RETROFITTING AN ADVANCED TECHNOLOGY CONTROL SYSTEM INTO THE COST COMPETITIVE LIGHT HELICOPTER COMMERCIAL MARKET

As part of an on-going effort to maintain the competitive edge of the Allison 250 series turboshaft engines for commercial helicopter applications, Allison Engine Company and Chandler Evans have developed a single channel Full Authority Digital Engine Control (FADEC) system that expands the aircraft ability to perform Category A takeoffs and will reduce cost of ownership and pilot work load for the highly competitive small engine market. Cost-of-ownership improvements are achieved through improved engine protection features, such as "hands-off" starting with an automatic start abort feature, which eliminates the risk of hot section overstress and has the potential for increasing the time between overhaul (TBO). In addition the control includes advanced diagnostics to identify failed control system components to the Line Replaceable Unit (LRU) level, thereby minimzing troubleshooting time and unnecessary removals. The control system itself is made affordable through its low parts count and overall simplicity. For example the required number of sensors is kept to the absolute minimum via judicious use of shared signals between the engines and the airframe. A hydromechanical backup control that minimizes complexity and cost provides a reversionary control channel in the event of a major electronics or multiple-sensor failure. Reductions in pilot workload come about through incorporation of proven modern control features, such as isochronous rotor speed governing, rotor power anticipation, automatic load sharing, automatic starting, and torque, temperature, and N1 limiting. In addition the FADEC provides for immediate detection of a failed engine with automatic enabling on twin engine aircraft of One Engine Inoperative (OEI) power ratings on the remaining powerplant. The control also provides cockpit indications when OEI power is being used. This paper describes the overall system design, the design philosophy, and main capabilities of the control. Results of engine and flight testing performed to date are discussed along with the status of control system qualification.