By David Shepardson

WASHINGTON (Reuters) -Delta Air Lines said on Thursday that it has updated the radio altimeters in its in-service airplane fleet to address potential 5G C-Band interference.

Transportation Secretary Pete Buttigieg warned in June that airlines could face operational constraints in bad weather if they had not updated airplanes ahead of a July 1 deadline. Delta had approximately 190 aircraft yet to be outfitted before the July 1 deadline, including all of its Airbus A220 fleet.

Delta said all planes in service have now been updated while a few aircraft are out of service for planned maintenance and will be equipped with 5G-compliant radio altimeters as they return to service. Delta said there was "no notable operational impact between July 1 and this week when the work was completed."

Concerns that 5G service could interfere with airplane altimeters, which give data on a plane's height above the ground and are crucial for bad-weather landing, led to brief disruptions at some U.S. airports last year as international carriers canceled some flights.

Last year, Verizon and AT&T voluntarily agreed to delay some C-Band 5G use until July as air carriers worked to retrofit airplane altimeters.

Reuters first reported in March that major U.S. wireless carriers agreed to some voluntary actions to address aviation safety concerns but also allow them to increase power levels to get to full C-Band use on July 1.

Buttigieg told Reuters in an interview on July 20 that the transition to make airlines 5G-compliant went better than expected, with minimal disruptions.

Buttigieg said that while airlines were largely prepared, the effort "took a lot of pressure." He added: "It took multiple moments where we had to really just make sure they could read our body language that we really were serious ... I don't think the airlines believed us early on."

As of late June, more than 80% of the domestic fleet serving U.S. airports had been updated, Buttigieg wrote to major airlines, but he noted "a significant number of aircraft still awaiting retrofit, including many operated by foreign air carriers. This means on bad-weather, low-visibility days in particular, there could be increased delays and cancellations."

(Reporting by David Shepardson in WashingtonEditing by Leslie Adler and Matthew Lewis)