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.

Aircraft-parts supplier Spirit AeroSystems Holdings Inc. on Tuesday said that there was “substantial doubt” about its ability to keep running as a “going concern,” as losses, debt, and difficulties at jet maker Boeing Co.

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.

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.

Spirit AeroSystems reported on Tuesday in a quarterly filing that “substantial doubt” exists about the company’s ability to continue “as a going concern.”

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.

Last month, Spirit Aero said it was burning through dwindling cash reserves, as a strike by Boeing's U.S. factory workers hammered the Wichita, Kansas-based supplier that produces the fuselage for the planemaker’s strongest-selling 737 MAX jet.