Listen to an Embraer technical briefing on the Legacy 500/450 fly-by-wire system with Fabrício Caldeira, flight control laws manager (first speaker), and chief project pilot Eduardo Camelier:For as long as there has been fly-by-wire on aircraft, there's been a debate about how to best utilize the electronic flight control system and where to draw the line between pilot freedom and hard and fast boundaries protecting the aircraft and its occupants. This debate is far from settled with the most famous dispute between Boeing and Airbus charting different courses through computer driven flight control actuation.
On the one hand, Boeing leaves the pilot's judgment at the forefront, allowing overspeeding, stalling and over-banking within the flight envelope. The aircraft will let the pilot know, loudly, that they are approaching, or in, a potentially unsafe condition for the aircraft. Additionally, Boeing aircraft include an auto-throttle system resulting in the back-driven motion of the throttle quadrant providing a tactile cue to pilots without referring to the EICAS.
For Airbus, fly-by-wire has resulted in hard limits on the aircraft flight envelope, preventing over-speeding, stalling, and over-banking of the aircraft. The maximum bank allowed is 67-deg, with nose-down pitch not exceeding 15-deg and a 2.5g limit. An auto-thrust system complements the A-floor protection by automatically spooling up the engines, limiting nose up pitch (angle of attack) to prevent the aircraft from stalling and providing best climb performance.
While these have generally been two polar points on the augmented flight control spectrum, Embraer has charted its own path to full fly-by-wire for its first implementation on its all-new 10-passenger mid-size Legacy 500 business jet, due for entry into service in late 2012, followed by the smaller Legacy 450 in 2013. The aircraft were designed around their respective flight control systems allowing Embraer to optimize the structural sizing based upon the built-in protections.
As it fleshed out the elements of its fly-by-wire philosophy, Embraer drew on the lessons learned from notable accidents over the years that involved human factors resulting from improper aircraft handling.
What Embraer has created is its own course for implementing fly-by-wire technology and the Legacy 500 is its first platform. The path that Embraer has laid out for itself will undoubtedly become a hallmark of its flight control philosophy and will find its way the Brazilian airframer's next generation commercial aircraft.
As it fleshed out the elements of its fly-by-wire philosophy, Embraer drew on the lessons learned from notable accidents over the years that involved human factors resulting from improper aircraft handling.
What Embraer has created is its own course for implementing fly-by-wire technology and the Legacy 500 is its first platform. The path that Embraer has laid out for itself will undoubtedly become a hallmark of its flight control philosophy and will find its way the Brazilian airframer's next generation commercial aircraft.
Photo Credit Embraer
The Legacy 500 finds a balance between the Airbus and Boeing philosophies as it introduces a mechanically-linked sidestick controlled all-axis closed-loop fly-by-wire for the aircraft's elevator, rudder, aileron and spoiler; a first for an aircraft of its size. Embraer first implemented fly-by-wire on the AMX in the 1980s and in the pitch and yaw axis on the E-Jet family.
So what's the difference between open and closed-loop flight controls?
Open-loop fly-by-wire controls resemble those of a conventional flight control system. When a pilot makes a control input, the flight control computer commands the surfaces to achieve a pre-determined deflection. Depending on many factors, the resulting response of the aircraft may vary. For example, on a Cessna 172, a turn of your yoke means the same deflection of your ailerons, but you'll get very different response if you're at 65kts on final or 120kts in cruise.
Closed-loop fly-by-wire controls result in a pilot input producing a pre-defined rate of roll, pitch or yaw, providing unchanged handling no matter the speed of the aircraft. The deflection of each control surface may differ throughout the flight envelope, but the resulting behavior of the aircraft is intended to be identical.
Embraer's three-axis closed-loop fly by wire allows the Legacy 500 to define the edges of its performance, while ensuring the structural safety of the aircraft. Ultimately, the 500's control laws are "flight path stable", meaning that the "system will maintain the airplane's flight path vector when sidestick is in neutral position" in the normal flight envelope with its autotrim function, says Embraer.
Embraer's three-axis closed-loop fly by wire allows the Legacy 500 to define the edges of its performance, while ensuring the structural safety of the aircraft. Ultimately, the 500's control laws are "flight path stable", meaning that the "system will maintain the airplane's flight path vector when sidestick is in neutral position" in the normal flight envelope with its autotrim function, says Embraer.The "normal flight envelope" does not establish hard operating limits, but rather "soft limits" because pilots can fly beyond them if desired, though continuous sidestick back pressure is required. The borders of this normal area includes 33-deg of bank angle, a velocity of 1.1 times the stall speed, maximum operating velocities (Vmo), and +30/-15-deg pitch angle. Releasing the sidestick outside of these limits will return to the normal flight envelop.
Where Embraer has imposed hard operating limits is not on pitch or bank angle, but rather on the aircraft's design limits and ensuring controllability and safe structural loads, as defined by "design diving speed, stall (maximum angle of attack), structural load factor and maximum sideslip." For example, in overspeed situations beyond Vmo, the aircraft will automatically return to the normal flight envelope.
In the yaw-axis the closed-loop system also prevents the pilot from over-stressing the vertical stabilizer, a system designed to prevent accidents such as American Airlines 587, in which an Airbus A300 severed its vertical tail fin after flying through the wake of a Boeing 747 on departure from John F. Kennedy International Airport.
"Even if the pilot tries [intentionally] to break off his vertical fin, he can't," says chief project pilot Eduardo Camelier. "If he tries to go from one sideslip to another fast, as hard as possible, he will never break that fin off, because the closed loop drives sideslip and not an open loop control that can go above the maximum structure."
Where Embraer has imposed hard operating limits is not on pitch or bank angle, but rather on the aircraft's design limits and ensuring controllability and safe structural loads, as defined by "design diving speed, stall (maximum angle of attack), structural load factor and maximum sideslip." For example, in overspeed situations beyond Vmo, the aircraft will automatically return to the normal flight envelope.
In the yaw-axis the closed-loop system also prevents the pilot from over-stressing the vertical stabilizer, a system designed to prevent accidents such as American Airlines 587, in which an Airbus A300 severed its vertical tail fin after flying through the wake of a Boeing 747 on departure from John F. Kennedy International Airport.
"Even if the pilot tries [intentionally] to break off his vertical fin, he can't," says chief project pilot Eduardo Camelier. "If he tries to go from one sideslip to another fast, as hard as possible, he will never break that fin off, because the closed loop drives sideslip and not an open loop control that can go above the maximum structure."
Additionally, what this means is Legacy 500 and 450 will be safely capable of a full 360-deg roll, as long as the aircraft remains within its normal operating envelope.
Embraer opted for an angle of attack (AoA) limiter for the Legacy 500 - first implemented on the E-170 and E-190 - which allows for a reduction in the stall speed as compared to a stick pusher, limiting airspeed to 1.23 times stall speed in approach and 1.13 at take off.
This was crucial for achieving safe performance into airports such as London City, says Fabrício Caldeira, Embraer flight control laws manager. This system also enables a max AoA for controlled flight into terrain (CFIT) avoidance and windshear escape along with full aft stick and full throttle to reach maximum climb rate.
Embraer says it replicated the conditions of Colgan Air 3407, in which a Bombardier Q400 crashed on approach to Buffalo International Airport following a stall and improper response to a stick pusher activation. The airframer found that its AoA limiter would not allow an E-Jet to enter a to stall in similar conditions.
For approaches down to CATII minimums, Embraer eliminated the requirement for a radio altimeter on the aircraft and developed a control law for approach and flare based on flap and landing gear position.
"During the flare what's natural for the pilot is to always command a positive sidestick deflection to the aft direction," says Caldeira. "So that's the reason why we had to switch control laws, because if we didn't switch the pilot would have to push the sidestick during the flare and that's not natural."
This was crucial for achieving safe performance into airports such as London City, says Fabrício Caldeira, Embraer flight control laws manager. This system also enables a max AoA for controlled flight into terrain (CFIT) avoidance and windshear escape along with full aft stick and full throttle to reach maximum climb rate.
Embraer says it replicated the conditions of Colgan Air 3407, in which a Bombardier Q400 crashed on approach to Buffalo International Airport following a stall and improper response to a stick pusher activation. The airframer found that its AoA limiter would not allow an E-Jet to enter a to stall in similar conditions.
For approaches down to CATII minimums, Embraer eliminated the requirement for a radio altimeter on the aircraft and developed a control law for approach and flare based on flap and landing gear position.
"During the flare what's natural for the pilot is to always command a positive sidestick deflection to the aft direction," says Caldeira. "So that's the reason why we had to switch control laws, because if we didn't switch the pilot would have to push the sidestick during the flare and that's not natural."
Embraer also incorporates a back-driven auto-throttle to enable tactile and visual cues for the crew, as well as vibrating tactile, aural and visual warnings in the case of an accidental dual input on the sidestick. In this instance, the inputs from pilot and co-pilot are summed together.
Camelier says a back-driven auto-throttle was selected after reviewing Airbus incidents, such as Air France 296 and TAM 3405 where there was "confusion related to what the auto-thrust was doing."
Camelier says a back-driven auto-throttle was selected after reviewing Airbus incidents, such as Air France 296 and TAM 3405 where there was "confusion related to what the auto-thrust was doing."
Further, the integration of the flight control system is aimed at reducing crew workload by incorporating features once exclusively reserved for much larger aircraft, by compensating for different configuration changes in the thrust, flaps and landing gear. For example, in the event of an engine shutdown resulting in asymmetric thrust, the aircraft will maintain a "slightly divergent heading and residual slideslip" with 80% required rudder while keeping the pilot in the loop.
Ultimately, all of these flight control system features are available in normal mode, though Embraer emphasized that the Legacy 500 and 450 are speed stable by design and normal mode system failure allows the aircraft to be controlled in direct mode like a traditional aircraft, though an open-loop operation for the control surfaces.
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