(Reuters) - American Airlines AAL.O shares fell as much as 11 percent on Thursday after the biggest U.S. carrier cut its estimate for a closely watched measure of performance, raising concerns that slowing global economic growth is hurting business travel.

FILE PHOTO: An American Airlines plane takes off from Los Angeles International airport (LAX) in Los Angeles, California, U.S. March 28, 2018. REUTERS/Mike Blake

The losses in American’s shares also weighed on the broader airlines sector as slowdown fears were exacerbated by lower fuel prices, which typically lead to competitive discounting.

Shares of Delta Air Lines Inc DAL.N fell as much as 5.5 percent, United Continental Holdings Inc UAL.N 7.6 percent and Southwest Airlines Co LUV.N 4.4 percent by midday.

“Legacies in particular have a clear risk ahead on unit revenues, and that includes American Airlines with everything from corporate, international to shutdown exposure,” Morgan Stanley analyst Rajeev Lalwani said.

“It’s happening a little earlier than anticipated per the American and Delta unit revenue guide downs in the fourth quarter, which is concerning.”

Delta Air Lines Inc DAL.N cut its quarterly unit revenue estimate last week due to lower-than-expected improvement in fares booked last minute.

A decline in fuel prices in 2016 and 2017 led U.S. airlines to discount ticket prices, hurting revenue. Investors have been concerned that the cycle may repeat itself if oil prices do not show an improvement, resulting in depressed fares.

American said it expects its fuel expense to reduce in the fourth quarter to between $2.22 and $2.27 per gallon, compared with $2.30 to $2.35 per gallon earlier.

The company now expects unit revenue, which compares sales to flight capacity, to increase about 1.5 percent in the quarter, compared with its earlier estimate of 1.5 percent to 3.5 percent rise. (bit.ly/2D1GRZi)

“We believe a change in messaging is needed at American. The company cannot continue to come out with a bullish message and then underperform all year,” Cowen & Co analyst Helane Becker said.

American also lowered full-year earnings per share expectation to a range of $4.40 to $4.60, from $4.50 to $5 earlier.

Analysts were expecting American to earn $4.62 per share in 2018, according to IBES data from Refinitiv.

American's shares have underperformed the broader airline sector - down 36 percent in the past 12 months compared with a 16.4 percent decline in the S&P 500 Airlines index .SPLRCAIR.