A Southwest Airlines Boeing 737-7H4 jet rolls for the gate after landing at Midway International Airport in Chicago, Illinois on April 6, 2021.

Kamil Krzaczynski | AFP | Getty Images

Southwest Airlines apologized to staff Friday after its unions complained about grueling flight schedules this summer, and said they would consider flight schedule changes for later this year.

The airline has added more flights to its schedule compared to some of its competitors to take advantage of increasing travel demand.

The increase in bookings was a welcome change compared to “a year ago, but we have to be honest with ourselves: It also put a strain on our operations and put a heavy burden on you all. And I’m really sorry,” wrote COO Mike Van de Ven in an employee notice viewed by CNBC.

Van de Ven said the airline is continuing to evaluate our fourth quarter flight schedules and we are seriously reviewing flight altitudes to ensure our flying matches the personnel required to operate in this more complicated COVID environment.

The Southwest Pilots Union said Thursday it is considering picket lines at U.S. airports over Thanksgiving and Christmas to protest over crowded schedules, shortages of hotels and dining options, and other issues. Flight attendants raised similar issues this week.

“We will not be following our traditional staffing models as we reassess our current situation in October, November and December,” he wrote. “You come first.

Southwest declined to comment further. The airline said earlier this week that it was struggling to hire new staff and offered roughly $ 300 worth of employee referral bonuses.

Earlier this month, the company lowered its revenue forecast, blaming the rapidly spreading Covid-19 Delta variant for a decline in bookings and an increase in cancellations.

This week, Spirit Airlines announced that it would cut part of its flight schedule for the remainder of the third quarter after more than 2,800 cancellations cost the company around $ 50 million.