New York CNN Business  — 

Southwest Airlines is prepping for a summer travel season without its Boeing 737 Max planes.

The airline said Thursday that it was extending changes to its flight schedule to account for the grounded aircraft’s absence. Southwest (LUV) last month modified its timetable up to June 7, but will now further tweak it through August 5.

The move aims to “reduce the amount of last-minute flight changes” and bolster schedule reliability, Tom Nealon, the airline’s president, said in a statement.

“While the timing for the return to service of the MAX remains unclear, what is very clear is our commitment to operate a reliable schedule,” he added.

The 737 Max was grounded in March after one of the planes flown by Ethiopian Airlines crashed, killing everyone on board. It was the second accident involving the jet model in less than six months, after another flown by Indonesia’s Lion Air crashed last October. Investigators are still probing both crashes, which killed 346 people in total.

The focus of the crash investigations is the plane’s automatic safety system, for which Boeing says it is testing a software fix. Boeing has halted deliveries of the 737 Max and announced plans to scale back production.

Airlines have started holding off on ordering 737 Max planes amid doubts around its safety. Southwest currently operates 34 of the jets.

Nealon said the airline is informing customers affected by its schedule changes so that it can “reaccommodate their flight plans well in advance of their travel date.”

“While the vast majority of our customers’ itineraries have remained unaffected, flight schedule changes have inconvenienced some of our valued customers, and, for that, I offer my sincerest apologies,” he said.

Chris Isidore contributed to this report.