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FBW EVOLUTIONS
By Fernando Alonso
INTRODUCTION
The FBW concept applied to all new Airbus Industrie aircraft since the launch of the A320 is now mature. This maturity has been the result of the consideration of customers’ requests stemming from the in-service experience and from the continuous research of new technological features to adapt and improve the core of the system to the particularities of the new models. In the years ahead, Airbus will be developing new and very large aircraft which introduce new constraints and technological challenges in the areas of Flight Controls and FBW.
Airbus Industrie and its partners actively participate in research activities to provide design solutions to the issues raised by our future programs. As was always the case, the technology will be introduced in as much as it is demonstrated that it provides the most overall efficient design and operational solution to a given problem.
This presentation is an overview of some of the most prominent research activities in the area of Flight Controls.
1. FLIGHT CONTROLS FOR FLEXIBLE AIRCRAFT
The design of very large aircraft such as the A340-600/-500 and the A3XX bring a new challenge to the structural designers. The optimization of aircraft performance, new demands on passenger comfort levels and good handling characteristics for the pilots can be conflicting objectives. For structural considerations, performance reasons and passenger comfort, the larger the aircraft the more flexible it should be. However the more an aircraft is flexible the more its natural vibration frequencies will decrease to the point where they may be close to the normal piloting frequencies. Furthermore, due to the large vibration amplitudes induced by external means (gusts, turbulence), the likelihood that the pilot may launch or increase the oscillations by an involuntary action on the sidestick is increased. This is clearly detrimental to the objective of good handling qualities.
A new concept for flight control laws has been developed by Airbus Industrie partner Aérospatiale to be applied for the next very large aircraft projects.
1.1. Evolution of flight control laws architectures
As the FBW aircraft have become larger and more flexible, the flight control laws architectures have gradually been adapted to provide the best possible handling characteristics and the highest level of passenger comfort.
On the A320/A319/A321 family, which can be considered as "rigid" aircraft, the control laws were simple since they only had to consider the "control objectives" i.e the pilot’s (or autopilot) control inputs. Due to the limited amplitude of the structural vibrations no filters were required. The principle is shown in the following figure.
Electrical flight control system for a rigid aircraft
On the A340-300/-200 and on the A330 this simple architecture had to be adjusted since the aircraft were larger and more flexible. The natural vibration frequencies were close to the piloting frequencies but sufficiently far from them such that simple filtering algorithms were enough to provide good handling characteristics. Due to the larger amplitudes and lower frequencies of the structural modes the feedbacks used for the flight control laws also had to be filtered and a specific "Comfort In Turbulence" function was developed as an add-on to the basic flight control law as shown in the following figure.
Turbulence sensitivity: the A340 solution
This architecture could be extrapolated further and applied to the A340-600/-500. But the amount of filtering required to prevent pilot/structure and feedback/structure coupling would be such that the aircraft’s handling would have deteriorated due to the increase in the time delays between the pilot’s input and the aircraft’s response and also due to the limitation of the feedbacks gains. Furthermore, to optimize the CIT function, additional specific sensors need to be installed along the fuselage of the A340-600/-500 ; these sensors make it possible to switch from an overall filtering approach to a more suitable mode-by-mode dumping approach. The CIT function can no longer remain an add-on item but will be an integral part of the flight control laws. This integration will provide a significant simplification of the flight control law architecture similar in level to that of the "rigid" aircraft where the "Control Objectives" are, in fact, replaced by "Control and Structural Objectives".
Flight control system: covas architecture
1.2. Flight control laws for the A340-600/-500
The principle of integrating the "Control and Structural Objectives" in the basic flight control laws for the A340-600/-500 has been validated during two flight test campaigns using A340 MSN 1. These campaigns validated the concept and demonstrated the feasibility of controlling the structural modes without degrading the handling qualities.
The damping of the structural modes of the A340-300 achieved with the integrated laws was similar to that achieved with the basic CIT law and there was no undesired coupling of the "structural" and "control" orders. This demonstrated the feasibility of integrating an efficient comfort improvement function with the flight control laws.
The aircraft’s response to pilot inputs throughout the flight envelope was never degraded. In some extreme maneuvers, the reduction of time delays due to the elimination of the filters improved the performance of the flight controls. The piloting accuracy was not affected by the integration of the "structural objectives". These results confirmed that, from an operational point of view, this new flight control law architecture will be totally transparent to the flight crew. The A340-600/-500 behavior in the various flight control laws will be similar to that of the A340-300/-200 and, therefore, this new concept of flight control law architecture will have no impact on operations, training and on the "Same Type Rating" concept for the entire A340 family despite the basic degradation of the unaugmented aircraft characteristics.
2. THE A340-600/-500 ELECTRICAL RUDDER CONTROL
2.1. Recall of A340-300/-200 rudder control
The transmission from the rudder pedals to the servo controls is via a single-path mechanical (cable) linkage. The yaw damper orders (turn coordination, Dutch roll damping ...) are added to the "mechanical" order in the differential mechanism such that the actual rudder deflection which is always the sum of the pedal input and the yaw damper order, is equal to the yaw control law order resulting from the lateral law computation. The rudder travel limitations required for structural reasons is performed by two mechanical units (RTLU and PTLU) each driven by two of the Secondary Flight Control Computers (SEC). A rudder trim actuator is added in the rear part of the aircraft which is also driven by the two SEC’s.
A340-300 Rudder control
The mechanical Back-Up control of the rudder allows the transient stabilization and control of the aircraft while the systems reconfigure following a total loss of the normal electrical power or of the EFCS computers.
Due to the Dutch roll damping characteristics of the A340 a Back-Up Yaw Damper Unit (BYDU) was added to provide good directional control and damping in this case. It is activated upon double Yaw Damper failure.
2.2. A340-600/-500 rudder control description
The increased flexibility of the A340-600/-500 compared to the -300 and the desire to optimize passenger comfort imposes severe requirements on the rudder control system architecture and on the performance levels of the rudder actuators themselves. Very small, accurate and rapid rudder deflections will be required to achieve the target comfort levels. These values are not easily achievable with the current A340 architecture due to inaccuracies introduced by the mechanical linkage. The kind of accuracy and speed required can be achieved more efficiently with a totally electrical rudder control (as is the case for the elevators and ailerons): rudder pedal position sensors will transmit pedal position to the EFCS computers which will compute the required rudder deflection and electrically send the command to the actuators. To further simplify the system, the rudder travel limitation units and yaw dampers actuators will be deleted and these functions will be integrated in the yaw control law within the EFCS computers.
In order to retain the concept of a mechanical linkage for Back-Up rudder control, a clutch would need to be developed. In normal operation, when the rudder is controlled (electrically) by the EFCS computers, the mechanical linkage should be de-clutched to eliminate degrading the performance of the electrical rudder control. In case of rudder Back-Up, this new unit would connect the rudder pedals to the rudder servo controls (thus re-establishing the mechanical pedal-rudder linkage). This solution would introduce additional and new failure cases (undue clutch operation, jamming ...) which would potentially deteriorate the availability of the "mechanical Back-Up rudder control" relative to that of the A340-300/-200.
To avoid this deterioration of the reliability and safety of the rudder Back-Up control it was decided to replace the "mechanical" Back-Up control by an electrical alternate rudder control, with sufficient "autonomy" "segregation" and "dissimilarity" relative to the normal control such as to provide equivalent "Back-Up" capability.
The schematic of the A340-600/-500 Rudder Control is shown in the following figure.
A340-600 electrical rudder control
This design has the following advantages:
Simplification and improved reliability due to the deletion of most of the mechanical parts (with the exception of the trim actuator which is relocated in the forward part of the aircraft).
Improved availability of the normal rudder control: Three paths compared to the "single-path" mechanical linkage.
Improved rudder performance for better control and passenger comfort.
Reduced maintenance costs and weight due to simplified design concept.
Pilot - aircraft interface is unchanged: The new design will be transparent to the crew.
2.3. Back-up rudder control on A340-600/-500
The Back-Up rudder control on the A340-600/-500 remains available to cover the same type of generic failures as on A340-300.
The Back-Up Control Module (BCM) is automatically activated in case of total loss of rudder control through the EFCS computers. It includes its own electrical power supply composed of two dedicated hydraulically powered electrical generators connected to the aircraft’s B and Y hydraulic systems. The BCM receives inputs from a specific gyro (for yaw damper) and dedicated pedal and rudder position sensors to control the rudder through the B or Y actuators.
Following the total loss of rudder control through the EFCS computers, the BCM will provide:
Rudder control
Dutch roll damping (as done by BYDU on A340-300)
Speed-scheduled rudder deflection limitation
3. NEW SOURCES OF HYDRAULIC POWER
A number of research projects are active to develop alternatives to the "conventional" hydraulic actuators used on aircraft. Airbus Industrie and its partners have taken a leading role in most of the international programs due to the potential use of these new equipment in future Airbus aircraft. One of the most promising concepts is the EHA (Electro-Hydrostatic Actuator).
3.1. What is an EHA?
A conventional actuator is connected to the central hydraulic system of the aircraft through pipes running from the aircraft’s hydraulic bay. It includes a servovalve which directs the hydraulic pressure (provided by the aircraft’s hydraulic pumps) to move the actuator shaft which is connected to the corresponding control surface (aileron, elevator, rudder, spoiler).
The EHA is connected to the aircraft’s electrical network (and not to the hydraulic system). It has a self contained electrical hydraulic pump, reservoir and accumulator which generate the hydraulic power required to move the same actuator shaft connected to the control surface. Therefore, the EHA is a real hydraulic actuator since the power to move the actuator shaft is hydraulic; however, since the hydraulic system is self-contained, it only requires electrical power from the aircraft’s network to operate.
EHA will allow to decentralize hydraulic system architectures. Kilometers of hydraulic pipes, pumps, reservoirs and accumulators which generate hydraulic power in a central location and then distribute this energy throughout the aircraft will be replaced by electrical wires and compact actuators which produce the energy required near the place where it is needed.
3.2. Status of EHA research
The research activities on EHA have been running for the past 5 - 6 years with the dedicated involvement of Airbus Industrie and its partners. The major milestones are:
1992: Inflight evaluation (50 FH) on A320 MSN 1 of an aileron EHA
1996: 80 flight hours evaluation of an aileron EHA on A321 MSN 364
1997 - 98: Over 500 hours operation of an EHA on the A320 Iron Bird
1998 - 99: Development and inflight evaluation of a high power EHA for A330/A340 inner aileron
The EHA technology has developed rapidly and there are five manufacturers in Europe able to support any new project.
Currently the research activity is focusing on actuators for flight control surfaces. Other applications such as cargo door actuators, braking actuators, thrust reverser actuators, nosewheel steering and landing gear extension/retraction actuators are being considered. The feasibility of EHA for "small" actuators (A320/A321 ailerons) has been demonstrated as well as the capability of these actuators to sustain extreme environmental conditions and electromagnetic interferences (EMI).
If the remaining tests to be completed during 1998-99 are satisfactory, the EHA technology for flight controls actuation should be mature for application on any future project.
3.3. Benefits of EHA
3.3.1. Simplification of the hydraulic architecture
The design objectives of hydraulic systems for large civil transport aircraft go much beyond the obvious requirement of providing hydraulic power to all consumers: Flight Controls, Flaps/Slats, braking, steering, landing gear operation ... Constraints are imposed on the designer by either the Regulatory Requirements, the configuration/size of the aircraft, the maintainability expectations of customers and last, but not least, the available technology. As an example, the design of hydraulic systems must consider aircraft operation in failure conditions: The hydraulic distribution must ensure that following a single or double (engine burst scenario) hydraulic failure the aircraft’s flight controls remain available even if degraded or with reduced power. Due to the extremely low probability of a total hydraulic failure on modern aircraft designs, the Certification Regulations impose no requirement in case of a total hydraulic failure; yet there have been a couple of accidents caused by the total loss of hydraulic power. Finally, as the size of the aircraft increases, the hydraulic requirements also increase leading to either very large and complex hydraulic systems or to the need of adding additional systems.
EHA applied to flight controls will be a significant technological breakthrough in hydraulic system design for the following reasons:
The total hydraulic failure can be covered by using EHA.
The consequences of single or multiple hydraulic failures will be minimized by the use of EHA as Back-Up actuators on flight control surfaces.
The use of EHA on all flight controls will simplify the hydraulic distribution network and allow to reduce the number of central hydraulic systems.
The size of central hydraulic systems can be reduced.
On an aircraft of the size of the future Airbus A3XX, the use of EHA offers new technological solutions for the design of the hydraulic systems. On this kind of aircraft if EHA were not available there would be debate as to whether three or four hydraulic systems would be necessary. Each of these two alternatives has some advantages but also some disadvantages. The design of the A3XX hydraulic/flight control system
is considering the application of EHA as a means of simplifying the central hydraulic systems and being able to survive a total hydraulic failure. The following schematic provides one of the possible solutions for the A3XX Flight Controls architecture with the use of EHA.
A3XX flight controls with EHA
3.3.2. Weight reduction
Up to 1 ton could be gained in the weight of an A340 if all the hydraulic actuators and circuits were to be replaced by EHA. This is, in the short term, a somewhat theoretical and unrealistic scenario since the technology is not yet mature to eliminate all central hydraulic circuits from a large transport aircraft. However a more realistic scenario might be the elimination of one hydraulic system and replacing it by EHA on the various control surfaces. With an A340-type architecture, the weight saving would be in the order of 250 kg.
3.4. Operational impact of EHA
The introduction of EHA technology on aircraft will have no operational impact: It will be transparent to the flight crew.
4. IMPROVED ENGINE-OUT HANDLING
Some flight crews have frequently stated that in order to minimize the yaw excursions induced by power changes during engine-out approaches, they prefer to disengage the Autothrust (ATHR) to ease the lateral and directional control of the aircraft. During the development campaign for the A330-200 a new "Engine-Out Yaw Damper" has been developed. The objective of this new feature is to improve the aircraft’s handling during engine-out operations.
4.1. Description
When an engine failure (real or simulated) is detected by the Flight Control Computers, the gains of the yaw damper are switched from the current values to a new set which, in fact, increase the directional stability of the aircraft. In simple terms, when these new gains are active, the yaw damper reacts to lateral asymmetries quicker in order to minimize yaw variations.
This new feature, due to its simplicity, has required minimum changes to the Flight Control Computers since it uses the already existing engine-out detection algorithms and Yaw Damper laws. The modification to the PRIM software has been limited to the introduction of the new set of Yaw Damper gains and the development of the switching logic from the all-engine to the engine-out gains.
4.2. Operational impact
Aircraft handling with all engines operating is unchanged.
Yaw excursions due to asymmetric thrust changes are significantly reduced thus improving the engine-out handling. During engine-out approaches these yaw variations have been reduced to the point where these approaches can be flown just as comfortably with or without ATHR.
This new law provides the same capability to de-crab during cross-wind landings as with all engines operating and thus imposes no additional operational restrictions.
Flight crew training is unaffected. We expect that pilots will use ATHR during engine-out approaches and find the comfort level and ease of flying significantly improved relative to the current standards.
As a by product of this new feature, the built-in engine failure compensation of the AI FBW laws will be slightly improved. As with the current laws, should an engine fail on take-off, if there is no pilot action on the rudder pedals, the aircraft will settle down at a certain bank angle and sideslip. With the new EO Yaw Damper the aircraft will stabilize at a smaller bank angle and sideslip but will still provide unmistakable clues of the engine failure. As with the previous flight control laws standards the aircraft’s response to an engine failure will prompt a pilot action which, with such behavior, need not be immediate. This is in line with the AI strategy to provide good engine failure identification and automatic compensation without the need to develop sophisticated engine failure detection algorithms which, inevitably, reduce the integrity and robustness of the system and may lead to the non-detection of real engine failures in specific failure situations.
4.3. Status
The new Engine-Out Yaw Damper is now undergoing extensive testing on the development A330-200. It will be evaluated by AI, JAA/FAA and Customer pilots and will be tested during cross-wind take-offs and landings with one engine inoperative. If the results of this evaluation campaign are conclusive this new feature will be introduced on the PRIM computers of the A330-200/RR (PRIM P3). This computer standard is currently scheduled to be retrofitted to the entire A330-200 fleet which will thus be the first model to benefit from this improvement. It is our objective to apply this EO Yaw Damper to the entire A330 and A340 family at the next possible opportunity.
5. ENHANCED DUAL INPUT AWARENESS
All Airbus FBW aircraft are fitted with uncoupled sidesticks. This choice of the sidestick was taken prior to the launch of the A320 due to the obvious benefits it provided for the operation of a Fly-by-Wire aircraft. The choice of non-coupled sticks, also taken at that time, was dictated by the available technology. With the technology of the eighties the only real choice was to couple both sidesticks mechanically; this would have unduly increased the complexity of the system and would have left the aircraft prone to failure cases (stick jamming) and common point typical of (and unavoidable) "mechanical" aircraft. With over 10 million FH of the AI FBW aircraft and over 15000 pilots qualified the initial debate on the merits and drawbacks of non-coupled sidesticks has ceased because flight crews have learnt to appreciate the advantages brought by the current system.
The feed-back received at Airbus from operators and pilot unions and the routinely analysis of in-service events indicate that there are still two areas where some pilots would appreciate some enhanced feed-back of "what the other pilot is doing". These areas are:
- Training flights.
- In a high stress environment there have been prolonged and undetected dual input situations.
Airbus Industrie continues to explore all possible features available which might provide a reply to these concerns. Research activities have been conducted over the past months in the following areas:
a) Evaluation of electronically coupled sidesticks
b) Evaluation of features which may prevent prolonged dual input situations
5.1. Electronically coupled sidesticks
Electronically coupled sidesticks (so called active sidesticks) offered by various aerospace equipment manufacturers have been evaluated and analyzed by Airbus Industrie. Despite the diversity of the proposed solutions, all the systems rely on high speed torque motors to move one sidestick to the same position of the other sidestick. All suppliers provide adjustment capabilities (forces, thresholds, displacements...) to match the sidestick characteristics and feel to the current Airbus sidesticks.
The evaluations of electronically-coupled sidesticks covered the following issues: System safety analysis, integration to the AI FBW/AUTOPILOT architecture, pilot interface and simulator tests in normal/abnormal/emergency scenarios.
Our underlying premise during this evaluation was that the coupled sidesticks should not degrade the reliability, the feel, the consequences of failure cases and the pilot interface of the current non-coupled sidesticks.
The main items identified during this activity were:
a) The increased complexity of the coupled sidestick makes it very difficult to closely match the current feel of the Airbus sidestick which is appreciated by flight crews.
The "copy" function of the active sidestick is accurate. The implementation of coupled sidestick would require a modification of the current simple principle of "algebraical addition of both sidesticks orders".
c) The current capability for the PNF to instinctively take control of the a/c, with or without the use of the priority pushbutton, must be kept.
The identification of the sidestick movements of a trainee under normal circumstances is improved. There is a clear benefit for the training scenario.
e) Due to the small deflections of the Airbus sidestick in high stress situations it is difficult to clearly identify what the PF is doing with the sidestick, namely if the motions are small and rapid.
f) When the PNF makes a small correction to the PF, here again in abnormal or emergency situations, the input from the PNF will, in most of these cases, go undetected.
g) Motion of the PNF sidestick can be distractive and in most circumstances can drive the attention away from the most important parameters or cues to be monitored.
h) There is a clear risk for injury if the sidestick hits the hand of either pilot following any runaway of the control system.
i) The active sidesticks introduce a number of new failure situations (runaway, jam...) and a common point which are significantly more severe than with the current AI system. To minimize the impact of these failure cases major modifications to the current flight control system architecture would be required.
Based on the analysis of these results it was concluded that:
a) The current technology for active sidesticks provides a marked improvement relative to that available at the time of launch of the A320: it improves the PNF awareness of the PF actions during training flights.
b) With characteristics of the AI sidestick, dual input situations can remain undetected with coupled sidesticks specially under stress situations (which is where the dual inputs tend to occur).
c) The increased complexity of the sidestick assembly and of the integration to the AI FBW architecture will unavoidably reduce the overall reliability of the system.
d) Work should be launched to develop alternative means to improve the detection of dual inputs situations since it was demonstrated that the coupled sidestick cannot provide a 100% detection rate.
5.2. Dual input detection enhancements
5.2.1. Description
As a complement to the active sidestick evaluations, various new features have been developed to improve the crew awareness of dual input situations on the FBW aircraft. The main objective of these features is to provide warnings which will prevent long duration dual input situations.
The following visual, aural and tactile cues have been tested with the active participation of pilots from Airbus, airlines (Cathay, DLH, Sabena), unions (ALPA, SNPL, German Cockpit) and Airworthiness Authorities (CEV, CAA, FAA, Transport Canada).
VISUAL CUE
When both sidesticks are deflected simultaneously (for more than 0.5 sec), the CAPT and F/O captions of the Sidestick Priority Light on both glareshields are illuminated flashing in green. As soon as the priority p/b is pressed on either sidestick, the glareshield lights revert to the classical priority configuration (CAPT and arrow or F/O and arrow). The principles of this visual indicator are identical for all FBW aircraft.
AURAL CUE
A "DUAL INPUT" audio message is triggered when both sidesticks have been simultaneously deflected for a certain time. The timing of the audio message has been adapted on the A320/A321/A31 and A330/A340 families due to the different systems architecture such that, on all cases, the message is triggered after the illumination of the glareshield lights. In this way, there is a degree of sequencing such that the aural warning will only be triggered if the dual input situation is prolonged.
The "DUAL INPUT" audio is repeated every 5 sec. It has the lowest priority of all the audio (voice) messages but can be generated simultaneously with any other non-voice audio warnings.
When the priority p/b is pressed on either sidestick the warning is canceled except if it has already started (i.e. it cannot be interrupted).
TACTILE CUE (BUZZER)
The sidestick is fitted with a small electrical motor which rotates an unbalanced weight and thus generates a vibration of the sidestick. The level of vibration depends on the rotation speed and on the weight of the rotating mass.
When both sidesticks are deflected for a certain time, the buzzer on both sidesticks are activated to produce a series of intermittent vibrations. Since the level of buzzer vibration could not be increased at will due to various reasons (e.g. rattling noise on the lateral console) the principle of intermittent "shots" was selected to improve the detection of the buzzer activation.
The timing of the buzzer activation has been adapted on all the FBW aircraft such that the buzzer is activated simultaneously or slightly after the illumination of the glareshield lights.
When the priority p/b is pressed on either sidestick the buzzer is stopped immediately.
5.2.2. In-flight evaluation campaign
The in-flight evaluation process was split into two phases: An initial period devoted to the definition of the various features and a demonstration phase where the visual, aural and tactile cues were presented to non-AI pilots in various scenarios. The tests were conducted on the in-house test A340 and A330-200 aircraft.
The initial development and definition phase concentrated on:
a) Adjustment of the activation thresholds
b) Relative timing of the three features
c) Adjustment of the buzzer vibration levels and frequencies
d) Adjustment of the quality and level of the audio message
c) Simulator and in-flight evaluation in various normal, abnormal and emergency scenarios
d) Definition of an evaluation program for non-AI pilots (airlines, unions, Authorities)
During the demonstration phase the modifications were presented to pilots from airlines, Airworthiness Authorities and Unions. In each flight the guest pilot flew the aircraft and the AI pilot made unannounced dual inputs. The dual inputs were performed in the following scenarios:
• Precise tracking tasks in the normal flight envelope (tight turns, manual ILS approach)
• Take-off rotation
• Flare for landing and GA
• High speed dives entering the HSP and triggering the Overspeed Warning
• Stall Warning in Alternate law
• Low speed maneuvering during approach
• Maneuvering in the AOA-prot range (Avoidance maneuvers and GPWS pull-ups).
At the end of the flight the guest pilot debriefed the flight and gave his overall opinion of the different features presented; the AI pilot indicated the number of cases where the dual input situation went undetected.
5.2.3. Results of the in-flight evaluation campaign
There was a clear consensus of the evaluating pilots on the following issues:
• The sidestick buzzer does not interfere in the normal piloting tasks
• The illumination of the glareshield lights may go unnoticed specially in daylight conditions
• The sidestick buzzer is probably the only useful cue in high stress situations
• All of the features improve the dual input detection capability
There was also a general consensus that these features, even if frequently requested by the pilot community, are not really mandatory since nothing, other than crew discipline, can prevent dual input situations. The proposed features will reduce the likelihood that these situations last while remaining undetected but will not prevent them.
5.2.4. Conclusion
a) The definition and inflight evaluation of the aural and tactile warnings is completed.
b) These features will be presented for Certification.
c) The visual, aural and tactile cues will enhance the detection of dual input situations. These features will not prevent dual inputs, but the likelihood of having prolonged dual input situations will be greatly reduced.
d) These features may be proposed as Standard Options grouped as follows:
1) Light only (option already available)
2) Light + Audio
3) Light + Buzzer
