Imagine you’re departing a high elevation airport on a warm day, surrounded by terrain, into poor visibility. You’ve carefully and methodically reviewed performance charts, and meet obstacle clearance requirements should you encounter an engine failure during climb out. Despite your calculations affirming you’re safe to proceed, you mentally review the single-engine climb scenario you rehearsed countless times in the simulator. You’re prepared, but wouldn’t it be reassuring to know if you do experience an engine failure the airplane will automatically configure itself for optimal climb performance?
Flight control engineers have examined this very scenario, and developed fly-by-wire (FBW) control laws that provide automatic compensation for asymmetric thrust associated with a failed engine. During such an event in a FBW aircraft, the pilot simply adjusts the airplane for peak performance, which the FBW system maintains so the pilot can remain situationally aware and execute emergency procedures.
If you follow the super midsize and large business jet markets, you’ve likely heard of FBW. As the technology is increasingly implemented in newly certified business jets, many are still learning what it is, what it does, and why it’s significant.
In basic terms, FBW replaces traditional mechanical cable and pulley systems linking cockpit flight controls to aircraft control surfaces (ailerons, rudder, elevator and spoilers) with a network of computers, electrical wires, servos and actuators. When a maneuver is commanded by the pilot, FBW computers convert the command into a digital signal which is sent to hydraulic actuators on the corresponding control surface(s). Essentially, the pilot tells the FBW computers where he wants the airplane to go, and the system responds accordingly.
Traditional aircraft are controlled through a series of complex cables, pulleys and push rod actuators.
The term fly-by-wire is most commonly used term to describe the technology, however it’s often misunderstood. For that reason, most aircraft manufactures prefer the term “digital flight control system” (DFCS). To differentiate FBW from conventional flight control systems, think of traditional aircraft as “fly-by-cable-and-pulleys” (FBCP).
While relatively new to business aviation, FBW has been around for decades. After years of research, and several successful hybrid (combined digital and mechanical control) FBW applications, in May, 1972, NASA flew the first FBW aircraft not equipped with backup mechanical controls. This highly modified F-8 Crusader successfully demonstrated its safety, efficiency and performance benefits while setting the groundwork for technology that would be used in later NASA programs such as the Space Shuttle. Spring-boarding the technology to the forefront of military aviation, by the late 1970s FBW was in full scale production on the era’s latest fighter jets including General Dynamic’s F-16 and Dassault’s Mirage 2000. Today’s most advanced jet fighters, such as Lockheed Martin’s F-22 and F-35, are FBW aircraft.
Its commercial aviation debut came in 1976 with the supersonic Concorde. Since then, Boeing introduced FBW on its 777 and 787 and Airbus on its A320 and A380. FBW has migrated to regional airliners as well. For example, Bombardier’s recently certified and controversial CSeries and Embraer’s soon to be certified E2 lineup.
Certified in 2008, Dassault’s Falcon 7X became the first FBW equipped business jet. Recognizing its safety, performance and manufacturing advantages, other business jet manufacturers quickly followed suit. Gulfstream certified the FBW equipped G650 in 2012. Embraer followed with the Legacy 500 in 2014. On track for certification in late 2018, Bombardier’s Global 7000 will be its first FBW business jet. Gulfstream’s newest two designs, the G500 and G600 will also feature FBW. Development of Textron Aviation’s first full FBW aircraft, the Citation Hemisphere, is underway. First flight is expected in 2019.
Of the many reasons aircraft manufacturers are undertaking the considerable effort and investment to develop FBW, one is the reduction in mechanical complexity. Aircraft engineers no longer need to route cables through a complex series of pulleys scattered throughout the airframe. Lighter than its cable and pulley predecessors, FBW applications save weight allowing engineers more freedom to incorporate additional interior features and/or larger fuel tanks.
Most FBW aircraft feature side stick flight controls in lieu of traditional control columns. Weighing less than their mechanical counterparts, side stick controls open up considerable cockpit real estate. Most pilots I’ve spoken with report the side stick is intuitive, and an easy transition.
Photo credit: Dassault Aviation
From a safety standpoint, the most notable FBW benefit is “envelope protection” whereby pilot commands are monitored by flight control computers to ensure aerodynamic and structural limits, AKA the “flight envelope”, are not exceeded. System logic varies according to the manufacturer, however in general FBW aircraft are resistant to aerodynamic stalls. Some aircraft, such as the Legacy 500, are simply impossible to stall, provided the system is functioning normally.
During my time selling new aircraft at Embraer Executive Jets, I had numerous opportunities to participate in Legacy 500 pilot demonstrations. After climbing to a safe altitude, the airplane would be slowed and configured to simulate a take-off / climb situation. Number 1 engine power was then reduced to flight idle, simulating an engine failure on climb out. Amazed at the lack of pilot intervention required to maintain coordinated flight, the most common reaction was simply, “WOW!!” Embraer’s control laws are so precise, its engineers could program the aircraft to compensate for 100% of the asymmetric thrust. To ensure the pilot is always “in the loop”, Embraer programmed 80% asymmetric thrust compensation, requiring the pilot to trim out the remaining 20%.
FBW system logics are designed to automatically compensate for asymmetric thrust in the event of an engine failure. The crew is alerted of the condition and prompted by the FBW computer to trim for optimal sideslip.
Envelope protection reduces aerodynamic forces on the airframe allowing engineers to incorporate smaller and lighter airframe structures. For example, the Legacy 500 has a smaller horizontal stabilizer than its similarly sized and FBCP competitor, Challenger 350. Smaller airframe structures weigh less and reduce drag, which requires less power and improves fuel economy.
Most aircraft manufacturers have determined FBW is a worthy undertaking given its performance, safety and design advantages. Development comes at considerable cost. As manufacturers look to recoup their FBW investments, the technology will remain exclusive to the midsize and large jet markets for the foreseeable future. That said, we should anticipate fly-by-wire in the light jet market in the decades ahead.