YC-14

The Air Force Boeing YC-14 Short Take-Off and Landing (STOL) jet transport technology demonstrator flew for the first time on August 9, 1976, from Boeing Field in Seattle, WA, during the period between Phase I and Phase II of the NASA DFBW F-8 program. Two proto­types were built with the second aircraft flying in October 1976. The YC-14 is noteworthy in that it was the first aircraft to fly with a fault – tolerant multichannel redundant digital fly-by-wire flight control system.

A mechanical backup flight control capability was retained. The full authority triply redundant digital fly-by-wire flight control system, designed by the British Marconi Company, performed computational commands for pitch, roll, and yaw that were used to control the eleva­tor, aileron, and rudder actuation systems. The reconfigurable computer architecture divided the basic control path into three subfunctional ele­ments with these elements replicated to provide fault tolerance. The inter­nal element redundancy management function was intended to detect and isolate faulty elements and perform the necessary reconfiguration. The input signal selection methodology was intended to guarantee that all three computers used the same numbers and thus produced identi­cal output values. During normal operation, the overall system output value was selected as the midvalue of the three individual values. The system would continue to operate in the event of a failure of one com­puter by taking the average of the output of the two remaining comput­ers. If they disagreed, both were disabled and the aircraft reverted to the backup manual control system.[1179]

The YC-14 was also noteworthy in that it used optical data links to exchange data between the triply redundant computers. The optical com­munications medium was chosen to eliminate electromagnetic inter­ference effects, electrical grounding loop problems, and the potential propagation of electrical malfunctions between channels. Optical cou­pling was used to maintain interchannel integrity. Each sensor’s out­put was coupled to the other channels so that each computer had data from each of the other sensors. Identical algorithms in each computer were used. They consolidated the data, enabling equalization and fault detection/isolation of the inputs. The computers were synchronized to avoid sampling time differences and to assure that all computers were receiving identical data inputs.[1180]

An important observation involving redundant computer-controlled fly-by-wire flight control systems was derived from the YC-14 flight-test experience. As noted above, the system was designed to ensure that all computers used the same sensor input values and should therefore produce identical outputs. However, a significant fault in the digital
flight control software was encountered during flight-testing that had not been detected during ground laboratory testing. The software fault resulted in incorrect tracking of control law computations in each of the three flight control channels, with each channel performing signal selections on a different set of values. This resulted in different input data for the three channels. Although the discrepancies between each channel’s inputs were small, the cumulative effect led to large tracking errors between flight control channels when airborne.[1181]

Following cancellation of the Air Force YC-14A program in 1979, the two prototypes were placed in storage at Air Force’s Aerospace Maintenance and Regeneration Group (AMARC) at Davis-Monthan AFB, AZ, in April 1980. The first prototype is now displayed at the Pima Air and Space Museum in Tucson, AZ.