Airline stocks including American Airlines Group Inc (NASDAQ:AAL) and United Airlines Holdings Inc (NASDAQ:UAL) are losing altitude Thursday in sympathy with Delta Air Lines Inc (NYSE:DAL), which fell on second-quarter earnings results.

What Happened With DAL: Delta reported second-quarter revenue of $16.658 billion, beating the consensus estimate of $15.452 billion. The company reported second-quarter adjusted earnings of $2.36 per share, missing estimates of $2.37 per share, according to estimates from Benzinga Pro.

Delta also guided for third-quarter revenue growth of 2% to 4% and adjusted earnings of $1.70 to $2 per share versus estimates of $2.06 per share. The company's guidance fell short of analyst estimates as rising costs and increased capacity weigh on profitability, per CNBC.

Don't Miss: Delta Air Lines Q2 Earnings: EPS Miss, Revenue Beat, Underwhelming Q3 EPS Outlook And More

Why It Matters: American and United shares are trading lower in sympathy as investors fear the pressure on profits will extend across the industry.

American is estimated to report earnings for the second quarter on July 22, although a date has not yet been set by the company. Analysts expect the airline to report earnings of $1.08 per share and revenue of $14.457 billion, according to estimates from Benzinga Pro.

United is due to report second-quarter financial results on July 17. The company is expected to report earnings of $3.99 per share and revenue of $15.14 billion, according to estimates from Benzinga Pro.

Delta, American and United are three of the largest airlines in the world measured by aircraft, capacity and scheduled revenue passenger miles. Delta shares were down 7.5% at last check.

AAL, UAL Price Action: American shares were down 6.41% at $10.45 and United shares were down 6.05% at $44.33 at the time of publication.