Shadow
Shadow is a feature of FlightGear. You can enable it in the Rendering Options dialog.
Shadow mapping
Shadow mapping is a technique that use a texture where distance of objects from the light has been recorded. During rendering, the distance to the light of the fragment rendered is compared to the shortest distance of the scene to the light, and if it is higher, the fragment is drawn using only ambient light. Otherwise, it is fully lit.
History
The old shadow feature became deprecated as of 1.9.0 and it was re-implemented during the shift to OSG. At some point scenery object shadows was reintroduced, probably when scenery objects like for example vegetations and buildings became rendered instantiated rather then as separate objects. Aircraft shadows were reintroduced in FlightGear 2024.1.