A letter from the national union representing 61,000 pilots has sent a letter directly to President Donald Trump warning him that the government shutdown is threatening the “safety and security of airspace.”

“The nation’s airspace system is a complex transportation network that involves government and industry partnerships to function properly, and the disruptions being caused by the shutdown are threatening the safe operations of this network,” stated the letter sent last Wednesday by Joe DePete, president of the Air Line Pilots Association, on behalf of union members.

The letter has surfaced just as passengers are reporting growing lines at airports and as Transportation Security Administration officers are reportedly calling in sick to work at paying jobs. Forbes reported Monday that some checkpoint waits at New York’s La Guardia Airport and Sea-Tac Airport in Washington state were as long as 90 minutes. The shutdown is also affecting staffing levels in air traffic control towers, according to Forbes.

DePete urged Trump in his letter to “take the necessary steps to immediately end the shutdown of government agencies that is adversely affecting the safety, security and efficiency of our national airspace system.”

The union president noted the importance of the Department of Homeland Security and the Transportation Department in safe flying.

“When any of their responsibilities are placed on pause due to a shutdown there are safety, security and efficiency gaps that immediately emerge,” DePete wrote.

The partial shutdown affects some 800,000 federal workers, but about half of those are considered essential workers — including agents of TSA, which is part of DHS. They are expected to perform their jobs without pay during the shutdown. That has driven some TSA workers to use sick time to work elsewhere to pay the bills, according to the union.

TSA officials have conceded that the number of TSA workers on the job was down, but said Friday that the reduction hadn’t yet affected passenger checkpoint wait times. That appears to be changing as air travel begins to pick up again after the traditional slow time after the winter holidays.

This TSA line at SeaTac is out of this world. I’ve NEVER been in a line this long before. — mika (@MikaaSande) January 6, 2019

DePete also noted in his letter that fewer Federal Aviation Administration safety inspectors are working — “fewer than are needed in order to ensure the air traffic control infrastructure is performing at its peak levels.”

The shutdown also impacts aircraft manufacturing oversight activities that have either stopped or are “significantly reduced,” DePete wrote. “These safety and oversight inspections will potentially allow for the introduction of safety issues that put passengers and airline crews at risk.”

The shutdown began after Trump refused to sign any legislation that doesn’t include $5.7 billion for his wall on the U.S.-Mexico border. The shutdown is now in its third week.

The White House did not immediately respond to a request for comment about the pilots union letter.