NASA 

National Aeronautics and 

Facts

Space Administration 

Dryden Flight Research Center 

P.O. Box 273

Edwards, California 93523 

Voice 661-276-3449 

FAX 661-276-3566 

pao@dfrc.nasa.gov FS-2002-09-024 DFRC 

F-8 Digital Fly-By-Wire Aircraft 

The F-8 DFBW aircraft in flight. (NASA photo ECN 347811) 

The Digital Fly-By-Wire (DFBW) concept uses an electronic flight-control system coupled with a digital computer 

to replace conventional mechanical flight controls. 

The first test of a DFBW system in an aircraft was in l972 on a modified F-8 Crusader at the Flight Research 

Center, Edwards, Calif. (now Dryden Flight Research Center). It was the forerunner of the fly-by-wire flight 

control systems now used on the space shuttles and on today’s military and civil aircraft to make them safer, more 

maneuverable and more efficient. It was safer because of its redundancies and because, for military aircraft, wires were less 

vulnerable to battle damage than the hydraulic lines they replaced. It was more maneuverable because computers could 

command more frequent adjustments than a human pilot and designers could do away with features that made the plane 

more stable and thus harder to maneuver. For airliners, computerized flight control could also ensure a smoother ride than 

a human pilot alone could provide. Finally, digital fly-by-wire was more efficient because it was lighter and took up less 

volume than hydraulic controls and thus either reduced the fuel required to fly with the extra weight and/or permitted 

carrying more passengers or cargo. It also required less maintenance than older systems. 

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Background

In the first few decades of flight, pilots controlled aircraft 

through direct force — moving control sticks and rudder 

pedals linked to cables and pushrods that pivoted control 

surfaces on the wings and tails. 

As engine power and speeds increased, more force was 

needed and hydraulically boosted controls emerged. Soon, all 

high performance and large aircraft had hydraulic-mechanical 

flight-control systems. These conventional flight-control 

systems restricted designers in the configuration and design of 

aircraft because of the need for flight stability. 

As the electronic era evolved in the 1960s, so did the idea of 

aircraft with electronic flight-control systems. Wires replacing cables and pushrods would give designers greater flexibility in configuration and in the size and placement of components such as tail surfaces and wings. A fly-by-wire system 

also would be smaller, more reliable, and in military aircraft, 

much less vulnerable to battle damage. A fly-by-wire aircraft 

would also be much more responsive to pilot control inputs. 

The result would be more efficient, safer aircraft with improved performance and design. 

The Aircraft 

By the late 1960s, engineers at Dryden began discussing how 

to modify an aircraft and create a digital fly-by-wire testbed. 

Support for the concept at NASA Headquarters came from 

Neil Armstrong, former research pilot at Dryden. He served 

in the Office of Advanced Research and Technology following his historic Apollo 11 lunar landing and knew electronic 

control systems from his days training in and operating the 

lunar module. Armstrong supported the proposed Dryden 

project and backed the transfer of an F-8C Crusader from the 

U.S. Navy to NASA to become the Digital Fly-By-Wire

(DFBW) research aircraft. It was given the tail number 

"NASA 802." 

Wires from the control stick in the cockpit to the control 

surfaces on the wings and tail surfaces replaced the entire 

mechanical flight-control system in the F-8. The heart of 

the system was an off-the-shelf backup Apollo digital flightcontrol computer and inertial sensing unit which transmitted 

pilot inputs to the actuators on the control surfaces. 

On May 25, 1972, the highly modified F-8 became the first 

aircraft to fly completely dependent upon an electronic flightcontrol system. The pilot was Gary Krier. 

The first phase of the DFBW program validated the fly-bywire concept and quickly showed that a refined system — 

especially in large aircraft — would greatly enhance flying 

Dryden research pilot Gary E. Krier. (NASA photo ECN 3091 

qualities by sensing motion changes and applying pilot 

inputs instantaneously. 

The Phase 1 system had a backup fly-by-wire system in the 

event of a failure in the Apollo computer unit, but it was 

Apollo computer interface box used in the F-8 DFBW program. 

(NASA photo EC96 43408-1) 

never necessary to use the system in flight. 

In a joint program carried out with the Langley Research 

Center in the second phase of research, the original Apollo 

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system was replaced with a triple redundant digital system. 

It would provide backup computer capabilities if a failure 

occurred. 

The DFBW program lasted 13 years. The final flight — 

the 210th of the program — was made April 2, 1985, with 

Dryden Research Pilot Ed Schneider at the controls. 

Research Benefits 

The F-8 DFBW validated the principal concepts of the allelectric flight control systems now used on nearly all 

modern high performance aircraft and on military and 

civilian transports. A DFBW flight-control system also is 

used on the space shuttles. 

NASA 802 was the testbed for the sidestick-controller used 

in the F-16 fighter, the first U.S. high-performance aircraft 

with a DFBW system. 

Among other electronic devices flown on the DFBW F-8 

were an angle-of-attack limiter and maneuver leading- and 

trailing-edge flaps, features commonly used on today’s new 

generation of aircraft. 

In addition to pioneering the Space Shuttle’s fly-by-wire 

flight-control system, NASA 802 was the testbed that 

explored pilot induced oscillations (PIO) and validated 

methods to suppress them. PIOs occur when a pilot overcontrols an aircraft and a sustained oscillation results. On 

the last of five free flights of the prototype Space Shuttle 

Enterprise during approach and landing tests in l977, a PIO 

developed as the vehicle settled onto the runway. The 

problem was duplicated with the F-8 DFBW and a PIO 

suppression filter was developed and tested on the aircraft 

for the Shuttle program office. 

The aircraft was used to develop a concept called Analytic 

Redundancy Management, in which dynamic and kinematic relations between various dissimilar sensors and 

measurements are used to detect and isolate sensor failures. 

In another series of successful tests, a software back-up 

system (Resident Backup System) was demonstrated as a 

means to survive common software faults that could cause 

all three channels to fail. This system has been subsequently used on many experimental and production aircraft 

systems. 

The Dryden project also worked with the British Royal 

Aircraft Establishment using the DFBW F-8 to produce 

ground-based software to use when researchers are investigating flight controls in high-risk flight environments. 

During contingencies, pilots can disengage the ground 

control software and switch to backup on-board controls. 

DFBW research carried out with NASA 802 at Dryden is 

now considered one of the most significant and successful 

aeronautical programs in NASA history. 

Digital fly-by-wire is now used in a variety of airplanes 

ranging from the F/A-18 to the Boeing 777 and the space 

shuttles. 

Specifications 

The F-8 aircraft was originally built by LTV Aerospace, 

Dallas, Texas, for the U.S. Navy, which made it available 

to Dryden as a test vehicle. 

NASA 802: Navy Bureau #145546 

Powerplant: Pratt and Whitney J57 turbojet 

Wingspan: 35 feet 2 inches (350 square feet) 

Overall length: 54 feet 6 inches and height is 15 feet 9 

inches 

Flown as the DFBW testbed by NASA from l972 to l985. 

Fleet F-8s were the first carried based planes with speeds 

in excess of 1,000 mph. LTV won the Collier Trophy for 

its design and development. Total production was 1,261. 

Three view of DFBW aircraft. 

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