Testing flight software is very intense. 50 KSLOC of C will take a few dozen man years to develop.
As a side note on terminology, QA means a very different thing in the aerospace industry. The function of QA in this world is to witness the tests and attest that we did indeed run the test as written. Usually in the form of stamping a printed procedure as it is followed. This mirrors the process used to test the mechanical and electrical parts. The testing of the software is called verification and validation testing not QA.
As a side note on terminology, QA means a very different thing in the aerospace industry. The function of QA in this world is to witness the tests and attest that we did indeed run the test as written. Usually in the form of stamping a printed procedure as it is followed. This mirrors the process used to test the mechanical and electrical parts. The testing of the software is called verification and validation testing not QA.