The airlines will require all customers over the age of 2 to wear face coverings on board and at airports, with no exceptions. They say people who don't comply won't be allowed to fly as next week.
One pilot messaged that the problem was "egregious" and he had "basically lied to the regulators (unknowingly)" when he had told the FAA that the flight control system was safe.
The pilots say they stand to lose $100 million in income. Southwest has canceled more than 30,000 flights since the FAA grounded the aircraft in March after two crashes killed 346 people.
The FAA says the issue that affected Southwest, Delta and other carriers has been resolved. But with hundreds of flights affected, the delays could linger.
Some U.S. airlines are scrambling to rebook passengers after the FAA announced it was grounding the Boeing 737 Max fleet. Airlines affected include Southwest, American and United.
The carrier says the Aircraft Mechanics Fraternal Association "has a history of work disruptions." The union says the airline is "scapegoating" its technicians.
Peanuts have long been a part of Southwest's marketing campaigns. The carrier says it is ditching the snack to protect passengers with peanut allergies.
On April 17, an engine that exploded caused the death of one passenger and injured others. A survivor has filed a lawsuit against the carrier, alleging she now suffers from PTSD and other conditions.