AFAIK this group at NASA doesn't write any lines of code at all. The Shuttle runs 4 dSpace ControlDesk Realtime controllers (similar to a F1 car), and the programming environment is graphical. You design models, you don't write code. A compiler writes the code automatically from this model. The automotive industry works similar. You design the engine, the gearbox, the drivetrain with toolboxes and connect them together. Lots of tools exist for quality control, correctness control and performance control. The error rate is significantly lower.
For some edge case they do write C code, maybe about 5%