The International Air Transport Association, which represents more than 100 carriers that fly to the US, said Tuesday that “supply chain issues make it unlikely that all aircraft can be upgraded by the 1 July deadline, threatening operational disruptions during the peak northern summer travel season.”

Buttigieg told airlines on a call the department does not plan to extend the deadline and urged them to work aggressively to continue retrofitting aeroplanes.

Concerns that 5G service could interfere with aeroplane altimeters, which measure a plane’s height above the ground and are crucial for bad-weather landing, led to brief disruptions at some US airports last year as international carriers cancelled some flights.

Last year, Verizon and AT&T voluntarily agreed to delay some C-Band 5G usage until July 1 as air carriers worked to retrofit aeroplane altimeters.

On March 31, four major US wireless carriers told the Federal Communications Commissions (FCC) they had agreed to some voluntary actions to address aviation safety concerns and allow full use of the C-Band wireless spectrum for 5G use.

The agreement with Verizon, AT&T, T-Mobile US and UScellular followed extensive discussions with the Federal Aviation Administration (FAA), allowing carriers to increase power levels to get to full C-Band use by July 1.

Separately, the FAA on Tuesday proposed seven airworthiness directives (ADs) for many Boeing aircraft due to the potential for 5G C-band interference.

The proposed directives impact 4,800 US registered aeroplanes and 14,600 worldwide. They require revising aircraft flight manuals by June 30 to prohibit some landings and include specific operating procedures for calculating landing distances and certain approaches when in the presence of 5G C-band interference.

Boeing said Tuesday it “continues to work with suppliers, regulators, the airlines and telecom companies to ensure long-term stability and help mitigate operational restrictions where possible.”