AVIONICS
AN AVIONICS SYSTEM OF EPIC PROPORTIONS
PART TWO OF TWO
Welcome back to the second part of my series on “An Avionics System of EPIC Proportions.” Although the
Honeywell Primus EPIC system is found on many fixed-wing aircraft, to date, the only helicopter platform it is installed on is the AW-139. As discussed in our February/March issue, the Primus EPIC system on the AW-139 employs a distributive architecture. Part one was an introduction to that architecture and a discussion on the system’s major LRUs. This second part on the Primus EPIC system discusses the major subsystems of the system. All photos and diagrams shown are courtesy of AgustaWestland. The key subsystems are: • Automatic flight control system (AFCS)
• Communications • Indicating and recording • Navigation
• Flight management system (FMS) • Attitude heading reference system (AHRS) • Weather radar • Radio altimeter
• Central maintenance system
Automatic Flight Control System (AFCS) The AFCS installed in the AW-139 can have an autopilot, yaw damper and flight director. In fact, this is a dual system in that there are two autopilots, two yaw dampers and one flight director. A second flight director is an available option. Under normal flight operations, both autopilots and yaw
dampers are active at the same time. Honeywell refers to this as an “active-active” system. Both autopilots and yaw dampers drive their own actuators to provide complete control throughout the helicopter’s flight envelope. A single fault/ failure of any LRU/module will cause the dual system to transfer complete control to the side that does not have the fault/failure and continue to operate. All flight director (FD) modes remain engaged as does the autopilot and yaw damper. The ability to detect and annunciate a failure, transfer operation from both systems to the remaining good system, keep the flight director modes engaged and the remaining autopilot active is called “fail- operational.” The AFCS software is housed in modules located in the modular avionics units (MAUs.)
R. Fred Polak
AFCS Configurations The AFCS for the AW-139 is available to the end user in three configurations: Basic three-axis system – This provides basic autopilot and stability augmentation system (SAS) functions. It also provides dual pitch, roll, yaw and trim control functions. No flight director is provided. This configuration is suitable for visual flight rules (VFR) operations. Three-axis system with a two-cue flight director – This configuration provides all of the autopilot and yaw damper functions listed above, along with pitch and roll flight director modes that can be coupled to the autopilot. This configuration is suitable for instrument flight rules (IFR) operations. Four-axis system with a three-cue flight director – This is the full up configuration that provides all the functions of the three-axis system but adds a servoed collective drive (the fourth axis). It also adds a third cue for the servoed collective. With the installation of the Honeywell flight management system (FMS), the flight director now has the additional capability of flying lateral navigation (LNAV) and vertical navigation (VNAV) commands from the FMS.
AFCS Controllers The AFCS in the AW-139 installation utilizes the following controllers: • Autopilot controller (PC-770)
• Remote instrument controller (RI-553) • Display controller (DC-840) • Guidance controller (GC-810)
Autopilot Controller (PC-770) The single autopilot controller allows the flightcrew to select and control both autopilot systems. The PC-770 enables the flightcrew to select which of the two systems is the “MASTER” system, perform the required pre-flight system test, and couple the FD to the AP. It also allows the flightcrew to select the SAS mode or attitude hold (ATT) mode of operation.
Remote Instrument
PC-770
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HelicopterMaintenanceMagazine.com April | May 2015
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