For the fourth month in a row, the U.S. airline industry is adding jobs.

The Associated Press quotes the U.S. Transportation Department that said the number of employees on staff at domestic airlines airlines is up almost 1 percent from the same time a year ago.

Delta, the top carrier at Minneapolis-St Paul International, added the most jobs, followed by US Airways, American and JetBlue. Two smaller carriers, Spirit Airlines and Allegiant Air, grew fastest in percentage terms, both posting double-digit gains. Regional airlines that fly smaller planes for the big airlines also added jobs; SkyWest, Republic and Mesa saw their work forces expand.

Bloomberg reported it's a significant turnaround for the industry, which has been troubled by bankruptcies, mergers and soaring jet-fuel prices in the last decade. Five years ago, European and Asian companies including Singapore Airlines and Lufthansa dominated the field, but now US airlines are now the world's most profitable carriers. According to data compiled by Bloomberg, Delta now ranks first in operating income and market value and led the world in 2013 operating income at $3.8 billion, a 34-fold gain from 2008.

Delta, American, United and Southwest, the top domestic carriers, returned to profitability by cutting flights, raising fares and charging fees for extras that were once provided as a courtesy to passengers.

Investors of airline stocks are being rewarded. The Bloomberg US Airlines Index more than quadrupled in the past five years, while a gauge of global carriers gained 60 per cent.

"Management's getting it figured out," said David Dotzenroth, a former commercial pilot now on the investment team at Wayzata Investment Partners, until recently one of the biggest shareholders for Atlanta-based Delta.