There are those who might have thought aerospace stock Boeing (BA) was back on track after months of disasters and setbacks.

SEATTLE—Ihssane Mounir stood before the skeptical and anxious crowd filled with hundreds of manufacturers whose fortunes rise and fall with Boeing BA-2.90%decrease; red down pointing triangle.

A shadow factory is what Boeing executives call a production line where engineers and mechanics work on fixing, maintaining or updating aircraft instead of building new ones. They exist for the ...