MIRAMAR, Fla. — Shares of Spirit Airlines Inc. plummeted Thursday after the company said it lost $45 million because of flight cancellations caused by pilots and gave a discouraging revenue forecast for the rest of summer.

In afternoon trading, the shares were down $9.84, or 20.1 percent, to $39.01.

Spirit said it canceled more than 850 flights in the second quarter during what the company had termed at the time an illegal work slowdown. A federal judge ordered pilots back to work the day after canceled flights led to a melee at the Spirit counter in a Florida airport.

The airline is negotiating with pilots over a new contract, and executives said Thursday that the problems in May should not change the Spirit’s long-term outlook.

In an investor update, however, the airline forecast that revenue for every seat flown one mile would fall by between 2 percent and 4 percent in the July-through-September quarter. That figure is watched closely as an indicator of average fares, and Spirit’s forecast was weaker than those of other U.S. airlines.

Spirit is a so-called ultra-low-cost carrier that charges low fares but imposes more fees than traditional airlines. Spirit has been growing quickly at airports dominated by bigger airlines, which has led those giants to fight back.

Delta, United and American have rolled out so-called basic economy fares that target Spirit’s budget-conscious customers. Without a sharp price advantage, Spirit could risk losing passengers to bigger airlines with more familiar names.

An analyst said that Spirit executives failed Thursday to adequately defend the business model of the ultra-low-cost carriers. Jamie Baker of J.P. Morgan said there is a “growing perception” that the discounters’ model might be inferior when used at hub airports against entrenched and profitable competitors.

Miramar, Florida-based Spirit said second-quarter earnings were $78.1 million, or $1.12 per share, a 7 percent increase from a year earlier.

Adjusted earnings, which exclude non-recurring costs, were $1.14 per share. Nine analysts surveyed by Zacks Investment Research had an average forecast of $1.11 per share.

Revenue rose 20 percent to $701.7 million, just below analysts’ prediction of $702.1 million.

The average Spirit customer paid $59.93 per flight on the ticket and $53.14 on fees.

Costs rose faster than revenue — up 23 percent from a year earlier — with fuel rising 26 percent.

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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 SAVE at https://www.zacks.com/ap/SAVE

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