T-45 Stability Augmented Steering System

Ryan Murphy, Patuxent River, MD, USA
Reid Wiseman, Patuxent River, MD, USA
Christina Stack, Patuxent River, MD, USA

Abstract

The ground handling characteristics of the T-45 Goshawk (a U.S. trainer variant of the Hawk aircraft) were identified as problematic since the early days of flight testing. deficiency report SA-162 addressed the “overly sensitive directional control characteristics” of the T-45 during landing rollout. This Part IK deficiency was not corrected during initial developmental testing and has presented itself in over 12 runway departures or loss of controls within the last two years. The runway departures typically occur when students who are not familiar with the landing characteristics, make large corrective inputs for aircraft heading during the landing rollout. The lowest region of directional stability has been observed in the 60-85 kt range.

A variety of potential solutions were attempted to remedy the deficiency over the years. Initially, full time nose wheel steering (NWS) was added to the aircraft. Although some improvement was noted, directional control during landing rollout remained an issue. Following the incorporation of full time NWS, several iterations of pulse width modulation NWS and active yaw damping during landing rollout were evaluated. None of these solutions provided sufficient improvement for fleet incorporation. Following these unsuccessful attempts to finding a ground handling solution, NAVAIR and Boeing initiated a ground handling study. This study resulted in a proposal to provide yaw rate feedback to the NWS to improve ground handling during landing rollout. This system became the Stability Augmented Steering System (SASS).

An engineering company that primarily investigates the handling qualities of racecars, Systems Technology, Inc., completed the study. Study results determined the geometry of the landing gear on the small airframe, combined with the material composition of the tires, created the unstable ground handling characteristics of the aircraft. Unable to practically redesign the geometry of the landing gear and the lack of suitable tire material to remake the tires, it was necessary to design a new system that could limit the departure characteristics by trying to limit the yaw that may be experienced during the landing rollout. This system had to operate in both the T-45A and the T-45C aircraft, and therefore designed as an independent system requiring limited information from the aircraft.