As an airliner prepares to land, a bird takes off at the Gravelly Point park that's just off the end of the runway near Reagan National Airport.

Airlines can't get federal officials to sign off on new planes and routes amid the U.S. government shutdown, while keeping federal aviation workers furloughed or unpaid altogether threatens the country's key aviation sector, industry members said Thursday.

Some 800,000 government workers have been furloughed or are working without pay since the partial government shutdown began on Dec. 22. About 420,000 of them, including Transportation Security Administration screeners and air traffic controllers, have been deemed essential and are required to work. TSA officers are set to miss their first paycheck on Friday.

"As the partial government shutdown continues, the human and economic consequences are increasing and doing greater harm," aviation industry members wrote in a letter to President Donald Trump, House Speaker Nancy Pelosi and Senate Majority Leader Mitch McConnell.

They said the shutdown is impacting a wide swath of the sector and could hurt federal agencies' ability to hire and retain key aviation employees such as customs and TSA screeners and federal testing of new mechanics.

Airlines need federal safety inspectors to debut new aircraft. For example, Delta Air Lines scheduled a launch Jan. 31 of its brand-new Airbus A220, a plane it's using to court business travelers with bigger seats and windows. It needs to fly with FAA safety inspectors before the aircraft can be introduced to the public. Additionally, Southwest Airlines is awaiting government approval to begin service to Hawaii.

"This partial shutdown has already inflicted real damage to our nation's aviation system and the impacts will only worsen over time," said the letter, signed by unions representing pilots, flight attendants and mechanics, and a trade group that represents American Airlines, United Airlines and FedEx, among other large carriers.

Investors are starting to take note. Jamie Baker, airline analyst at J.P. Morgan Chase, said even if the government reopens soon, it could ding first-quarter airline revenues and said "idled workers are increasingly unlikely to plan lavish summer vacations."