Boeing and the aerospace supply chain have navigated a costly labor strike with surprising ease. One of Boeing’s key suppliers, however, has run into significant trouble. Tuesday, fuselage supplier Spirit AeroSystems filed its quarterly report with the Securities and Exchange Commission.

Spirit AeroSystems, a key supplier to US planemaker Boeing, warned on Tuesday there was “substantial doubt” that the company would be able to continue as a going concern.

Boeing's (BA) key supplier and acquisition target, has raised substantial doubt about its ability to continue as a going concern amid mounting losses, rapid cash burn and production challenges.  Its debt balance as of September 26 was $4.

The designer and manufacturer for commercial and defense aircraft said its liquidity has been affected in part by the strike at Boeing and limitations on that company’s ability to increase production.

Spirit AeroSystems is warning that its business viability remains in doubt, saying its cash reserves are drying up due to Boeing's sharply reduced production rates and to other factors driving steep losses on its Airbus and Boeing programme work.

"We retain a range of options and levers to address our financial and spacing constraints and are working with our customers on these matters," a Spirit spokesperson said.

Wichita-based Spirit AeroSystems says it will soon end the furloughs of some of its employees and has no immediate plans to implement additional furloughs or layoffs after Boeing workers approved a contract offer on Monday.

A major Boeing (BA) supplier is worried it won’t be able to continue as a business for much longer. Fuselage builder Spirit AeroSystems (SPR) worries it won’t have enough cash to continue operations.

U.S. supplier Spirit AeroSystems said on Tuesday in a quarterly filing that substantial doubt exists about the company's ability to continue as a going concern.