(NewsNation) —  Fatigue, both acute and cumulative, has become the most critical safety threat for Southwest Airlines, or so says the Southwest Airlines Pilots Association.

The group sent an open letter to Southwest executives last week highlighting safety concerns. SWAPA reports that fatigue numbers have climbed exponentially since last summer “with no meaningful attempts by management to mitigate them.”

They say fatigue rates among airline staff increased by 350% in August and September with more than 600% in October. April is reportedly already setting new fatigue records.

According to Southwest, since the airline implemented schedule revisions in November, they have seen a decline in the number of pilots calling in fatigued.

“We did experience an increase in monthly averages for March with approximately 60 Pilots calling in fatigued per 10,000 duty periods. The increase is expected, as it’s common to experience an elevated level of fatigue calls during irregular operations and in March, the industry faced weather and airspace delays that resulted in disruptions across the network,” a Southwest spokesperson told NewsNation.

SWAPA says some Southwest pilots have reported being unable to get hotel rooms for proper rest following flight reassignments or delays. Last year, the group says they told Southwest’s VP of Flight Operations about more than 100 cases where pilots were not provided with their federally mandated minimum rest opportunities. They claim there has been no corrective action taken to date.

Where do the high reports of fatigue stem from? The union believes it’s Southwest’s scheduling processes “that have now escalated out of control,” sparking record high reassignment rates for pilots.

“Reassignment rates have remained at record highs for months on end,” the SWAPA letter to Southwest executives reads. “During April 1-3, more than half our Pilots didn’t fly their originally assigned schedule each day. Reassignment rates this year are routinely in the range of 30% to 50% after climbing as high as 85% late last year. Since last summer, our Pilots have lost more than 18,000 days off when the Company forced them to work on a day when they weren’t previously scheduled.”

For its part, Southwest says safety is its top priority, and promises to conintue monitoring its procedures and internal controls.

“Reports connected to Crew Fatigue are showing positive trends with our rates returning to favorable 2019 levels. Southwest has a Culture of Safety that proactively manages risks and encourages all Employees to report any concerns,” a Southwest representative told NewsNation in a statement. “We, along with the industry, continue our progress toward returning our operation back to pre-pandemic service and staffing levels, which will foster improvements in our ability to handle unexpected operational disruptions, minimizing the impact on Employees and Customers.”

The SWAPA letter on behalf of pilots comes a little over a month after Delta Air Lines pilots picketed at Atlanta’s airport to protest fatiguing schedules.

“We’ve flown record amounts of overtime during the pandemic to help Delta operate its schedule and get our passengers safely to their destinations. In many cases, pilots are flying long after their day or trip was supposed to end. Delta cannot continue to operate the schedule at redline with no room for error,” Capt. Jason Ambrosi, chairman of the Delta Master Executive Council of the Air Line Pilots Association, Int’l., said last month.

Negative impacts of fatigue on pilots, per SWAPA, may include impaired judgment, lack of concentration, reduced in-flight attention, decreased reaction time and slower hand-eye coordination.

SWAPA says immediate action to address Southwest’s apparent scheduling issues is imperative.