United said that has tried to minimize the impact of the mid-March grounding by using other spare aircraft. But the peak summer travel season made those changes too difficult, the company said. The airline operates a fleet of 14 Boeing 737 Max 9 aircraft.
"Moving forward, we'll continue to monitor the regulatory process and nimbly make the necessary adjustments to our operation and our schedule to benefit our customers who are traveling this summer," United said.
Boeing has slowed production of the Max and plans to submit a software fix to the Federal Aviation Administration in the next couple weeks. Boeing's CEO said it's completed 96 flights totaling over 159 hours of air time with its new fix.
Investigators and lawmakers have scrutinized the airplane maker's software system malfunction, from the original design to the training and safety certifications.
WATCH: 737 Max not coming back any time soon