare wars and a glut of seats.

Delta’s president, Glen Hauenstein, said the airline goes into 2018 with momentum from higher revenue because of strong demand and rising business fares.

The company expects that trend to continue. It predicted that revenue for each seat flown one mile, a figure that investors watch closely, will rise by 2.5 percent to 4.5 percent in the first quarter — positive, but slightly slower growth than in the fourth quarter.

The Atlanta-based airline said it expects 2018 earnings of between $6.35 and $6.70 per share, citing benefits from the tax law. Analysts surveyed by FactSet were expecting $5.92 per share. Delta said rising revenue and the new tax law will partly offset higher jet fuel prices.

Delta, the second-largest U.S. airline behind American, plans to boost passenger-carrying capacity by 3 percent in the first quarter compared with the same period in 2017. That kind of growth is likely to be modest enough to avoid spooking investors, who worry that airlines are dumping too many seats on the market. Delta just ordered 100 new Airbus jets for its 999-plane fleet, but they don’t start arriving until 2020.

Delta said it earned $572 million in the fourth quarter, down 8 percent from $622 million a year earlier. The airline took a $60 million hit in income from a power outage that shut down the Atlanta airport, its biggest hub, on Dec. 17 just as the Christmas travel rush was beginning.

Excluding what the airline considers one-time gains and expenses, the profit was 96 cents per share. That topped the forecast of 88 cents per share that analysts expected, according to a survey by Zacks Investment Research.

Revenue rose 8 percent to $10.25 billion, also topping forecasts.

In premarket trading, Delta’s shares were up $2.03, or 3.6 percent, to $57.90. Airline stocks rallied the day before after American and United both hinted that revenue for the fourth quarter would be stronger than had been expected.

Shares of Delta Air Lines Inc. have fallen slightly since the beginning of the year, while the Standard & Poor’s 500 index has risen roughly 3 percent. Delta has climbed almost 10 percent in the last 12 months.

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Elements of this story were generated by Automated Insights (http://automatedinsights.com/ap) using data from Zacks Investment Research. Access a Zacks stock report on DAL at https://www.zacks.com/ap/DAL