Legacy designers tap latest technology for fly-by-wire jets
The high level of technology employed in Embraer’s newest business jet family, the Legacy 450 and 500 program, enables detailed exploration and development to take place much further ahead of first flight, according to Eduardo Camelier, chief test pilot for the Brazilian manufacturer.
“We are ‘flying’ this airplane a lot more before first flight than we did the 170,” he said. Camelier was project pilot for the 170 program and is intimately familiar with the airliner’s fly-by-wire (FBW) system and the development and testing needed to certify that airplane.
It is much easier, with powerful cheap computers, to replicate an airplane by building an engineering test simulator and especially so with FBW designs. The Legacy 450 and 500 are even better suited to simulation, being Embraer’s first design and also the first business jets ever to use a full closed-loop FBW flight control system on all flight controls. On the 170, Camelier said, “We started flying simulation about a year before the first flight. [With the 450/500], we’re already flying maybe two or three years before first the flight. We’re doing a lot more testing on this airplane.”
The Legacy 500 will be the first of Embraer’s new family to fly. First metal was cut in April, and the landing gear has already been assembled. By the middle of next year, the fuselage and wings should come together and the new jet is scheduled to take off during the second half of next year. “We are advancing well on this project,” said Luis Carlos Affonso, Embraer executive vice president executive jets.
Both the 450 and the stretched fuselage 500 share the same Honeywell HTF7500E engines, Rockwell Collins Pro Line Fusion avionics suite with synthetic and enhanced vision and Honeywell Ovation Select cabin management and entertainment system. The jets have flat-floor cabins that feature 6,000-foot cabin altitude at maximum altitude, 40 cu ft of internally accessible heated and pressurized baggage space, a B/E Aerospace vacuum lavatory and club seats that fold into beds.
“There are no airplanes with the combination of characteristics of these airplanes in the market today,” said Affonso. And the technology, from FBW to the avionics and cabin amenities, is just as important to buyers as performance, he explained. “They want to have an airplane that is updated so it preserves residual value into the future. The customer associates technology with safety. It’s on top of their minds. Also they perceive, in terms of fuel efficiency, the ability to optimize routes and spend less fuel and be more green, and they associate new technologies with environmental friendliness.”
FBW is not new. The 1970s-era F-16 fighter was designed with an analog FBW system and airliners–like most Airbuses and Boeing’s 777–employ FBW flight control systems. FBW in business aviation is rarer, with the only certified aircraft so far being Dassault’s Falcon 7X. The Gulfstream G650 has FBW flight controls and should be the next FBW business jet to be certified.
Embraer’s 170/190 family and thus the business jet-derived Lineage 1000 also features FBW, but the Legacy 450 and 500 will be the lowest cost FBW business jets–at $15.25 million and $18.4 million, respectively–and also the first to feature full closed-loop FBW systems. Like the 7X and Airbuses, the Legacy 450/500 will have sidestick controllers. The Lineage 1000 and G650 have traditional yokes, as do Boeing’s 777 and new 787.
Fabrício Reis Caldeira is flight control laws manager at Embraer and a key player in the design of the Legacy 450/500. “We are bringing a technology now available only in larger and more expensive airplanes to the midsize, midlight category of business jet,” he said. “The reason we adopted the sidestick is the reduction in weight, maintenance and spare parts compared to a yoke. It provides a better view of displays in the cockpit.”
In the FBW Legacys, he explained, “pilot inputs are transmitted to the flight control computer through a digital databus, and the flight control computer also receives information from the airplane’s sensors and then provides the command to the remote electronic unit that commands the flight control surface.”
In the 450/500 FBW system, there are only two modes of operation: normal and direct. Normal uses the flight control computers and senses pilot input then commands the flight control surface to move to deliver the result requested by the pilot. Direct mode is used if there is a failure causing the flight control computers not to be available. The controls are still actuated electronically–there is no mechanical backup system–but the pilot’s commands are relayed directly and the flight controls move in proportion to the pilot’s control of the sidestick, instead of the computers selecting the amount of movement in the controls.
What makes the 450/500 FBW system unique is that it is used to drive all the flight controls–rudder, spoilers, flaps, ailerons and elevator–and that it is a closed-loop system. Closed-loop means that the pilot isn’t directly controlling the rate of flight control surface movement. In an open-loop system, the pilot’s movement of the stick, yoke or rudder pedals moves the flight control surface a corresponding amount, which means, said Caldeira, “that there is a direct link between the pilot pedal input and the rudder [or other open-loop flight control]. If the pilot applies full pedal, it means that the rudder is going to move to full authority. When you have a closed loop control, if a pilot pushes the pedal, it provides a sideslip angle command. And then the surface moves but it moves in a more damped way so the pilot achieves the sideslip command that he wants.”
Test pilot Camelier provided more detail. “Normally in a closed-loop control, you’re controlling rates. It you put the sidestick to the left, you’re commanding some roll rate instead of just commanding ailerons and roll spoilers. In our airplane, if [the sidestick] is in the neutral position, it’s commanding zero roll rate. Even if I have a gust or hardover of one of the spoiler panels, of course there’s the transient that’s going to happen, but right after the transient, the airplane will control the rolling motion. That means even with the hardover of a spoiler, it will command a little bit of roll and then it stops because the sidestick is in zero. It’s always commanding zero roll rate. So instead of always having input [equal] control to the surface, input means you’re commanding a rate that you want. And the airplane will do whatever it has to do to give you the rate.”
The only other aircraft with full closed-loop FBW in all axes are the Airbus A380 and Boeing 787, according to Caldeira. The Falcon 7X has mostly closed-loop controls, except for the rudder. The Airbus A340-600 has a FBW rudder, but it is open-loop, he said. The Boeing 777 has closed-loop FBW in the pitch axis and open-loop FBW in the lateral and yaw axes. The Embraer 170/190 uses FBW in pitch and yaw and hybrid FBW in the roll axis, with FBW spoilers and conventional ailerons.
The most sophisticated element of the 170/190 FBW system is an angle-of-attack (AOA) limiter to prevent stalls, Caldeira said. This also helps with a tailstrike protection feature during takeoff and landing and for enabling steep approaches at certain constrained airports, such as London City. Embraer engineers learned a lot about flight controls on the 170/190, he said, and “then we convinced the management that we could do the next step–the first Embraer full fly-by-wire airplane.”
FBW design allows engineers to add envelope protection (see chart on left), one of the benefits of the new technology. But there are different philosophies for envelope protection. Boeing’s 787 has partial envelope protection, according to Caldeira, while Airbus and Dassault employ full protection. The 170/190, with its AOA limiter, offers partial envelope protection.
Embraer’s philosophy is to allow the pilot a lot of latitude within the envelope, and the pilot of a Legacy 450/500 will have the option to command the maximum maneuvering capability, as long as envelope restrictions aren’t violated. The 450/500 will be able to roll at 30 degrees per second, for example, and the pilot can roll at that rate as long as controllability and structural integrity limits are adhered to. In the Airbus, Caldeira said, “for flaps down, they limit the roll rate authority to 7.5 degrees per second. We didn’t want to do that.”
Where an Airbus autothrottle stays in one position for the entire flight, the Legacy 450/500 autothrottle moves as engine power changes. “By looking at the thrust lever position, you know what the engine’s N1 is,” he said. “We think that’s positive feedback, that the pilot knows what the thrust on the engine is by putting his hand on the thrust lever.”
Another feedback feature is the tactile warning in the sidestick, which tells both pilots that they are providing input to both sticks at the same time. There are also audio (“dual input”) and visual warnings. When this happens, say, during a high workload situation when the pilots might not notice the audible or visual warning but will feel the stick vibrate, the pilot who should be flying presses the priority button and takes control.
When both pilots are pushing on their sidesticks, their inputs are summed but that sum is limited to the maximum possible input. The last person to press the priority button has control. Pressing and holding the priority button for 20 seconds gives control to that sidestick.
“The main reason for the priority,” said Caldeira, “is, for example, if you have a jam on one sidestick. It’s very rare but we have to show that it could happen.”
Rudder pedals are mechanically linked to each other, but send electronic signals to the flight control computers. Like the sidestick, the pedals don’t provide positional feedback. To help pilots in an engine-out situation, the rudder is automatically programmed to kick in with 80 percent of the needed control for optimum sideslip. This helps the pilot easily maintain control, but also by not going to 100 percent, the slight turn to one side gives a cue as to which engine has failed. “We want the pilot to recognize the failure,” Caldeira said.
Flying the FBW
If the Legacy 500 flight test simulator is any indication, flying the jet will be easy, requiring just a light touch on the sidestick. Changing the flight path angle is just a matter of a finger-push fore and aft or side to side. The stick is spring-loaded to the center position.
Whatever flight path vector the pilot selects, the airplane automatically stays there, as long as it is within the flight envelope. There is no trim, and in turns, pitch and adverse yaw compensation are automatic, too. “Instead of having constant input to the sidestick,” said Camelier, “you’re flying the sidestick almost as if you were a trim button. So what you do is make small inputs for the minor corrections you want, but never continuously inputting to the sidestick.”
The normal envelope boundaries are 33 degrees bank angle, 1.1 times stall speed, VMO and +30/-15 degrees pitch angle. The limit flight envelope is larger, but the pilot must keep pushing on the sidestick to get into the limit envelope region. To do a 45-degree bank, for example, the pilot simply has to hold the sidestick away from neutral. If the sidestick is released and allowed to go back to neutral, the airplane will return to the boundary or 33 degrees bank angle and stay there.
The edges of the limit envelope are defined as maximum structural load factor, maximum design speed, stickshaker maximum and AOA that doesn’t allow the airplane to stall. There are no hard limits for bank and pitch, and a pilot could roll the 450/500, as long as it is done without jeopardizing controllability or structural integrity. If the jet was in a high-speed condition where it’s not safe to roll, the pilot could not roll.
The real benefits of the 450/500’s FBW system are not just weight savings and easier pilot workload but also performance and comfort. By limiting AOA, for example, the stall speed can be lower because the system does not allow the airplane to stall, but also it doesn’t need a stick pusher, which forces the nose down before stalling and extracts a performance penalty.
The AOA limiter also could help prevent accidents like the Colgan Air Q400 crash in Clarence Center, N.Y., where the pilot reacted by pulling the yoke aft when the pusher fired. “Instead of having something commanding the airplane to go down, the nose of the airplane just stays there. The maximum AOA that can provide a good lift and also good controllability,” said Caldeira.
For windshear or a CFIT-avoidance maneuver, the pilot simply has to pull the sidestick full aft and add full throttle or push the go-around button on the throttle. This provides maximum climb rate without jeopardizing structural integrity, Caldeira said. “We believe by following this procedure, the pilot’s going to achieve a better climb rate than in a conventional airplane, because in a conventional airplane the pilot doesn’t know the structural limit. So he may pull too hard; then, afraid of jeopardizing the structural integrity, he releases; then he pulls too hard again and he releases. With this simple reliable procedure, the pilot is just going to pull up to the aft stop of the sidestick, command full throttle, and then he’s going to know he’s at the limit of what the airplane can give.”
Pilot workload is generally lower in the FBW 450/500 because the system automatically compensates for configuration changes like power changes or flaps and landing gear movement. At different weights, the flight characteristics of ordinary airplanes change, but the FBW system compensates so it feels the same no matter what the controls need to do to deliver the requested performance.
FBW responds more quickly to control inputs, according to Caldeira, because there is no delay as in mechanical controls, especially when an autopilot servo is involved. “By having a faster response with the actuation system,” he said, “you get a more damped response of the airplane to gust or turbulence.”
The 450/500 will be able to fly faster because it can fly closer to the structural limits without compromising safety, Caldeira explained. If speed gets too high, the FBW system moves the elevator to slow the airplane down to Vmo.
“Even if you try to hold full sidestick to the front,” said Camelier, “it won’t let you go above a certain speed above maximum design speed.”
Another benefit of the closed-loop FBW system is that it helps prevent excessive sideslip from heavy rudder use. “When the pilot gets desperate,” Caldeira explained, “he can apply the pedals from one side to the other. Normally these airplanes are not designed to take this type of load.
“With the fly-by-wire system we are in much better shape because even if the pilot applies pedals from one side to the other, it doesn’t mean that [the rudder] is going to move from one side to the other. The problem is that when you do this type of maneuver [in other airplane types] you can add the load generated by the sideslip, and when you provide the pedal to the other direction, you add to the load provided by the rudder as well. So you can have a much higher load on the fin than the airplane is designed for.”
“Even if the pilot tries to break off his vertical fin, he can’t,” said Camelier. “If he tried to go from one sideslip to another as fast as possible, he would never be able to break that tail off because the closed loop drives the sideslip.”
The digital electronic nature of the 450/500 allows engineers to tap into rich sources of data enabled by the FBW computers, including structural integrity monitors. When a pilot runs into heavy turbulence or makes an extra hard landing in a 170/190, the operator has to send data to Embraer to be analyzed to see if an inspection might be needed. With the built-in integrity monitors on the 450/500, a crew alerting system message will say whether or not further inspection might be
necessary after a hard landing or turbulence encounter, saving lots of back-and-forth analysis with the factory.