Digital Fly By Wire

Well into the 1980s, mechanically controlled aircraft were the norm. A vehicle’s control surfaces were operated through cables and pushrods physically connecting the aircraft’s wings and surfaces to the pilot’s control sticks and rudder pedals.

But a transformation began taking root as early as 1970, when a Dryden team visited NASA Headquarters and proposed an advanced aircraft controlled by a fly-by-wire system—with no mechanical backup.

In a fly-by-wire system, a computer collects sensor data from the pilot’s controls and uses those signals to move the aircraft’s control surfaces accordingly. It’s control-by-computer as opposed to control-by-cable.

Dryden researchers had developed significant experience in electronic flight controls through the development of experimental aircraft. In fact, the Lunar Landing Training Vehicle NASA used to train the Apollo spacecraft commanders employed an analog fly-by-wire system with no mechanical backup—making it the first genuine fly-by-wire vehicle.

But these systems all used analog computers, not digital ones, and at the time there were no flight-qualified digital computers for airplanes. The aircraft the Dryden team proposed looked like it might be too futuristic to pursue at the time.

One Giant Leap for Aircraft

In an unexpected development, the idea gained a well-known figure as a supporter: Neil Armstrong, himself a former Dryden test pilot. Armstrong had recently flown to the Moon and back with his life entrusted to the Apollo Primary Guidance, Navigation, and Control System (PGNCS, pronounced “pings”)—a fully digital guidance computer.

Through Armstrong’s support, and that of a US Navy vice admiral, Dryden acquired a trio of F-8C Crusaders from the Navy and, working with Draper, the Center installed an adapted extra Apollo PGNCS on one of the planes, which became the Digital Fly-by-Wire (DFBW) research aircraft. Another of the F-8s was converted into an “Iron Bird” ground-based simulator for testing the flight software and training pilots, and the third F-8 was used to familiarize test pilots with the aircraft.

More than 200 successful test flights later, the team had proven that, not only could a digital computer could be used to fly an aircraft, but it could be done practically in commercial and government aircraft designed and flown around the world.

“We Were Able to Transfer a Lot of Technology”

“Some of the techniques we developed at that time are still being used, and that spawned the digital fly-by-wire revolution,” says Ken Szalai, who was a young engineer at Dryden at the time and went on to become center director before retiring in 1998. “We communicated with all of the major airframe manufacturers and were able to transfer a lot of the technology.”

The first commercial airliner to fly with DFBW was the Airbus 320 in 1987, followed by Boeing’s 777 in 1994. Today, the technology features in a number of aircraft from both manufacturers.

For commercial aircraft, the technology replaces heavy mechanical systems, allowing airlines to benefit from greater fuel efficiency or carry more passengers and cargo. The heightened responsiveness of DFBW-enabled aircraft allows pilots to provide a smoother flight, and the system’s redundancies help ensure safe operation of the vehicle. Mechanical maintenance needs are also reduced, saving costs and time spent on upkeep and repairs of the mechanical systems and reducing the chance of failures.

 “Digital fly-by-wire has unshackled designers from the rules of the 1950s and 1960s, so you end up with vehicles like the Space Shuttle, the B-2 bomber, and the F-117. You couldn’t have these kinds of aircraft without a fly-by-wire system,” says Szalai.

“One of the values of flight research is that reality meets with dreams and visions,” he says. “They meet and they clash. You have to solve the real world issues. You can’t always take a picture of the spinoff. Sometimes it’s showing that you can do something.”