Advanced Distributed Flight Control System … Journal of Engineering Trends and Technology (IJETT) – Volume-42 Number-8 - December 2016 Advanced Distributed Flight Control System

International Journal of Engineering Trends and Technology (IJETT) – Volume-42 Number-8 - December 2016

Advanced Distributed Flight Control System Architecture with Prognosis for High

Performance Fighter Aircraft V Madhusudana Rao1, Vijay V Patel2, NNSSRK Prasad3

1Group Director (Flight Control Computers),2Group Director( Flight Control LAWS) and 12IFCS Directorate, 3Technology Director( LCA Pay loads), Avionics Directorate,

Aeronautical Development Agency(ADA), PB No: 1718, Vimanapura Post, Bangalore-560 017,INDIA Abstract— There is a constant demand for improved performance of fighter aircraft as there is a rapid change in threat scenario. There is need for advanced flight control system to meet this demand for providing improved performance with ease of maintenance. This paper brings out the advanced distributed digital flight control system with built in prognosis features. It also compares the architectural features with traditional centralized flight control system. This also brings out system level prognosis implementation strategies and leading to improved platform/system safety. Keywords — Fighter aircraft, distributed digital Flight Control System, System Architecture, Prognosis, System Safety.

I. INTRODUCTION

The fighter aircraft systems are becoming too complex as large number of functionalities are getting integrated to improve the system performance for the constant demand. The flight control system of an aircraft plays a vital role in achieving required agility, manoeuvrability and good handling qualities. Flight control systems evolved from simple mechanical systems to fly-by-wire flight control systems (Ref. Fig. 1). Modern fighter aircraft with unstable configuration uses fly-by-wire (FBW) flight control system and fly-by-wire means completely electrically signaled, computer configured control system.

The FWB was developed in 1970’s as analogue system and later transformed in to digital. The

supersonic Concorde aircraft is the first civil aircraft equipped with an analogue fly-by-wire and Mirage fighter aircraft employs hybrid (Analogue and Digital) fly-by-wire system. Boeing 777 civil aircraft[1] uses digital fly-by-wire flight control system. Some of the advantages of fly-by-wire flight control system are 1. Increased safety and reliability, 2. Improved handling qualities, 3. Reduced pilot workload, 4. Fuel efficiency, 5. Overall airframe weight reduction, 6. Overall cost reduction.

Most of the traditional aircraft use centralised architecture (Ref Fig. 3) for flight control systems. In this architecture, flight control computer is an ultra dependable real time computer having required computational power, it also interfaces directly with sensors, cockpit controls and flight control actuators.

Fig. 2. Typical flight control system with

centralized configuration. All FCS sub-systems (Sensor, Actuators, etc,) are

powered by DFCC, to have common power reference/ground for the complete system. The chassis reference ground is used in order to have better ground reference for the complete system. The limitations of centralized configuration are like, increased hardware design complexity in DFCC due

Fig 1. Fly-by-wire Flight Control System- Block Schematic.

Digital Flight

Control Computer

(DFCC)

Other Aircraft

Systems

Avionics interface

Secondary Actuators (leading

Edge Slat)

Accelerometers

Rate Sensors

Air data Sensors

Primary Actuators (Eleven

Actuators)

Cockpit Sensors (Rudder and pilot stick),

switches, etc..

Avionics bus interface

Source: airguardian.net


ISSN: 2231-5381 http://www.ijettjournal.org Page 417

to implementation of all interfaces (sensor excitation and signal conditioning, actuators drive electronics and in lime monitoring, etc), handling of large electrical power by DFCC to power complete FCS units like sensors, actuators etc,. This makes computer bulky, heavier with forced air cooling design and leading to maintainability issues on aircraft. Some of the other limitations are, lack of modularity for ease of upgradation with technology insertion and scalability, complexity in system maintenance, so on.. These limitations can be overcome with architectural improvements and by adopting suitably new technologies. These system limitations are addressed through a distributed architecture for advanced FCS.

II. ADVANCED DISTRIBUTED DIGITAL FLIGHT CONTROL SYSTEM.

The world’s top advanced 5th generation aircraft are, 1. The world’s premier fifth generation fighter aircraft, Lockheed Martin F-22 Raptor-USA. 2. Lockheed F-35 Lightning II. 3. Chengdu J-20, Black Eagle–China. 4. Sukhoi PAK FA( T-50)-Russia. 5. KAI KF-X- South Korea, 6. Mitsubishi ATD-X Shinshin-Japan, 7. TAI TFX / F-X –Turkey. 8. HAL Sukhoi PMF/FGFA-India & Russia, 9. Shenyang J-31 (F-60)-China. These aircraft have incorporated some or all features like super maneuverability with stealth, air superiority including ground attack, electronic warfare, signals intelligence roles and also features like a lethal, survivable and flexible multi-mission etc,.

The architecture and selection of FCS subsystems will differ based on the features to be incorporated in advanced fighter aircraft. For example, stealth feature demands flush air data system, concealed actuators and internal weapons, etc., similarly super maneuverability mostly demands integrated propulsion and flight controls with thrust vectoring capability. The flight control system complexity grows with number of feature to be implemented. Then it will become too complex to realize Digital Flight Control Computer (DFCC) with complete computational and interface functionality (sensors, actuators, etc,.) required for an advanced fighter, considering most of features. This higher level of complexity at FCS, demands to have distributed architecture for flight control system of 5th generation fighter aircraft. With this, the hardware and software complexity get distributed across various subsystems/units. The digital flight control computers have to perform core flight control system computations, redundancy management, decision making and supporting Pilot Vehicle Interface(PVI) requirements through avionics bus interface. This simplifies the hardware requirements for digital flight control computer, enabling to use COTS [2] hardware with standard bus interfaces to take advantage of shorter development cycle and ease of long term maintenance. The other issues

related to COTS hardware like reliability, fault tolerance features, etc,. to be addressed before selecting for this critical requirement.

Realization of digital flight control computer with custom hardware is also relatively easy as it houses only computational and minimal interface blocks. The design enables for realizing fully conduction cooled unit eliminating forced air cooling.

State of art fighter aircraft use various architectures for avionics and flight control systems. Architecture proposed is an “advanced FCS architecture” based on traditional Mil-Std-1553B bus with high speed (10Mbits/s or 20Mbits/s) version.

III. SYSTEM ELEMENTS OF PROPOSED DISTRIBUTED FCS ARCHITECTURE.

In a distributed architecture, system elements should be evolved based on logical grouping of system functions and its complexity level (Ref. Figure 4). The system bus proposed here is High Speed Mil-Std-1553B. However, if there is higher bandwidth requirement, Fiber Channel-Avionics Environment which operates at 1Gbps or AFDX® bus[3] can be used. The major subsystems or elements of advanced flight control system are described below.

A. Digital Flight Control Computer(DFCC) B. Smart Sensor System(SSS) C. Smart Actuation System(SAS) D. Smart Flush Air Data System(SFADS) E. Cockpit Interface System( CIS) F. Aircraft Interface Computer(AIC) G. Prognosis and fault detection System (PFDS).

A. Digital Flight Control Computer (DFCC). DFCC is ultra dependable, fault tolerant real time computer with quad redundancy to meet stringent PLOC requirements of 10-07/h over one hour mission.

There are multiple latest microprocessors/ microcontrollers (M/s Motorola PowerPC family, etc,.) or System on Chips-SoC (M/s Xilinx, Zync series or M/s Altera family SoC, ARM- based Hard Processor System, etc,.) are available for developing advanced digital flight control computers. Power supply module can be build with high efficiency (>95%) DC/DC converters (like M/s SynQor etc,.) for reducing unit level thermal power dispation so that system can be realised with fully conduction cooled chassis.

DFCC will have cross channel data link for exchanging the data across the channels, This interface can be RapidIO or Fiber channel-Avionics Environment(FC-AE) or any other suitable and reliable high speed serieal bus. DFCC will have


avionics bus interface (as RT on Avionics bus) for communicating to avionics system.

All other information required for computing and commanding actuators will be obtained on high speed digital bus of FCS. Digital Flight Control Computer will be design to host application softwares like extensive Buit In Tests (BIT) like Pilot intiated Built In Test- PBIT, Pilot in Loop Built In Test (PLBIT), Maintainance BIT(MBIT), and Continous BIT(CBIT), schedular/executive, synchronization module, intialization software, sensor data acquisation (from smart sensor unit on bus), health monitering and Redundency Management softaware, voting algorithems[4] (Meadian Select-MS/ Mean of the Medial Extremes-MME/ Weighted Average Voting-WAV), computation and command generation to actuators, air data computations etc. DFCC acts like bus controller for the complete set of Line Replacable Units (LRUs) on the FCS bus. The prognosis features will be incorporated in a digital flight control computer.

Number of IOs are limited on front panel, DFCC can be either with high density military grade circular connectors from M/s Amphenol Aerospace, M/s ODU AMC, M/s Glenair, etc,. or it could be ARINC series 600 or 700 connectors from M/s Amphenol, M/s Souriau, etc,. for ease of installation and removal on the aircraft.

B. Smart Sensor System (SSS). Flight control system needs primary sensors for measurement of body accelerations and rates. There are fighter aircraft uses accelerometer sensor assembly (ASA) and Rate Sensor Assembly (RSA) separately on aircraft. It was found that not so significant advantage by separating them (at node and anti-node). Availability of high computational processors and advances in sensors technologies (MEMS technology based sensors), enable to integrate quad ASA, RSA and other associated sensors in a single package and perform real time computations (like real time null drift and gain non linearity compensations, sensor data associated algorithms, etc,.) to generate all necessary FCS sensor parameters from smart sensor system (SSS) and provide IMU (Inertial Measurement Unit) like functionality. Hence it is proposed to have integrated sensor package with quad RTs on FCS bus. The prognosis features will be incorporated in smart sensor system.

C. Smart actuation System (SAS). The platform defines type of actuators required and its number

for meeting flight control system requirements. It is proposed to have dual actuator computers housing

Fig. 4 Advanced Fly-By-Wire Flight Control System Architecture Proposed



necessary actuator electronics and required computations as part of smart actuation system. Actuators are logically grouped into two groups for better aircraft performance in failure conditions. This also reduces load on actuator drive electronics requirement inside actuator computer, enabling to realize the compact unit with fully conduction cooled chassis. These smart actuation system units are quard redundant computers having four RTs on FCS bus. Platform level requirements like stealth feature will demand for concealed actuators. Thrust vectoring for super manoeuvrability needs interface to the engine actuators. The prognosis features will be incorporated in smart actuation system. D. Smart Flush Air Data System(SFADS). Platform level requirement of stealth demands flush air data system. The accurate measurement of air data is critical to the flight control, guidance, and post flight analysis of most modern atmospheric flight vehicles. The entire airdata state can be described by five parameters: Mach number (M), angle of attack(AoA), angle of sideslip(AoSS), either pressure altitude or free-stream static pressure, and the true airspeed. Using these five parameters, all other airdata parameters of interest can be calculated.

Historically, airdata have been measured through the use of intrusive booms or probes that penetrate the flow away from the influence of the vehicle body to measure total and static pressure, angle of attack, angle of sideslip, and free-stream temperature. However, specialized requirements of advanced vehicles make using intrusive conventional airdata measurement systems highly undesirable. Advanced vehicles include hypersonic vehicles, for which the heating environment is extremely hostile. Stealth vehicles, which require a minimal radar cross section and high-performance vehicles that operate beyond the poststall flight regime. The flush air data sensing (FADS) system concept, in which airdata are inferred from nonintrusive surface pressure measurements. The FADS concept relies on a mathematical model that relates measured forebody surface pressures to the airdata state. The current method uses a nonlinear pressure model and a nonlinear solution algorithm to solve for the airdata state from the measured pressures.

Flush air data system need to interface with flush airdata sensors for pressure measurement by providing required excitation and execute pressure model for airdata parameters computation. This system will has quard bus interface to FCS bus. The prognosis features will be incorporated in smart flush air data system. E. Cockpit Interface System (CIS). This system is fully responsible for all cockpit interfaces. Cockpit interface computer is the part of this system. This system will have typical interfaces with pilot control stick, rudder pedals, discrete

switch interfaces, cockpit panels (Flight Control Panel-FCP, Auto pilot panel (APP), Cockpit Warning Panel-CWP, Flight Test Panel(FTP), etc,.) and indicators. This system provides excitation to pilot control stick and rudder pedal. CIS will provide brightness control for day and night flying. It will be a quadruplex system interfacing to FCS bus as RT. This will be located close to the cockpit for reduced cable harness and better performance. The prognosis features will be incorporated in Cockpit Interface System. F. Aircraft Interface System(AIS). This system will interface from FCS to all required subsystems on aircraft. The AIS will have following typical interfaces with the aircraft systems.

Table 1: Typical Aircraft to AIS Sl. no

System/Interface Remarks

1 Weight on Wheels (WoW) status

For detecting on ground or in-air status

2 FADEC and TQA For Auto throttle including back drive interface

3 In Flight Refueling(IFR) system.

IFR Initiation status to FCS CLAW

4 Internal weapon release initiation/doors status.

Status to FCS- CLAW( Control Laws)

5 Break Management & Nose Wheel Steering (NWS)System interface

Anti Skid algorithms execution and control in addition to NWS control.

6 DFCC Power transients monitoring

To isolate, power interruptions internal or external to FCS system

7 Ground Crew Interface

For Daily Inspection and Post flight analysis.

This system will be responsible for interfacing all required aircraft subsystems and placing necessary data on FCS Bus.

G. Prognosis and fault detection System (PFDS) : This system will not participate in any of the flight control system functionality. However it monitors complete data (either with FCS system sensors or additional PFDS sensors) generated during execution of the flight control system and any additional data required will be obtained directly by interfacing or through relevant subsystems. Required prognosis algorithms will be running in this computer to detect likely faulty system or faulty system. Prognosis will be carried out on sensors, actuators, and all other electronics subsystems of FCS, PFDS integrates this and infer. It is proposed to have prognosis[5] on associated systems of FCS like Electrical and hydraulic systems. This integrated approach improves in assessing FCS health and enhancing system safety.

This system monitors and records the complete FCS bus transactions for the post flight analysis. This is quadruplex system interfacing to FCS bus.



This also will have interface with Integrated Vehicle Health Monitoring(IVHM)[6] system for overall aircraft level health monitoring and management. H. FCS Architecture Optimization: Flight Control System demands quadruple redundancy for meeting PLOC requirement of 1x10-7/hour. A detailed analysis was carried out through Fault Tree Analysis (FTA) method and found that relatively the weakest link in whole FCS system is sensors reliability towards meeting FCS PLOC (Probability of Loss Of Control). Considering this data, when examined, the FCS sensor assembly/package realization with MEMS sensors will have large MTBF compare to usage of conventional sensors. With this improvement in sensors package reliability, overall FCS PLOC requirement can be met with TMR architecture based computer by suitably augmenting for fault detection at input and output stage[] like inline monitoring features at output etc, in the computer.

IV. IMPROVED MAINTENACE AND ENHANCED SAFETY WITH PROGNOSIS.

On condition maintenance is adopted for flight control system during operational stage. Though it has advantage like need not check the health of every FCS component very frequently on aircraft during usage. Checks will be carried out only during scheduled maintenance. The failure of the component is not known in advance till component is failed and same is indicated by the system. In corporation of system prognosis and fault detection system in distributed architecture (Ref Fig 4) help to detect the likely hood of failure of the system/component by continuously collecting heath parameters/data and applying different prognosis techniques, likely failure can be detected in advance before the component completely fails on the aircraft. The prognosis techniques are like Prognosis based Health Monitoring(PHM) for residual life estimation of structural components, vibration signature based fault assessment for rotary/actuator components, sensor health assessment based parameter drift in tolerance band[7], etc, can be used early detection of faults in flight control system Line Replacement Units (LRUs). This early detection of likely faulty component helps in effective spares planning and reducing Aircraft on Ground(AoG) leading to improved system availability. As component failures are detected prior to component fails on the aircraft with suitable prognosis techniques, this will improve the safety of the system significantly.

V. PERFORMACE BENEFITS OF DISTRIBUTED FLIGHT CONTROL SYSTEM.

The major advantages and disadvantages of distributed flight control system w.r.t centralized system are given in table 2.

Table 2. Performance Benefits Sl.no

Parameter / Area Remarks

1 Design and realization Easy 2 Weight Less 3 Computational power

and Growth potential Improved

4 Obsolescence Easy to address 5 Software complexity Less due to

distributed across. 6 System safety Improved due to

prognosis 7 Cost Relatively higher

CONCLUSIONS

It is feasible to design and implement distributed digital flight control system with prognosis using advance technologies which can provide numerous benefits compare to traditional centralised system.

. ACKNOWLEDGEMENTS

Authors are indebted to Dr. Girish S Deodhare for suggestions and inputs. A special thanks to Dr. D Sita Rama Raju for his encouragement, inputs and helpful guidance. We would also like to thank FCC team of IFCS, ADA for their support.

REFERENCES [1] Y.C(Bob) Yeh, “Design Considerations in Boeing 777 Fly- By-Wire Computers”, Flight Systesm, P.O Box 3707, M/s 02-KA, Settle, WA 98124-2207. [email protected] [2] Savio N. Chu. 1999 ”Design of a fault tolerant COTS Based Bus Architecture”. Jet Propulsion Laboratory, Pasadena. IEEE Transactions on Reliability, Vol.48,No.4, Dec 1999. [3] Christian M. Fuchs. 2012 “The Evolution of Avionics Networks From ARINC 429 to AFDX”. Network Architectures and Services, August 2012, DOI:10.2313/NET-2012-08-1_10. [email protected] [4] Zahra latifi, and abbas Karini “A TMR Generic Voting

Algorithm for fault tolerant Medical Robot”, Department of Computer Engineering, Arak Branch, Islamic Azad University, Markazi, IRAN, Procida Computer Science 42(2014)301-307. www.sciencedirect.com

[5] Edward Balaban., “An Approach to Prognostic Decision Making in the Aerospace Domain” NASA Ames Research Center, Annual Conference of the prognosis and Health management Society. 2012 [6] Ashok N.Srivastava. 2009, Integrated Vehicle Health Management, “Automated detection, diagnosis, prognosis to enable mitigation of adverse events during flight”. NASA Technical Plan, Version 2.03, Nov 2009. [7] Brian A. Tuchband and Michael G. Pecht, “The Use of Prognostics in Military Electronic Systems” Center for Advanced Life Cycle Engineering (CALCE) Prognostics and Health Management Consortium University of Maryland, College Park, MD 20742 USA +1-301-405-5323 · [email protected]

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Advanced Distributed Flight Control System … Journal of Engineering Trends and Technology (IJETT) – Volume-42 Number-8 - December 2016 Advanced Distributed Flight Control System