Southwest shares tumble after airline warns of persistently low fares

Investors are fretting over Southwest Airlines' warning that it's struggling with persistently low airfares. The no-frills carrier said Wednesday that it expects revenue for each seat it flies a mile, a key industry gauge of how much airlines can charge for a seat, to be flat compared with the first three months of 2017. The airline had previously expected a revenue increase of 1 to 2%. Shares of the airline lost close to 5% to end at $57.78. Revenue trends outside peak demand periods, such as holidays, "have been below the company's previous expectations primarily due to the competitive fare environment that continues to pressure passenger revenue yields," Southwest said in a filing. It also blamed a "sub-optimal" Q1 flight schedule on the retirement of its Boeing 737-300 fleet, which it is replacing with Boeing's new 737 MAX line, a more fuel-efficient model. Southwest said it still expects to grow revenue per seat mile in 2018. <br/>