Category NASA’S CONTRIBUTIONS TO AERONAUTICS

NASA’s work with the RPV concept came of age when the agency applied RPV technology to support the Research, Development, Test, and Evaluation (RDT&E) of a new Air Force fighter, the McDonnell – Douglas (subsequently Boeing) F-15 Eagle. In 1969, the Air Force selected McDonnell-Douglas Aircraft Corporation to build the F-15, a Mach-2- capable air superiority fighter airplane designed using lessons learned during aerial combat over Vietnam. The prototype first flew in July 1972.

In the months leading up to that event, Maj. Gen. Benjamin Bellis, chief of the F-15 System Program Office at Wright-Patterson Air Force Base,

OH, requested NASA assistance in testing a three-eights-scale model F-15 RPRV to explore aerodynamic and control system characteristics of the F-15 configuration in spins and high-angle-of-attack flight. Such maneu­vers can be extremely hazardous. Rather than risk harm to a valuable test pilot and prototype, a ground pilot would develop stall/spin recov­ery techniques with the RPRV and pass lessons learned to test pilots fly­ing the actual airplanes.

In April 1972, NASA awarded McDonnell-Douglas a $762,000 con­tract to build three F-15 RPRV models. Other contractors provided electronic components and parachute-recovery equipment. NASA technicians installed avionics, hydraulics, and other subsystems. The F-15 RPRV was 23.5 feet long, was made primarily of fiberglass and wood, and weighed 2,500 pounds. It had no propulsion system and was designed for midair recovery using a helicopter. Each model cost a little over $250,000, compared with $6.8 million for a full-scale F-15 aircraft.[907] Every effort was made to use off-the-shelf components and equipment readily available at the Flight Research Center, including

hydraulic components, gyros, and telemetry systems from the lifting body research programs. A proportional uplink, then being used for instru­ment-landing system experiments, was acquired for the RPRV Ground Control Station (GCS). The ground cockpit itself was fashioned from a general-purpose simulator that had been used for stability-and – control studies. Data-processing computers were adapted for use in a programmable ground-based control system. A television cam­era provided forward visibility. The midair recovery system (MARS) parachute mechanism was taken from a Firebee drone.[908] The first F-15 RPRV arrived at the Flight Research Center in December 1972 but wasn’t flown until October 12, 1973. The model was carried to an altitude of about 45,000 feet beneath the wing of a modified B-52 Stratofortress known as the NB-52B. Following release from the launch pylon at a speed of 175 knots, ground pilot Einar Enevoldson guided the craft through a flawless 9-minute flight, during which he explored the vehi­cle’s basic handling qualities. At 15,000 feet altitude, a 12-foot spin – recovery parachute deployed to stabilize the descent. An 18-foot engage­ment chute and a 79-foot-diameter main chute then deployed so that the RPRV could be snagged in flight by a hook and cable beneath a helicopter, and set down gently on an inflated bag.[909] Enevoldson found the task of flying the RPRV very challenging, both physically and psychologically. The lack of physical cues left him feeling remote from the essential reassuring sensations of flight that provide a pilot with situational feed­back. Lacking sensory input, he found that his workload increased and that subjective time seemed to speed up. Afterward, he reenacted the mission in a simulator at 1.5 times actual time and found that the pace seemed the same as it had during the flight.

Researchers had monitored his heart rate during the flight to see if it would register the 70 to 80 beats per minute typical for a piloted test flight. They were surprised to see the readings indicate 130 to 140 beats per minute as the pilot’s stress level increased. Enevoldson found flying the F-15 RPRV less pleasant or satisfying than he normally did a difficult or demanding test mission.[910] "The results were gratifying,” he wrote in his postflight report, "and some satisfaction is gained from the

success of the technical and organizational achievement—but it wasn’t fun.”[911] In subsequent tests, Enevoldson and other research pilots explored the vehicle’s stability and control characteristics. Spin testing confirmed the RPRV’s capabilities for returning useful data, encouraging officials at the F-15 Joint Test Force to proceed with piloted spin trials in the preproduction prototypes at Edwards.[912] William H. "Bill” Dana piloted the fourth F-15 RPRV flight, on December 21, 1973. He collected about 100 seconds of data at angles of attack exceeding 30 degrees and 90 sec­onds of control-response data. Dana had a little more difficulty control­ling the RPRV in flight than he had in the simulator but otherwise felt everything went well. At Enevoldson’s suggestion, the simulator flights had been sped up to 1.4 times actual speed, and Dana later acknowl­edged that this had provided a more realistic experience.

During a postflight debriefing, Dana was asked how he liked flying the RPRV. He responded that it was quite different from sitting in the cockpit of an actual research vehicle, where he generally worried and
fretted until just before launch. Then he could settle down and just fly the airplane. With the RPRV, he said, he was calm and cool until launch and then felt keyed up through the recovery.[913] The first of several inci­dents involving the MARS parachute gear occurred during the ninth flight. The recovery helicopter failed to engage the chute, and the RPRV descended to the ground, where it was dragged upside down for about a quarter mile. Fortunately, damage was limited to the vertical tails, can­opy bulge, and nose boom. The RPRV was severely damaged at the end of the 14th flight, when the main parachute did not deploy because of failure of the MARS disconnect fitting.

Rather than repair the vehicle, it was replaced with the second F-15 RPRV. During the craft’s second flight, on January 16, 1975, research pilot Thomas C. McMurtry successfully completed a series of planned maneuvers and then deployed the recovery parachute. During MARS retrieval, with the RPRV about 3,000 feet above the ground, the towline separated. McMurtry quickly assumed control and executed an emer­gency landing on the Edwards Precision Impact Range Area (PIRA). As a result of this success and previous parachute-recovery difficulties, further use of MARS was discontinued. The RPRV was modified with landing skids, and all flights thereafter ended with horizontal touch­downs on the lakebed.[914] The F-15 RPRV project came to a halt December 17, 1975, following the 26th flight, but this did not spell the end of the vehicle’s career. In November 1977, flights resumed under the Spin Research Vehicle (SRV) project. Researchers were interested in evalu­ating the effect of nose shape on the spin susceptibility of modern high – performance fighters. Flight-testing with the F-15 model would augment previous wind tunnel experiments and analytical studies. Baseline work with the SRV consisted of an evaluation of the basic nose shape with and without two vortex strips installed. In November 1978, following nine baseline-data flights, the SRV was placed in inactive status pending the start of testing with various nose configurations for spin-mode determi­nation, forebody pressure-distribution studies, and nose-mounted spin – recovery parachute evaluation. Flights resumed in February 1981.[915]

When the SRV program ended in July 1981, the F-15 models had been carried aloft 72 times: 41 times for the RPRV flights and 31 times for the SRV. A total of 52 research missions were flown with the two aircraft: 26 free flights with each one. There had been only 2 ground aborts, 1 aborted planned-captive flight, and 15 air aborts prior to launch. Of 16 MARS recoveries, 13 were successful. Five landings occurred on the PIRA and 34 on the lakebed.[916] Flight data were correlated with wind tunnel and mathematical modeling results and presented in vari­ous technical papers. Tests of the subscale F-15 models clearly demon­strated the value of the RPRV concept for making bold, rapid advances in free-flight testing of experimental aircraft with minimal risk and max­imum return on investment. R. Dale Reed wrote that, "If information obtained from this program avoids the loss of just one full-scale F-15, then the program will have been a tremendous bargain.”[917]

Indeed it was: spin test results of the F-15 model identified a poten­tially dangerous "yaw-trip” problem with the full-scale F-15 if it had an offset airspeed boom. Such a configuration, the F-15 RPRV showed, might exhibit abrupt departure characteristics in turning flight as angle of attack increased. Subsequently, during early testing of F-15C aircraft equipped with fuselage-hugging conformal fuel tanks (like those subse­quently employed on the F-15E Strike Eagle) and an offset nose boom, Air Force test pilot John Hoffman experienced just such a departure. Review of the F-15 RPRV research results swiftly pinpointed the prob­lem and alleviated fears that the F-15 suffered from some inherent and major flaw that would force a costly and extensive redesign. This one "save” likely more than paid for the entire NASA F-15 RPRV effort.[918]

The NACA’s applied research was initially restricted to wind tunnel work. The wind tunnels had their own problems with supersonic flow, as shock waves formed and disturbed the flow, thus casting doubt on the model test results. This was especially true in the transonic regime, from Mach

0. 8 to 1.2, at which the shock waves were the strongest as the super­sonic flow slowed to subsonic in one single step; this was called a "nor­mal” shock, referring to the 90-degree angle of the shock wave to the vehicle motion. Free air experiments were necessary to validate and improve wind tunnel results. John Stack at NACA Langley developed a slotted wind tunnel that promised to reduce some of the flow irregu­larities. The Collier Trophy was awarded for this accomplishment, but validation of the supersonic tunnel results was still lacking. Pending the development of higher-powered engines for full-scale in-flight experi­ments, initial experimentation included attaching small wing shapes to NACA P-51 Mustangs, which then performed high-speed dives to and beyond their critical Mach numbers, allowing seconds of transonic
data collection. Heavy streamlined bomb shapes were released from NACA B-29s, the shapes going supersonic during their 30-45-second trajectories, sending pressure data to the ground via telemetry before impact.[1055] Supersonic rocket boosters were fired from the NACA facil­ity at Wallops Island, VA, carrying wind tunnel-sized models of wings and proposed aircraft configurations in order to gain research data, a test method that remained fruitful well into the 1960s. The NACA and the United States Air Force (USAF) formed a joint full-scale flight-test program of a supersonic rocket-powered airplane, the Bell XS-1 (subse­quently redesignated the X-1), which was patterned after a supersonic

0. 50-caliber machine gun projectile with thin wings and tail surfaces. The program culminated October 14, 1947, with the demonstration of a controllable aircraft that exceeded the speed of sound in level flight. The news media of the day hailed the breaking of the "sound barrier,” which would lead to ever-faster airplanes in the future. Speed records popularized in the press since the birth of aviation were "made to be broken”; now, the speed of sound was no longer the limit.

But the XS-1 flight in October was no more a practical solution to supersonic flight than the Wright brothers’ flights at Kitty Hawk in December 1903 were a director predecessor to transcontinental passen­ger flights. Rockets could produce the thrust necessary to overcome the drag of supersonic shock waves, but the thrust was of limited duration. Rocket motors of the era produced the greatest thrust per pound of engine, but they were dangerous and expensive, could not be throttled directly, and consumed a lot of fuel in a short time. Sustained supersonic flight would require a more fuel-efficient motor. The turbojet was an obvious choice, but in 1947, it was in its infancy and was relatively ineffi­cient, being heavy and producing only (at most) several thousand pounds of static thrust. Military-sponsored research continued on improving the efficiency and the thrust levels, leading to the introduction of after­burners, which would increase thrust from 10-30 percent, but at the expense of fuel flows, which doubled to quadrupled that of the more normal subsonic cruise settings. The NACA and manufacturers looked at another form of jet propulsion, the ramjet, which did away with the complex rotating compressors and turbines and relied on forward speed of the vehicle to compress the airflow into an inlet/diffuser, where fuel

would then be injected and combusted, with the exhaust nozzle further increasing the thrust.

NASA’s early research in SVS concepts almost immediately influenced broader perceptions of the field. Working with NASA researchers who reviewed and helped write the text, the Federal Aviation Administration crafted a definition of SVS published in Advisory Circular 120-29A, describing it as "a system used to create a synthetic image (e. g., typically a computer generated picture) representing the environment external to the airplane.” In 2000, NASA Langley researchers Russell V. Parrish, Daniel G. Baize, and Michael S. Lewis gave a more detailed definition as "a display system in which the view of the external environment is pro­vided by melding computer-generated external topography scenes from on-board databases with flight display symbologies and other informa­tion from on-board sensors, data links, and navigation systems. These systems are characterized by their ability to represent, in an intuitive manner, the visual information and cues that a flight crew would have in daylight Visual Meteorological Conditions (VMC).”[1136] This definition can

be expanded further to include sensor fusion. This provides the capability to blend in real time in varying percentages all of the synthetically derived information with video or infrared signals. The key requirements of SVS as stated above are to provide the pilot with an intuitive, equivalent-to – daylight VMC capability in all-weather conditions at any time on a tacti­cal level (with present and near future time and position portrayed on a head-up display [HUD] or primary flight display [PFD]) and far improved situation awareness on a strategic level (with future time and position portrayed on a navigation display [a NAV display, or ND]).

In the earliest days of proto-SVS development during the 1980s and early 1990s, the state of the art of graphics generators limited the terrain portrayal to stroke-generated line segments forming polygons to repre­sent terrain features. Superimposing HITS symbology on these displays was not difficult, but the level of situational awareness (SA) improve­ment was somewhat limited by the low-fidelity terrain rendering. In fact, the superposition of HITS projected flight paths to include a rectilinear runway presentation at the end of the approach segment on basic PFD displays inspired the development of improved terrain portrayal by sug­gesting the simple polygon presentation of terrain. The development of raster graphics generators and texturing capabilities allowed these sim­ple polygons to be filled, producing more realistic scenes. Aerial and sat­ellite photography providing "photo-realistic” quality images emerged in the mid-1990s, along with improved synthetic displays enhanced by constantly improving databases. With vastly improved graphics generators (reflecting increasing computational power), the early concept of co-displaying the desired vertical and lateral pathway guid­ance ahead of the airplane in a three-dimensional perspective has evolved from the crude representations of just two decades ago to the present examples of high-resolution, photo-realistic, and elevation-based three­dimensional displays, replete with overlaid pathway guidance, provid­ing the pilot with an unobstructed view of the world. Effectively, then, the goal of synthetically providing the pilot with an effective daylight, VMC view in all-weather has been achieved.[1137]

Though the expressions Synthetic Vision Systems, External Vision Systems (XVS), and Enhanced Vision Systems (EVS) have often been used interchangeably, each is distinct. Strictly speaking, SVS has come

to mean computer-generated imagery from onboard databases com­bined with precise Global Positioning System (GPS) navigation. SVS joins terrain, obstacle, and airport images with spatial and navigational inputs from a variety of sensor and reference systems to produce a real­istic depiction of the external world. EVS and XVS employ imaging sen­sor systems such as television, millimeter wave, and infrared, integrated with display symbologies (altitude/airspeed tapes on a PFD or HUD, for example) to permit all-weather, day-night operations.[1138]

Confusion in terminology, particularly in the early years, has char­acterized the field, including use of multiple terms. For example, in 1992, the FAA completed a flight test investigating millimeter wave and infrared sensors for all-weather operations under the name "Synthetic Vision Technology Demonstration.”[1139] SVS and EVS are often combined as one expression, SVS/EVS, and the FAA has coined another term as well, EFVS, for Enhanced Flight Vision System. Industry has undertaken its own developments, with its own corporate names and nuances. A number of avionics companies have implemented various forms of SVS technologies in their newer flight deck systems, and various airframe manufacturers have obtained certification of both an Enhanced Vision System and a Synthetic Vision System for their business and regional aircraft. But much still remains to be done, with NASA, the FAA, and industry having yet to fully integrate SVS and EVS/EFVS technology into a comprehensive architecture furnishing Equivalent Visual Operations (EVO), blending infrared-based EFVS with SVS and millimeter – wave sensors, thereby creating an enabling technology for the FAA’s planned Next Generation Air Transportation System.[1140]

The underlying foundation of SVS is a complete navigation and situational awareness system. This Synthetic Vision System consists mainly of integration of worldwide terrain, obstacle, and airport data­bases; real-time presentation of immediate tactical hazards (such as weather); an Inertial Navigation System (INS) and GPS navigation capability; advanced sensors for monitoring the integrity of the data­base and for object detection; presentation of traffic information; and a real-time synthetic vision display, with advanced pathway or equivalent guidance, effectively affording the aircrew a projected highway-in-the – sky ahead of them.[1141] Two enabling technologies were necessary for SVS to be developed: increased computer storage capacity and a global, real-

time, highly accurate navigation system. The former has been steadily developing over the past four decades, and the latter became available with the advent of GPS in the 1980s. These enabling technologies uti­lized or improved upon the Electronic Flight Information System (EFIS), or glass cockpit, architecture pioneered by NASA Langley in the 1970s and first flown on Langley’s Boeing 737 Advanced Transport Operating System (ATOPS) research airplane. It should be noted that the research accomplishments of this airplane—Boeing’s first production 737—in its two decades of NASA service are legendary. These included demonstra­tion of the first glass cockpit in a transport aircraft, evaluation of trans­port aircraft fly-by-wire technology, the first GPS-guided blind landing, the development of wind shear detection systems, and the first SVS – guided landings in a transport aircraft.[1142]

The development of GPS satellite navigation signals technology enabled the evolution of SVS. GPS began as an Air Force-Navy effort to build a satellite-based navigation system that could meet the needs of fast-moving aircraft and missile systems, something the older TRANSIT system, developed in the late 1950s, could not. After early studies by a variety of organizations—foremost of which was the Aerospace Corporation—the Air Force formally launched the GPS research and development program in October 1963, issuing hardware design con­tracts 3 years later. Known initially as the Navstar GPS system, the con­cept involved a constellation of 24 satellites orbiting 12,000 nautical miles above Earth, each transmitting a continual radio signal containing a

precise time stamp from an onboard atomic clock. By recording the time of each received satellite signal of a required 4 satellites and comparing the associated time stamp, a ground receiver could determine position and altitude to high accuracies. The first satellite was launched in 1978, and the constellation of 24 satellites was complete in 1995. Originally intended only for use by the Department of Defense, GPS was opened for civilian use (though to a lesser degree of precision) by President Ronald Reagan after a Korean Air Lines Boeing 747 commercial airliner was shot down by Soviet interceptors in 1983 after it strayed miles into Soviet territory. The utility of the GPS satellite network expanded dra­matically in 2000, when the United States cleared civilian GPS users to receive the same level of precision as military forces, thus increasing civilian GPS accuracy tenfold.[1143]

Database quality was essential for the development of SVS. The 1990s saw giant strides taken when dedicated NASA Space Shuttle mis­sions digitally mapped 80 percent of Earth’s land surface and almost 100 percent of the land between 60 degrees north and south latitude. At the same time, radar mapping from airplanes contributed to the dig­ital terrain database, providing sufficient resolution for SVS in route and specific terminal area requirements. The Shuttle Endeavour Radar Topography Mission in 2000 produced topographical maps far more precise than previously available. Digital terrain databases are being produced by commercial and government organizations worldwide.[1144]

With the maturation of the enabling technologies in the 1990s and its prior experience in developing glass cockpit systems, NASA Langley was poised to develop the concept of SVS as a highly effective tool for pilots to operate aircraft more effectively and safely. This did not hap­pen directly but was the culmination of experience gained by Langley research engineers and pilots on NASA’s Terminal Area Productivity (TAP)
and High-Speed Research (HSR) programs in the mid – to late 1990s. By 1999, when the SVS project of the AvSP was initiated and funded, Langley had an experienced core of engineers and research pilots eager to push the state of the art.

Before an aircraft can get into the winter sky and safely avoid the threat of icing, it first must take off from what the pilot hopes is a long, wide, dry runway at the beginning of the flight, as well as at the end of the flight. Likewise, NASA’s contributions to air safety in fighting the tyr­anny of temperature included research into ground operations. While NASA did not invent the plow to push snow off the runway, or flame­throwers to melt off any stubborn runway snow or ice, the Agency has been active in studying the benefits of runway grooves since the first civil runway was introduced in the United States at Washington National Airport in December 1965.[1260]

Runway grooves are intended to quickly channel water away from the landing strip without pooling on the surface so as to prevent

hydroplaning. The 3-mile-long runway at the Shuttle Landing Facility is probably the most famous runway in the Nation and known for being grooved. Of course, there is little chance of snow or ice accumulating on the Central Florida runway, so when NASA tests runway surfaces for cold weather conditions it turns to the Langley Aircraft Landing Dynamics Facility at NASA’s Langley Research Center (LaRC) in Hampton, VA. The facility uses pressurized water to drive a landing-gear-equipped platform down a simulated runway strip, while cameras and sensors keep an eye on tire pressure, tire temperature, and runway friction. Another runway at NASA’s Wallops Flight Facility also has been used to test various sur­face configurations. During the mid-1980s, tests were performed on 12 different concrete and asphalt runways, grooved and non-grooved, includ­ing dry; wet; and snow, slush, and ice-covered surface conditions. More than 200 test runs were made with two transport aircraft, and more than 1,100 runs were made with different ground test vehicles. Ground vehi­cle and B-737 aircraft friction tests were conducted on grooved and non­grooved surfaces under wet conditions. As expected, grooved runway surfaces had significantly greater friction properties than non-grooved surfaces, particularly at higher speeds.[1261]

As the 1970s came to an end, the U. S. military fleet of high-performance fighter aircraft had been transformed from departure-prone designs to new configurations with outstanding stability and departure resistance at high angles of attack. Thanks to the national research and develop­ment efforts of industry and Government following the Dayton sympo­sium in 1971, the F-14, F-15, F-16, and F/A-18 demonstrated that the peril of high-angle-of-attack departure exhibited by the previous gener­ation of fighters was no longer a critical concern. Rather, the pilot could exploit high angles of attack under certain tactical conditions without fear of nose slice or pitch-up. At air shows and public demonstrations, the new "supermaneuverable” fighters wowed the crowds with high – angle-of-attack flybys, and more importantly, the high-alpha capabilities provided pilots with new options for air combat. High-angle-of-attack
technology had progressed from concerns over stall characteristics to demonstrated spin resistance and was moving into a focus on poststall agility and precision maneuverability.

Reflecting on the advances in high-angle-of-attack technology of the 1970s and concepts yet to be developed, technical managers at Langley, Dryden, and Ames began to advocate for a cohesive, integrated research program focused on technologies and innovative ideas. The Agency was in an excellent position to initiate such a program thanks to the unique ground – and flight-testing capabilities that had been developed and the expertise that had been gathered by interactions of the NASA researchers with the real-world challenges of specific aircraft programs. At Langley, for example, researchers had been intimately involved in high-angle-of – attack/departure/spin activities in the development of all the new fight­ers and had accumulated in-depth knowledge of the characteristics of the configurations, including aerodynamics, flight control architecture, and handling characteristics at high angles of attack. Technical exper­tise and facilities at Langley included subscale static and dynamic free – flight model wind tunnel testing, advanced control-law synthesis, and computational aerodynamics. In addition, extensive peer contacts had been made within industry teams and DOD aircraft development offices.

At Dryden, the world-class flight-test facilities and technical expertise for high-performance fighter aircraft had been continually demonstrated in highly successful flight-test programs in which potentially hazardous testing had been handled in a professional manner. The Dryden staff was famous for its can-do attitude and accomplishments, including the conception, development, and routine operation of experimental air­craft; advanced flight instrumentation; and data extraction techniques.

Meanwhile, at Ames, the aeronautical research staff had aggressively led developments in high-performance computing facilities and com­putational aerodynamics. Computational fluid dynamics (CFD) codes developed at Ames and Langley had shown powerful analysis capabil­ity during applications to traditional aerodynamic predictions such as cruise performance and the analysis of flow-field phenomena. In addi­tion to computational expertise, Ames had extensive wind tunnel facili­ties, including the huge 80-by 120-Foot Tunnel, which had the capability of testing a full-scale fighter aircraft as large as the F/A-18.

From the perspective of the three technical managers, the time was right to bring together the NASA capabilities into a focused program directed toward some of the more critical challenges in
high-angle-of-attack technology.[1308] The research program that evolved from the planning meetings grew into one of the more remarkable efforts ever undertaken by NASA. The planning, advocacy, and conduct of the program was initiated at the grassroots level and was managed in a most remarkable manner for the duration of the program. Within NASA’s aero­nautics activities, the program brought an enthusiastic environment of cooperation—not competition—that fostered a deep commitment to team spirit and accomplishments so badly needed in research endeav­ors. The personal satisfaction of the participants was widely known, and the program has become a model for NASA intercenter relationships and joint programs.[1309]

The first task in planning the program was to identify major techni­cal issues facing the high-angle-of-attack community. Foremost among these was the understanding, prediction, and control of aerodynamic phenomena at high-angle-of-attack conditions, especially for aircraft configurations with strong vortical flows. Achieving this goal involved detailed studies of separated flow characteristics; measurement of static and dynamic phenomena in ground-test facilities as well as flight; cal­ibration of flow predictions from CFD methodology, wind tunnels, and flight; and the development of CFD codes for high-angle-of-attack condi­tions. In addition, the analysis and prediction of aerodynamic phenom­ena associated with structural fatigue issues for vertical tails immersed in violently fluctuating separated flows at high-angle-of-attack condi­tions became a major element in the program.

The second research thrust in the proposed program was directed toward an exciting new technology that offered unprecedented levels of controllability at high angles of attack—thrust vectoring. The thrust­vectoring concept had been developed in early rocket control applica­tions by placing vanes in the exhaust of the rocket vehicle, and extensive NASA-industry-DOD studies had been conducted to develop movable nozzle vectoring concepts for aircraft applications. The introduction of the superb fighters of the 1970s had demonstrated a new level of design achievement in stability at high angles of attack, but another nemesis remained—inadequate control at high-angle-of-attack conditions at
which conventional aerodynamic control surfaces lose effectiveness because of separated flow. The problem was particularly critical in the lack of ability to create crisp, precise roll control for "nose pointing” at high angles of attack. For such conditions, the ability to roll is dependent on providing high levels of yaw control, which creates sideslip and roll­ing motion because of dihedral effect. Unfortunately, conventional rud­ders mounted on vertical tails become ineffective at high angles of attack.

During the early 1980s, researchers at the Navy David Taylor Research Center pursued the application of simple jet-exit vanes to the F-14 for improved yaw control.[1310] Teaming with Langley in a joint study in the Langley Differential Maneuvering Simulator, the researchers found that the increased yaw control provided by the vanes resulted in a dramatic improvement in high-angle-of-attack maneuverability and dominance in simulated close-in air combat. Inspired by these results, Langley researchers evaluated the effectiveness of similar vanes on a variety of configurations during free-flight model testing in the Langley Full Scale Tunnel. Following investigations of modified models of the F-16, F/A-18, X-29, and X-31, the researchers concluded that thrust vec­toring in yaw provided unprecedented levels of maneuverability and control at high angles of attack. In addition, providing feedback from flight sensors to the vane control system enhanced dynamic stability for the test conditions.

Another technology that had matured to the point of research appli­cations was the control of strong vortical flow shed from the long pointed forebodies of contemporary fighters at high angles of attack. As previ­ously mentioned, Dryden and the Air Force Flight Dynamics Laboratory had conducted a joint program to evaluate the effects of blowing on the nose of the X-29A for enhanced control. Competing concepts for vorti­cal flow control had also received attention during NASA and industry research programs, including investigations at Langley of deflectable forebody strakes that could be used to control flow separation on the forebody for enhanced yaw control.

Perhaps the most contentious issue in planning the integrated NASA high-angle-of-attack program was whether a research air­craft was required and, if so, which aircraft would make the best testbed for research studies. Following prolonged discussions (the Ames
representative did not initially endorse the concept of flight-testing), the planning team agreed that flight-testing was mandatory for the program to be relevant, coordinated, and focused. Consideration was given to the F-15, F-16, X-29, and F/A-18 as potential testbeds, and after discussions, the team unanimously chose the F/A-18, for several reasons. The ear­lier Navy F/A-18 development program had included extensive support from Langley; therefore, its characteristics were well known to NASA (especially aerodynamic and aeroelastic phenomena, such as vortical flow and vertical tail buffet). During spin-testing for the development program, the aircraft had displayed reliable, stall-free engine operations at high angles of attack and excellent spin recovery characteristics. The F/A-18 was equipped with an advanced digital flight control system that offered the potential for modifications for research flight tests. Finally, the aircraft exhibited a remarkably high-angle-of-attack capability (up to 60 degrees in trimmed low-speed flight)—ideal for aerodynamic tests at extreme angles of attack.

The intercenter planning team presented its integrated research program plan to NASA Headquarters, seeking approval to pursue the acquisition of an F/A-18 from the Navy and for program go-ahead. After Agency approval, the Navy transferred the preproduction F/A-18A Ship 6, which had been used for spin testing at Patuxent River, MD, to NASA Dryden, where it arrived in October 1984. This particular F/A-18A had been stripped of several major airframe and instrument components following the completion of its spin program at Patuxent River, but it was still equipped with a multi-million-dollar emergency spin recov­ery parachute system and a programmable digital flight control com­puter ideally suited to NASA’s research interests. The derelict aircraft was shipped overland to Dryden and reassembled by a team of NASA and Navy technicians into a unique high-angle-of-attack research air­plane known as the F/A-18A High-Alpha Research Vehicle (HARV).[1311] The HARV was equipped with several unique research systems, includ­ing flow visualization equipment, a thrust-vectoring system using exter­nal postexit vanes around axisymmetric nozzles, and deployable nose strakes on a modified fuselage of forebody. Additional aircraft systems included extensive instrumentation, integrated flight research con­trols with special flight control hardware and software for the thrust-

vectoring system, interface controls for the forebody strakes, and safety backup systems including a spin recovery parachute.

The High-Angle-of-Attack Technology program (HATP) was funded and managed under an arrangement that was different from other NASA programs but was extremely efficient and productive. Headquarters pro­vided program management oversight, but recommendations for day – to-day technical planning, distribution of funds, and technical thrusts were provided by an intercenter steering committee consisting of mem­bers from each of the participating Centers. In recognition of its techni­cal expertise and accomplishments in high-angle-of-attack technology, Langley was designated the technology lead Center. Dryden was desig­nated the lead Center for flight research and operations of the HARV, and Ames and Langley shared the technical leadership for CFD and experi­mental aerodynamics. In subsequent years, the NASA Lewis Research Center (now the NASA Glenn Research Center) joined the HATP for experiments on engine inlet aerodynamics for high-angle-of-attack con­ditions. The HATP included aerodynamics, flight controls, handling qual­ities, stability and control, propulsion, structures, and thrust vectoring.[1312]

The HATP program was conducted in three sequential phases, centering on high-angle-of-attack aerodynamic studies (1987-1989), evaluation of thrust vectoring effects on maneuverability (1990-1994), and forebody flow control (1995-1996), with 383 research flights. In the first activities, aerodynamic characteristics obtained from flight-test results for the baseline HARV (no vectoring) were correlated with wind tunnel and CFD predictions, with emphasis on flow separation predic­tions and vortical flow behavior on the fuselage forebody and wing-body leading-edge extension (LEX).[1313]

The first HARV research flight was April 17, 1987. Flown in its base­line configuration, the HARV provided maximum angles of attack on the order of 55 degrees, limited by aerodynamic control. At the time the flight studies were conducted, CFD had not yet been applied to real
aircraft shapes at high angles of attack. Rather, researchers had used computational methods to predict flow over simple shapes such as pro­late spheroids, and the computation of flow fields, streamlines, and sep­aration phenomena for a modern fighter was extremely challenging. Many leaders in the NASA and industry CFD communities were pessi­mistic regarding the success of such a venture at the time.

The experimental wind tunnel community was also facing issues on how (or whether) to modify models to better simulate high-angle – of-attack aerodynamics at flight values of Reynolds numbers and to understand the basic characteristics of vortical flows and techniques for the prediction of flow interactions with aircraft structures. Using a highly innovative, Dryden-developed propylene glycol monomethyl ether (PGME) dye flow-visualization technique that emitted colored dye tracers from ports for visualization of surface flows over the HARV forebody and LEX, the team was able to directly compare results, ana­lyze separation phenomena, and modify CFD codes for a valid predic­tion of the observed on-surface flow characteristics. The ports for the PGME were later modified for pressure instrumentation to provide even more detailed information on flow fields. Additional instrumentation for aerodynamic measurements was initially provided by a nose boom, but evidence of aerodynamic interference from the nose boom caused the team to remove it and replace the boom with wingtip air-data probes. A rotating, foldout flow rake was also used to measure vortical flows shed by the LEX surfaces.[1314]

The results of the HATP flight – and ground-based aerodynamic stud­ies provided a detailed perspective of the relative accuracy of compu­tational flow dynamics and wind tunnel testing techniques to predict critical flow phenomena such as surface pressures, separation con­tours, vortex interaction patterns, and laminar separation bubbles.[1315] The scope of correlation included assessments of the impact of Mach and Reynolds numbers on forebody and LEX vortexes as observed in flight with the HARV, the Langley 7- by 10-Foot High-Speed Tunnel, the Langley

30- by 60-Foot Tunnel, the Navy David Taylor Research Center 7- by 10-Foot Transonic Tunnel, and the Ames 80- by 120-Foot Wind Tunnel.

A wide variety of subscale models of the HARV configuration was tested in the various wind tunnels, and a full-scale F/A-18 aircraft was used for testing in the Ames 80- by 120-Foot Tunnel. The test article was an ex-Blue Angel flight demonstrator, whose life had been exceeded,

that had been bailed to NASA for the tests. When the tunnel tests were conducted in 1991 and 1993, the aircraft had both engines, flowthrough inlets, and the wingtip missile launchers removed.

Using extensive instrumentation that had been carefully coordi­nated between ground and flight researchers gathered an unprecedented wealth of detail on aerodynamic characteristics of a modern fighter at high angles of attack. The effort was successful particularly because it had been planned with common instrumentation locations for pressure ports and flow visualization stations between wind tunnel tests and the flight article. More importantly, the high value of the data obtained was the result of one of the most successful aspects of the program—close communications and working relationships between the flight, wind tunnel, and CFD technical communities.

As NASA neared the end of the aerodynamic phase of testing for the HARV, growing concerns over buffeting of the vertical tail surfaces for military fleet F/A-18 aircraft led the Navy and McDonnell-Douglas to develop vertical longitudinal fences on the upper surfaces of the LEX to
extend the service life of the tails of fleet F/A-18s. Although the fences were not installed on HARV during the early aerodynamic studies, they were added during the second and third phases of the program, when extensive wind tunnel and HARV flight studies of the tail buffet phenom­enon were conducted. Resulting data were transmitted to the appropri­ate industry and service organizations for analysis of the F/A-18 specific phenomena as well as for other twin-tail fighter aircraft.

As the second phase of the HATP began, Dryden accepted major program responsibilities for the implementation of a relatively simple and cheap thrust-vectoring system for the HARV aircraft. The objec­tive of NASA’s research was not to develop a production-type thrust­vectoring engine/nozzle system, but rather to evaluate the impact of vectoring for high-angle-of-attack maneuvers, assess control-law require­ments for high-angle-of-attack applications, and use the control augmen­tation provided by vectoring to stabilize the aircraft at extreme angles of attack for additional aerodynamic studies. With this philosophy in mind, the program contracted with McDonnell-Douglas to modify the HARV with deflectable external vanes mounted behind the aircraft’s two F-404 engines, similar in many respects to the installations used by the Navy F-14 mentioned earlier and the Rockwell X-31 research aircraft.

For the installation, the exhaust nozzle divergent flaps were removed from the engines and replaced with a set of three vanes for each engine, thereby providing both pitch and yaw vectoring capability. The research teams at Dryden and Langley thoroughly studied the specific vane con­figuration, structural design, and control system modifications required for the project. The scope of activities included measurements of thrust – vane effectiveness for many powered model configurations at Langley, simulator studies of the effectiveness of vectoring on maneuverability and controllability at Langley, and hardware and software development— as well as the integration, checkout, and operations of the system—at Dryden. The implementation of the HARV thrust-vectoring hardware and software modifications proved to be relatively difficult, requiring the NASA research team to participate in the final design of the thrust­vectoring system. The HARV vectoring system followed the HATP objec­tive of providing thrust-vectoring research capability at minimal cost through external airframe modifications rather than a new production – type vectoring engine. With the massive external thrust-vectoring vane actuation system and the emergency spin recovery parachute system both mounted on the rear of the aircraft and necessary ballast added

to the nose of the aircraft to maintain balance, the weight of the HARV was increased by about 4,000 pounds.

In the thrust-vectoring phase of the HATP project, the conventional flight control system of the HARV was modified to include a research flight control system (RFCS) to influence control laws. The conventional F/A-18 control laws were used for takeoff, for landing, and as a backup in case of failure of the RFCS, whereas the second set of control laws were for high-angle-of-attack research flights. The design and implementation of the RFCS system was one of the more complex changes to the F/A-18 digital flight control system undertaken at that time.

First flight of the HARV with vectoring engaged occurred in July 1991, a few weeks after the X-31 research aircraft demonstrated pitch­vectoring capability at Edwards, but the HARV conducted the first mul­tiaxis vectoring flights shortly thereafter. Research flight-testing of the HARV equipped with thrust vectoring vividly demonstrated the anticipated benefits at high angles of attack that had been predicted by earlier free – flight model tests and piloted simulator studies. The precision and angu­lar rates available to the pilot were remarkable, and the enhanced stability and control at extreme angles of attack permitted precision aerodynamic
studies that had previously been impossible. Angles of attack as high as 70 degrees were flown with complete control in aerodynamic experiments.

During the late 1980s, three NASA-industry-DOD programs had been initiated to explore thrust-vectoring systems for high-angle-of – attack conditions. Each program had different objectives and focused on separate technologies. NASA’s HARV aircraft was designed to evaluate fundamental thrust-vectoring system control-law synthesis and use vec­toring to stabilize the aircraft at high angles of attack for aerodynamic experiments. The DARPA X-31A aircraft was conceived to demonstrate enhanced fighter maneuverability at poststall angles of attack under simulated tactical conditions. In addition, the Air Force F-16 Variable – Stability In-Flight Simulator Test Aircraft (VISTA) was modified into the F-16 Multi-Axis Thrust Vectoring (MATV) project with an objective of demonstrating the effectiveness of a production-type thrust-vectoring system. All three programs had different goals, and the three research aircraft underwent flight-testing at Edwards in the same time period.

The HATP participants conceived, developed, and assessed several control-law schemes, which included special configurations for longi­tudinal control at high angles of attack, lateral and directional control mixing strategies, automatic spin prevention, and spin recovery modes. Seventy-five spin attempts (at low power conditions) resulted in 70 fully developed spins with satisfactory recoveries, and the emergency spin recovery parachute was never fired in flight.

As the HARV conducted its thrust-vectoring research program, a critical issue emerged within the advanced fighter design community. With new configurations under consideration having extreme angle-of – attack capability and reduced longitudinal stability for performance and maneuverability enhancements, the issue of providing sufficient nose – down control effectiveness for recovery from high-angle-of-attack excur­sions became significant. NASA-DOD technical meetings had been held to discuss studies to assess the adequacy of theoretical and wind tunnel predictions, and it appeared that using the HARV flight capability with thrust vectoring would provide highly desirable data for design criteria for future fighters. In view of the urgency of the situation, Langley led a HATP element known as High-Alpha Nosedown Guidelines (HANG), which included extensive simulator studies and flights with the HARV.[1316]

Although the main objective of the HARV thrust-vectoring experi­ments was not air-to-air combat maneuvering, Dryden conducted flight tests to provide validation data for a proposed high-angle-of-attack flying qualities requirement MIL-STD-1797A by using basic fighter maneuvers and limited air combat maneuvering. Six NASA research test pilots from Dryden and Langley provided the major expertise and guidance for the HATP simulator and HARV flight-testing. Other guest pilots from NASA, the Navy, the Canadian Air Force, the United Kingdom, McDonnell-Douglas, and Calspan also participated in flight-test evaluations of the HARV vectoring capabilities.

The third and final phase of the HATP was directed to in-flight assessments of the effectiveness of controlling the powerful vortex flows shed by the fuselage forebody for augmentation of yaw control at high angles of attack. Ground-based research in NASA wind tunnels and sim­ulators had indicated that the most effective method for rolling an air­craft about its flight path for nose pointing at high angles of attack was through the use of yaw control. Unfortunately, conventional rudders suffer a severe degradation and control effectiveness at high angles of attack because of the impingement of low-energy stalled flows only ver­tical tail surfaces. Years of NASA research had demonstrated that the use of deployable fuselage forebody strakes was a potentially viable con­cept for yaw control augmentation. With a vast amount of wind tunnel data and pilot opinions derived from air combat simulation, the strake concept was ready for realistic evaluations in flight. Once again, the cohesive nature of the HATP was demonstrated when the strake hard­ware was designed and fabricated on a special F/A-18 forebody radome in machine shops at Langley and the control laws were developed at Langley and delivered to Dryden, where the flight computer interface and instrumentation were accomplished by the Dryden staff. The proj­ect, known as actuated nose strakes for enhanced rolling (ANSER), was evaluated independently and in combination with thrust vectoring.[1317]

Implementation of the ANSER concept on the thrust-vectoring – equipped HARV provided three control combinations. The aircraft could be flown with thrust vectoring only, thrust vectoring in longitudinal con­trol with a thrust-vectored and strake-blended mode for lateral control,
and a strake mode with thrust-vectoring control longitudinally and strakes controlling the lateral mode. As was the case for thrust vector­ing, the forebody strake flight results demonstrated that a significant enhancement of high-angle-of-attack rolling capability was obtained, particularly at higher subsonic speeds. In fact, at those speeds, the effec­tiveness of the strakes was comparable to that of thrust vectoring.

Several other subsystems were implemented on the HARV, including an instrumented inlet rake, extensive pressure instrumentation, aero – servoelastic accelerometers, thrust-vectoring vane loads and tempera­tures, and an emergency power backup system. Notably, although the power backup system was implemented to continue aircraft systems operation in the event of a dual-engine flameout or unrecoverable dual­engine stalls, it was removed later in the program when testing showed excellent high-angle-of-attack engine operations. In fact, 383 high-angle – of-attack flights were made without experiencing an engine stall.

Throughout the HATP program, NASA ensured that results were widely disseminated within industry and DOD. Major HATP technical conferences were held, with at least 200 attendees at Langley in 1990, at Dryden in 1992 and 1994, and a wrap-up conference at Langley in 1996.[1318] Hundreds of reports and presentations resulted from the program, and the $74 million (1995 dollars) activity produced cutting-edge tech­nical results that were absorbed into the Nation’s latest aircraft, includ­ing the F-22, F-35 and F/A-18E.

The XV-5 was a proof-of-concept lift-fan aircraft and thus employed a completely "manual” powered-lift flight control system. The lack of an integrated powered-lift system required the pilot to manually control the aircraft flight-path through independent manipulation of stick, engine power, thrust vector angle and collective lift. This lack of an integrated powered-lift management system (and in particular, the conversion controls) was responsible for most of the adverse handling qualities of the aircraft. An advanced digital fly­by-wire control system must provide level one handling qualities, especially for integrated powered-lift management.

CONVERSION SYSTEM DESIGN

The manually operated conversion system was the most exacting, interesting and potentially hazardous flight opera­tion associated with the XV-5. This type of "bang-bang” con­version system should not be considered for the SSTOVLF. Ideally, the conversion should consist of a fully reversible and continuously controllable process. That is, the pilot must be able to continuously control the conversion process. Good examples are the XV-15 Tilt Rotor, the X-22A and the AV-8 Harrier. Furthermore, the conversion of the SSTOVLF with an advanced digital flight control system should be fully decou­pled so that the pilot would not have to compensate for lift, attitude or speed changes. The conversion controller should be a single lever or beeper-switch that is safety-interlocked against inadvertent actuation. The conversion airspeed limit corridor must be wide enough to allow for operational flexi­bility and compensate for single-pilot operation where mis­sion demands can compete for pilot attention.

The conditions if not the mechanics that generate lightning are now well known. In essence, this atmospheric fire is started by rubbing particles together. But there is still no agreement on which processes ignite lightning. Current hypotheses focus on the separation of electric charge and generation of an electric field within a thunderstorm. Recent studies further suggest that lightning initiation requires ice, hail, and semifrozen water droplets, called "graupel.” Storms that do not pro­duce large quantities of ice usually do not develop lightning.[116] Graupel forms when super-cooled water droplets condense around a snowflake nucleus into a sphere of rime, from 2 to 5 millimeters across. Scientific debate continues as experts grapple with the mysteries of graupel, but the stages of lightning creation in thunderstorms are clear, as outlined by the National Weather Service of the National Oceanic and Atmospheric Administration (NOAA).

First comes charge separation. Thunderstorms are turbulent, with strong updrafts and downdrafts regularly occurring close to one another. The updrafts lift water droplets from warmer lower layers to heights between 35,000 and 70,000 feet, miles above the freezing level. Simultaneously, downdrafts drag hail and ice from colder upper layers. When the opposing air currents meet, water droplets freeze, releasing heat, which keeps hail and ice surfaces slightly warmer than the sur­rounding environment, so that graupel, a "soft hail,” forms.

Electrons carry a negative charge. As newly formed graupel collides with more water droplets and ice particles, electrons are sheared off the ascending particles, charging them positively. The stripped electrons col­lect on descending bits, charging them negatively. The process results in a storm cloud with a negatively charged base and positively charged top.

Once that charge separation has been established, the second step is generation of an electrical field within the cloud and, somewhat like a mirror image, an electrical field below the storm cloud. Electrical opposites attract, and insulators inhibit current flow. The separation of positive and negative charges within a thundercloud generates an electric field between its top and base. This field strengthens with fur­ther separation of these charges into positive and negative pools. But the atmosphere acts as an insulator, inhibiting electric flow, so an enor­mous charge must build up before lightning can occur. When that high charge threshold is finally crossed, the strength of the electric field over­powers atmospheric insulation, unleashing lightning. Another electrical field develops with Earth’s surface below negatively charged storm base, where positively charged particles begin to pool on land or sea. Whither the storm goes, the positively charged field—responsible for cloud – to-ground lightning—will follow it. Because the electric field within the storm is much stronger than the shadowing positive charge pool, most lightning (about 75 to 80 percent) remains within the clouds and is thus not attracted groundward.

The third phase is the building of the initial stroke that shoots between the cloud and the ground. As a thunderstorm moves, the pool of positively charged particles traveling with it along the ground gath­ers strength. The difference in charge between the base of the clouds and ground grows, leading positively charged particles to climb up taller objects like houses, trees, and telephone poles. Eventually a "stepped leader,” a channel of negative charge, descends from the bottom of the storm toward the ground. Invisible to humans, it shoots to the ground in a series of rapid steps, each happening quicker than the blink of an eye. While this negative leader works its way toward Earth, a positive charge collects in the ground and in objects resting upon it. This accumulation of positive charge "reaches out” to the approaching negative charge with its own channel, called a "streamer.” When these channels connect, the resulting electrical transfer appears to the observer as lightning.

Finally, a return stroke of lightning flows along a charge channel about 0.39 inches wide between the ground and the cloud. After the ini­tial lightning stroke, if enough charge is left over, additional strokes will flow along the same channel, giving the bolt its flickering appearance.

Land struck by a bolt may reach more than 3,300 °F, hot enough to almost instantly melt the silica in conductive soil or sand, fusing the grains together. Within about a second, the fused grains cool into ful­gurites, or normally hollow glass tubes that can extend some distance into the ground, showing the path of the lightning and its dispersion over the surface.

The tops of trees, skyscrapers, and mountains lie closer to the base of storm clouds than does low-lying ground, so such objects are commonly struck by lightning. The less atmospheric insulation that lightning must burn through, the easier falls its strike. The tallest object beneath a storm will not necessarily suffer a hit, however, because the opposite charges may not accumulate around the highest local point or in the clouds above it. Lightning can strike an open field rather than a nearby line of trees.

Lightning leader development depends not only upon the electrical breakdown of air, which requires about 3 million volts per meter, but on prior channel carving. Ambient electric fields required for lightning leader propagation can be one or two orders of magnitude less than the electrical breakdown strength. The potential gradient inside a developed return stroke channel is on the order of hundreds of volts per meter because of intense channel ionization, resulting in a power output on the order of a megawatt per meter for a vigorous return stroke current of 100,000 amperes (100 kiloamperes, kA).

In June 1995, the FAA announced its plans for a joint FAA-DOD-NASA initiative called the National Plan for Civil Aviation Human Factors. The plan detailed a national effort to reduce and eliminate human error as the cause of aviation accidents. The plan called for projects that would iden­tify needs and problems related to human performance, guide research programs that addressed the human element, involve the Nation’s top scientists and aviation professionals, and report the results of these efforts to the aviation community.[221]

NASA’s extensive involvement in human factors issues is detailed in another case study of this volume.

From new navigation instruments to updated air traffic control proce­dures, none of the developments intended to make safer skyways that was produced by NASA could be deployed into the real world until it had been thoroughly tested in simulated environments and certified as ready for use by the FAA. Among the many facilities and aircraft available to NASA to conduct such exercises, the Langley-based Boeing 737 and Ames-based complement of air traffic control simulators stand out as major contrib­utors to the effort of improving the National Airspace System.

NASA initiated and implemented this important human-based safety program in 1976 at the request of the FAA. Its importance can best be judged by the fact it is still in full operation—funded by the FAA and managed by NASA. The Aviation Safety Reporting System (ASRS) col­lects information voluntarily and confidentially submitted by pilots, controllers, and other aviation professionals. This information is used to identify deficiencies in the National Aviation System (NAS), some of which include those of the human participants themselves. The ASRS analyzes these data and refers them in the form of an "alerting message” to the appropriate agencies so that problems can be corrected. To date, nearly 5,000 alert messages have been issued.[377] The ASRS also educates through its operational issues bulletins, its newsletter CALLBACK and its journal ASRS Directline, as well as through the more than 60 research studies it has published.[378] The massive database that the ASRS main­tains benefits not only NASA and the FAA, but also other agencies world­wide involved in the study and promotion of flight safety. Perhaps most importantly, this system serves to foster further aviation human fac­tors safety research designed to prevent aviation accidents.[379] After more than 30 years in operation, the ASRS has been an unqualified success. During this period, pilots, air traffic controllers, and others have pro­vided more than 800,000 reports.[380] The many types of ASRS responses to the data it has collected have triggered a variety of safety-oriented actions, including modifications to the Federal Aviation Regulations.[381]

It is impossible to quantify the number of lives saved by this impor­tant long-running human-based program, but there is little dispute that its wide-ranging effect on the spectrum of flight safety has benefitted all areas of aviation.