Asymmetric TACS Failure With Centering Strips Installed
Tamarack SB1475 adds centering strips to the Tamarack Active Camber Surface (TACS) of ATLAS.
The TACS were initially designed to be aerodynamically over-balanced to provide low TACS hinge moments. In the event of an ATLAS fault, the system removes electrical power from the TCU and actuator. The actuators are readily back-driven when unpowered, so it was possible for the TACS to aerodynamically move to a deployed position, including asymmetric deployments. This has occurred in flight, producing undesirable roll inputs.
Certification flight testing included tests of failure conditions, such as single asymmetric deployments and dual (adverse) asymmetric TACS deployments. These failure conditions were shown to be controllable. However, in the interest of continual product improvement, the centering strips have been designed to mitigate the aerodynamic over-balance of the TACS and reduce the severity of TCU failures. This was approved by Cranfield Aerospace Solutions as Minor Mod 1475.
Flight testing was conducted to verify the performance of the TACS centering strips. Testing included a sample of simulated ATLAS failures to check the TACS centering strip performance in actual flight conditions. A proprietary ATLAS manual control module was installed to override normal ATLAS function and introduce simulated ATLAS failures (asymmetric TACS deployments) followed by simulated ATLAS faults.
During initial certification flight test in 2015, the dual asymmetric failures were simulated in level, high speed flight with autopilot ON. Those simulated failures produced a significant roll which caused the autopilot to disengage immediately. The rolling tendency could be readily corrected by implementing the published recovery procedures following a three second delay to simulate pilot recognition of the failure.
Flight testing with the centering strip modification included simulated dual asymmetric failures in level flight, at high speed, with autopilot ON. This video was taken during a test point simulating a dual asymmetric failure condition with the TACS centering strips installed. The video clearly shows that this failure condition with centering strips is not severe. For example, the autopilot remains engaged, and returns the airplane to level flight.
While no formal credit was sought for this improvement in terms of the system safety assessment, it is valuable to acknowledge the improvement which will reduce the severity of in-flight failures when SB1475 is implemented.
As of May 10, 2019 SB1475 has been delivered to ~95% of ATLAS customers worldwide. Centering strips are included in all new ATLAS installations.