Not only does flight control software typically never use garbage collection, it is also preferable to never allocate memory during the runtime -- all memory ever needed is allocated at program startup, and memory is not acquired or released until landing or some other major end event. Because memory issues are often the most significant causes of crashes, which you don't want to translate into, well.. a different kind of crash.
The creator of C++, Bjarne, wrote a nice paper on flight control software if you are interested in more.
