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I was wondering why Boeing didn't use VxWorks from Wind River, a highly reliable RTOS (we used VxWorks in a communications product I managed a few years ago). Avionics magazine published "Product Focus: COTS Operating Systems: Boarding the Boeing 787" on this topic in April 2005:

Real-time operating systems (RTOS) are the foundation software for computing systems. They manage the computers' other software programs and orchestrate these programs' requests for services. And they must be robust enough to deal with unexpected events without causing an aircraft to lose a flight critical function.
In the past, many aerospace companies developed their own, proprietary operating systems and software tools, which were optimized for specific functions. Honeywell, for example, created the digital "engine" operating system (DEOS)...
Collins and LynuxWorks now are partnering to achieve acceptance of the LynxOS-178 kernel and POSIX application programming interface (API) as a reusable software component (RSC) under the guidelines described in advisory circular 20-148, issued by FAA in December 2004. Among other things, the new approach will allow FAA to accept third-party utility software, even though the software is only a part of a larger application, such as a flight management system (FMS) or a flight control system. The third-party RSC developer has to partner with an avionics system developer as part of a technical standard order (TSO), type certificate (TC) or supplemental type certificate (STC) project, but the RSC developer controls the distribution of the RSC approval letter.
Wind River Systems describes a technology approach to RTOS reuse for some military customers. They will be able to move applications between various "platforms," which contain an operating system, development environment and other software. They can move between the "general purpose platform" and the "platform for safety critical" as soon as the latter software platform adopts the development environment used in the general purpose package. This is scheduled to occur later this year.
Today avionics manufacturers still use homegrown RTOS and stripped-down, single-purpose operating systems known as runtime executives. But they are becoming more comfortable with third-party, commercial off-the-shelf (COTS) products, as well.
Recent aircraft provide striking evidence of this trend. The Boeing 787 Dreamliner will use COTS operating systems by Green Hills Software and Wind River Systems in core avionics systems.
Smiths Aerospace chose Wind River Systems' VxWorks 653 RTOS for the B787's common core system (CCS), a cabinet that will host 80 to 100 applications, including Honeywell's FMS and health management software and Collins' crew alerting and display management software. Multiple utility management applications relating to landing gear, electrical power, hydraulics, environmental control and even "lavs and galleys" management also are hosted on the CCS, according to Mike Madden, Smiths' program director for B787 common core system. (CCS also includes the common data network and remote data concentrators.)
The Wind River RTOS is part of the CCS infrastructure software, which also includes the Smiths common operating environment and a certified configuration management tool set. Smiths is integrating the "architecture and configuration tool set," which incorporates software from Smiths, Wind River and Rockwell Collins.
Smiths also plans to use the RTOS on the B767 global tanker transport aircraft's avionics flight management computer, a traditional line replaceable unit with VMEbus cards. The FMS software and the related operating environment will be part of an STC, the RTOS' first FAA, DO-178B acceptance. The operating environment, including the RTOS, will be certified to DO-178B, Level B, but the artifacts "will be developed to Level A for use on future applications," says John Armendarez, Smiths' director of military air transport programs.
Green Hills' Integrity-178B is present on the Dreamliner, as well. Honeywell chose the RTOS for the B787's fly-by-wire flight control electronics. Integrity-178B is to run in the B787's flight control modules, which are distributed among the four flight control electronic cabinets the integrator will supply each 787. Outputs from the software in these flight control modules drive Honeywell actuator control electronics units, which in turn communicate with the actuators that move the control surfaces.
Honeywell chose Green Hills because it provides a DO-178B, Level A, certified time and space partitioned operating system with a tightly coupled development environment, says Don Morrow, Honeywell's director of Boeing business development. Both Green Hills and Wind River have effectively been "certified," Morrow says. "They already have convinced FAA that they have systems which are compliant with ARINC 653." This standard concerns RTOS "partitioning," the services the RTOS supplies to enable the running of multiple applications on the same processing resources.
Honeywell is particularly interested in commercially available tools, such as compilers, linkers and debuggers. The company had to create the operating system and tools for its highly integrated airplane information management system (AIMS) on the B777. "It costs a lot of money, and it's not our core business," Morrow says
Honeywell and Collins have not gotten out of the operating system business completely. Honeywell uses DEOS in the Primus Epic integrated avionics suite and in the flight control system of the Embraer 170 and 190 regional jets.