Due to the 737 Max grounding, the Dallas-based airline said it made the "difficult decision" to delay two new-pilot hire classes and two captain upgrade classes. These classes were scheduled to take place in September, October and December.

"Once we have more clarity on the return-to-service date of the MAX, and future MAX delivery timelines, we will look towards reinstating classes, as needed, to support the expected growth of our fleet," said a Southwest spokesperson.

The spokesperson added the airline plans to hire more than 450 first officers this year.

The issue is Southwest currently has a smaller fleet than anticipated. Not only is the carrier the largest operator of the 737 Max, with 34 in its fleet, it hasn't been able to accept delivery of dozens more 737 Max's that were scheduled for this year.

The Southwest Airlines Pilots Association, which represents the nearly 10,000 Southwest pilots, said the airline has been altering the pilot manning model which created an overmanning situation even before the 737 Max groundings.

"Management’s manning decisions were already decreasing pilot productivity below our historic productivity levels, and then the grounding of the MAX obviously exacerbated the problem greatly," a union spokesperson said, adding that pilot income and schedule flexibility has decreased as a result of the overmanning and 737 Max groundings.