Improvements in the Backside Approach-and-Landing Flight Control System of a Large STOL Aircraft

This paper describes recent updates to the Flight Path Control System of the C-17A Globemaster III transport aircraft as a STOL aircraft utilizing externally blown flaps during the backside approach-and-landing task. The Flight Control upgrades, which were in response to field incidents as well as flight testing in challenging high wind conditions, were designed to address multiple objectives. The primary objective of the changes was to reduce the risk of hard or short landings by providing increased quick control authority of flight path in the up-direction to slow the occasional excessively rapid descent of the aircraft more effectively toward the ground. A significant motivation for this objective was provided by a hard and short landing field incident resulting in substantial aircraft damage that was caused in part by a lack of time to arrest the sink rate. Additionally, hard landing annunciations exceeding aircraft structural design sink rate thresholds, which require aircraft inspections, occur too frequently in the field. An important secondary objective was to improve the control accuracy part of handling qualities to facilitate a reduction in the occurrences of critical approach-and-landing control scenarios. In addition to describing high-level innovative design features affecting the flight path control system, this paper quantifies the project’s flight test results of the final control law function in order to verify and validate the flight control system updates. The new flight control laws for the C-17A Flight Path Control System were part of the C-17A continuous delivery model (CDM) robust flight path control (RFPC) program and C-17A fiscal year 2015 (FY15), maintenance build (MX), software operational flight program (OFP). The Flight Test programs, intended to evaluate the RFPC software, were conducted by the Air Force Test Center at EAFB in California. The resulting data showed marked improvements over the Legacy software for approach-and-landing flight path control, and the evaluation pilots deemed the handling qualities as satisfactory. The handling qualities for the complete backside approach-and-landing flight path and aimpoint control task, up-direction flight path control effectiveness from off-nominal engine thrust, and human systems integration, were all deemed satisfactory by the evaluation pilots.