One of the unique challenges of working in the aerospace domain is the need to maintain onboard software for the long lifetimes of many aircraft. For both civil and military aircraft, software must often remain in the field for decades, during which time maintenance and updates may be needed to meet ever increasing demands. 

When such software is certified to DO-178 (DO-178A, B or DO-178C), MIL-HDBK (MIL-HDBK-516A, B or MIL-HDBKC) or related guidelines, this means verification environments must also be available for long periods of time.  

This can be particularly challenging when production and test equipment becomes harder to source and maintain. The Eurofighter Typhoon, for example, was developed during the 1980s and 90s. 

As decades have passed since the Typhoon’s launch in the early 2000s, the test rigs used for verification are now prone to breaking because of their age, and it’s becoming harder to source the MVME 135 & MVME 136 (Motorola 68020) microprocessors that are used in the aircraft’s flight control and radar systems. 

One of our customers was looking for an alternative option to maintaining test rigs for verifying software running on the 68020 to reduce verification costs and derisk verification. From that need, Rapita’s Sim68020 was born.

Simulation and emulation with Sim68020

The Sim68020 simulates the behavior of the Motorola 68K series of microprocessors, supporting execution, debugging, and verification of programs for 68K processors without having to use the real hardware. 

The Sim68020 produces branch traces that can be used to verify programs developed for the 68K processor for structural coverage analysis, execution time analysis and task-level scheduling analysis using zero-footprint RVS tools. 

While simulating the Motorola 68020 processor is a powerful first step, avionics systems are rarely that simple. Real-world embedded systems rely on a complex web of memory-mapped devices, custom ASICs, and communication interfaces. That’s why Sim68020 goes beyond just CPU simulation—it supports full-system emulation through a flexible and extensible plugin architecture.

Sim68020 also supports a virtual RTBx interface, allowing seamless migration to on-target testing when needed. This means the same RVS integration can be used both in simulation and on real hardware, reducing duplication of effort and accelerating verification timelines.

Sim68020 Demonstrates Autopilot Control in Microsoft Flight Simulator

To demonstrate the full-system emulation capabilities of Sim68020, we integrated it with Microsoft Flight Simulator and ran a custom autopilot demo. The software includes basic waypoint navigation and a Ground Collision Avoidance System (GCAS).

Through a custom plugin, Sim68020 connects to Microsoft Flight Simulator via a shared memory interface. This allows the emulated software to:

• Read real-time flight data such as altitude, airspeed, pitch, roll, and heading from the virtual aircraft.
• Send control commands to manipulate the aircraft’s ailerons, elevators, rudder, and throttle—enabling full control over flight dynamics and engine power.

The demo autopilot software uses this interface to follow a predefined set of waypoints, adjusting course and altitude as needed. The GCAS logic monitors terrain elevation and aircraft descent rate, issuing corrective commands to avoid potential collisions with the ground.

This setup enables closed-loop testing of control logic in a realistic, high-fidelity environment. Developers can observe how the 68020-targeted software responds to simulated weather, turbulence, and terrain, all without needing physical avionics hardware.

By combining Sim68020 with Microsoft Flight Simulator, we’ve created a powerful platform for demonstrating, testing, and validating embedded flight software—bridging the gap between legacy systems and modern simulation tools.

Conclusion

We developed the Sim68020 to meet a specific need for our customer, and it offers a much more cost-effective alternative to maintaining expensive test rigs, especially as it’s becoming much harder to source 68020 microprocessors.

Do you have a similar verification need for your avionics project? Let us know how we can help today.