This paper is divided into two major parts. The first part focuses on the YF-22 aircraft and includes an overview of the Advanced Tactical Fighter (ATF) prototype program, a review of the control law development process and a thorough analysis of the YF-22 PIO mishap. This review will include a discussion of the Air Force accident review board conclusions and the corrective action plan that was developed after the accident investigation. The second part of the paper will discuss the development of the F-22 control laws. The F-22 control law design process will be reviewed; including the design philosophy developed for the F-22 and the use of simulation and other analysis tools. Particular emphasis will be placed on the handling qualities and PIO metrics that have been used to shape the longitudinal axis flying qualities of the F-22. Introduction to the ATF Program The YF-22A was the Lockheed, General Dynamics and Boeing entry into the Air Force Advanced Tactical Fighter (ATF) demonstration and validation program. It was a prototype of a single seat, twin engine air superiority fighter that combined stealth, agility and sustained supersonic cruise capabilities. It was designed to optimize the blend of these aircraft characteristics for the demanding threat environment of the 21 century. Aircraft agility was deemed essential across the entire Mach/altrtude envelope of the aircraft. The YF-22 incorporated thrust vectoring nozzles to provide enhanced agility and extend the maneuvering envelope of the aircraft to extreme angles of attack. The external lines of the aircraft Copyright © 1996 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. were very comparable to the F-22, which is shown in Figure 1. The YF-22 was designed to meet the requirements of the ATF program and to reduce the risk for the Engineering Manufacturing Development (EMD) program. An extremely aggressive control law development schedule and flight test program led to the successful demonstration of the outstanding capabilities of the aircraft. As one might expect for a program whose ultimate goal was to demonstrate a number of new technologies, the YF-22 control laws started with a very conventional inner-loop design and added capabilities as aerodynamic and propulsion models matured. The basic command architecture was very similar to that used on the F-16. The initial control law version did not have active thrust vectoring. Many enhanced capabilities were added to the YF-22 control laws as the prototype program matured and flight test approached. The use of thrust vectoring nozzles to augment the aerodynamic pitch control power of the aircraft was incorporated into the control laws and the high angle of attack control laws were also added to the basic structure. For flight test purposes, a switch was provided in the cockpit to engage/disengage thrust vectoring. The flight envelope was initially cleared up to 20° angle of attack with vectoring disengaged. Test points were repeated with vectoring engaged. Envelope expansion into the high angle of attack flight regime with vectoring engaged and disengaged followed the initial buildup. In addition to the incorporation of thrust vectoring, other program objectives, such as the requirement to provide very high pitch rate capability in certain parts of the envelope, were added to the YF-22 control laws. Many of these features were designed to demonstrate capabilities at a specific point in the envelope and, 155 American Institute of Aeronautics and Astronautics due to schedule constraints, did not represent a full envelope, production aircraft design. YF-22 Control Law Development Philosophy The YF-22 control law feedback/feedforward gains were designed using fairly conventional tools. Design goals were based on accepted short period mode, roll mode and dutch-roll mode frequencies, damping ratios and mode shapes. An eigenstructure assignment algorithm was used to calculate the initial feedback gains for the longitudinal axis. However, by combining the pitching moment control power of the horizontal tails and thrust vectoring nozzles into a single, generalized controller, this technique essentially reduced to a pole placement algorithm. The short period mode frequency/damping and the integrator mode frequency were the primary design parameters. The short period mode design goals were also fairly conventional: • Control Anticipation Parameter (CAP)=1.0 • Damping Ratio (Q=0.8 Further refinements to the control law gains were developed from analysis of off-line simulation time histories (step/doublet pitch stick inputs) and comments from piloted evaluations using the fixed-base, YF-22 Handling Qualities Simulator (HQS). Command gradients and gain tweaks to shape rise times and mode characteristics were developed through piloted evaluations in the HQS. Due to the time constraints of the prototype program, much of the analysis of the handling qualities of the YF-22 was performed after the control laws were designed. That analysis generally consisted of stability margin predictions and time history analyses that were incorporated into the Flying Qualities Substantiation report and used to support safety and flight readiness reviews. Demonstration/Validation Flight Test Program The YF-22 development program was extremely successful and, in a very aggressive flight test program, the team was able to demonstrate the exceptional high angle of attack characteristics of the YF-22, as well as the incorporation of a number of other critical technologies. Supersonic cruise and internal weapon carriage/launch capabilities were also demonstrated. The maneuverability enhancements provided by pitch axis thrust vectoring were demonstrated at cruise altitudes (>10K ft). The flight test program commenced in the fall of 1990 and by December 1990 the YF-22 had demonstrated stabilized flight at 60° angle of attack. The brevity of the flight test program required an efficient and coordinated envelope expansion for aircraft loads, flutter and handling qualities. A very rapid response by the Vehicle Management System (VMS) team in Fort Worth allowed several Operational Flight Program (OFP) updates in support of the flight test program. The basic handling qualities of the aircraft were also validated. For all of the maneuvers flown during the initial flight test program the aircraft was very well behaved with predictable flying qualities. The aircraft generally received Level 1 CooperHarper ratings for both Up&Away (UA) and Power Approach (PA) handling qualities tasks. In April of 1991 the Lockheed team was awarded the EMD contract for the ATF program and a followon flight test program with the YF-22 began shortly thereafter. The primary objective of the follow-on test program was to expand the flutter and flying qualities envelopes of the aircraft. This phase of the flight test program ended prematurely when a pilot-induced-oscillation (PIO) occurred during a low approach, forcing a gear-up landing.