Final answer:
The DCU reversionary key is associated with an aircraft's Multi-Function Display (MFD), allowing critical flight information to be displayed in a backup mode when the primary DCU function is unavailable.
Step-by-step explanation:
The DCU reversionary key is typically applicable to the aircraft's Multi-Function Display (MFD). In instances where the Display Control Unit (DCU) encounters an issue or if the primary function of the DCU becomes unavailable, the reversionary mode can be engaged. This mode allows for essential flight information to continue to be presented on the MFD, though in a potentially degraded or limited format compared to the full capabilities of the system. In modern aircraft, this functionality is important as it provides a backup means to access critical flight data, thereby enhancing safety.
Typical information that might be available on the MFD in reversionary mode includes but is not limited to, attitude, airspeed, altitude, and heading. The exact features available can depend on the aircraft make, model, and specific configurations of the avionics.