There are Unix-like real-time OS'es that do have DO-178B certification. One of these is LynxOS-178 by LynuxWorks. This software is actually licensed from Rockwell-Collins, and uses their Virtual Machine Operating System (VMOS) technology, which has its own DO-178B Level A certification. The LynxOS-178 product was announced in 2002 but apparently VMOS has been around since 1988. They claim to have DO-178B Level C certification for one in-flight application and further claim they are certifiable to Level A, no doubt due to the VMOS licensing.
Green Hills Software also offers a product which is DO-178B Level A certifiable named Integrity-178. But this, too, is not a Linux OS.
Across the ocean, The British Health and Safety Executive issued a study in 2002, "RR 011 - Preliminary assessment of Linux for safety related systems", which concludes that the 2.4 vanilla kernel may be certifiable to their Safety Integrity Levels 1, 2, and 3 but probably not to Level 4. They noted that the OS was being studied for a railroad traffic control application. Linux may not be certified in the UK for their safety systems.
Perhaps some day a Linux OS version will achieve DO-178B certification at some level, or a European certification at an equivalent level, but it is a very costly effort. The documentation requirements from the FAA and it's functionally equivalent agencies in other countries for such certification covers a lot of ground and a vendor's documentation and claims would be independently checked. Someone will have to pay for that plus the various other expenses.
