In another example of the far-reaching effects of the coronavirus outbreak, the FAA today said that it is temporarily closing the tower at Midway Airport, the second largest airport in the Chicago area. In a statement, the FAA says that that several technicians at the facility tested positive for COVID-19. For a while, there was a ground stop in effect for Midway and no flights could take off to land at the airport.

In its statement, FAA notes that the airport remains open and that operations will continue at a reduced rate.

“The air traffic system is a resilient system with multiple backups in place. This shift is a regular execution of a longstanding contingency plan to ensure continued operations. Each facility across the country has a similar plan that has been updated and tested in recent years,” the FAA says in a statement. How exactly the airport will operate without a working control tower is unclear, though. Smaller airports often revert to being uncontrolled fields after the tower closes at night and have procedures in place for this. That’s not typically the case for large airports like Midway, though, which are staffed 24/7.

Update (4:55pm PT): the airport has now clarified how it will keep the tower staffed.

The largest airline at Midway is Southwest Airlines, though Delta Air Lines also flies there.

In a separate statement, the National Air Traffic Controllers Association, the labor union for aviation safety professionals, asks that all who work at the tower should be tested for the disease. “NATCA is adamant that the testing must occur immediately and is hopeful there will be no additional COVID-19 infections. We are encouraged that the FAA is pursuing the rapid testing of all personnel at [Midway] Tower,” the organization writes.