Aerial Refueling (AR) receiver capability for the E-2D is a developmental effort by the Navy and Northrop Grumman Corporation (NGC) to provide persistent Airborne Early Warning (AEW) and Command and Control for the Navy's carrier strike groups. The E-2D update results in numerous system and structural modifications. During AR risk reduction flight tests in 2011, the highly-coupled pitch and yaw response to power was highlighted as a risk to successfully executing AR. With a 1960s-vintage, cable-and-pulley mechanical flight control system, coupled with a limited-authority Automatic Flight Control System (AFCS), the high workload during AR was not a surprise. It was apparent that an additional stability augmentation mode would be needed to aid the pilot during AR; however, with the existing Stability Augmentation System limited to a simple yaw damping/roll coordination mode, it requires a more comprehensive rewrite of the existing augmentation control laws.The E-2 Integrated Test Team (ITT) set out to determine how to get an aircraft with a 1960s-era flight control system and level II handling qualities to reliably conduct day/night AR-a task the aircraft was never designed to do. In the late 1990s the E-2/C-2 Airborne Tactical Data System Program Office (PMA-231) implemented a digital Flight Control Computer (FCC) in the E-2C for the first time, replacing the legacy analog computer. This 1990s technology forms the foundation of the E-2D's Auxiliary Stability Augmentation System (AUX SAS) flight control mode. By combining the capability of the digital FCC with some creative engineering, the AFCS functionality was modified to create the first generation of the AUX SAS. The first generation AUX SAS consisted of a 3-axis stability augmentation mode that adjusted pitch, roll, and yaw damper algorithms. First flight tested in 2013, the first generation AUX SAS showed marked improvement in handling qualities over the unaugmented aircraft, especially in the longitudinal axis; however, handling qualities deficiencies remained. The control input to aircraft response proved to be almost 160 degrees out of phase, causing the pilot to easily excite a longitudinal pilot induced oscillation (PIO), especially during high-gain control input maneuvers like AR. With the fourth generation AUX SAS software, which includes a feedback loop for pitch attitude using the Embedded Global Positioning System Inertial Navigation System (EGI), phase lag is effectively reduced by changing the longitudinal control input from a rate command to an attitude command controller. The fourth generation AUX SAS will enter AR developmental tests in the last quarter of 2016.Use of the Naval Air Warfare Center's Manned Flight Simulator (MFS) proved critical to deficiency analysis and the rapid development of the AUX SAS versions. Analysis and piloted MFS sessions demonstrated vast improvements in expected handling qualities over the original AUX SAS design. Flight tests during the developmental test period will be the final word.
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