COTS APPROACH TO DYNAMIC FLIGHT DATA

ACQUISITION FOR HUMS/OLM APPLICATIONS

Author : Richard H J Fielding

ACRA CONTROL LTD Landscape House, Landscape Road,

Dublin 14, Ireland. www.acracontrol.com

+353-1-2951264, +44 1252 664 780 fielding@acracontrol.com

Summary : Rapidly advancing technologies in high speed analogue to digital conversion, ultra high density gate arrays and solid state storage media can permit high bandwidth data (such as vibration and video data} to be stored at real time rates at an affordable cost. This precludes the use for complex data reduction processes in flight so improving system reliability and qualification costs. The advantage of this approach is that it preserves all raw data enabling detailed analysis to be performed, post flight, by the many commercially available statistical and structural data analysis programs.

Keywords: FTI, OLM, FDR, FDAU, HUMS, ODR, Solid State Memory, Structural and Spectral Analysis

Introduction Traditionally aircraft data monitoring systems (HUMS, FDAUs, ODRs etc) have been tailored for each aircraft type based on what specific measurements and analysis tools are perceived to be required at the time. In most cases the specifications are either Cardinal Point (leaving the potential suppliers to make calculated guesses as to what might actually be required) or so specific that there is little flexibility allowed in the system design. The final result typically ends in a ‘black box’ system that might only ever be used on one specific aircraft type with bespoke hardware and software. Qualification of both hardware and software are expensive, flexibility is low and maintenance costs are high which often result in a poor return on investment. The time taken to debug and qualify (CAA and FAA approval included) the system means that many systems become obsolete before even getting into service. COTS Approach More and more HUMS and other DAU users are looking toward a Commercial-Off-The-Shelf (COTS) solution in order to take advantage of new acquisition, processing and data storage technologies, existing standards for sensor / bus interfaces and data distribution plus the myriad of high quality mathematical, statistical and structural analysis software that are freely available in the aerospace / industrial / commercial market. Use of COTS systems has some significant advantages over bespoke system designs –

Eleventh Australian International Aerospace Congress Sunday 13 – Thursday 17 March 2005

Melbourne, Victoria, Australia

AIAC-11 Eleventh Australian International Aerospace Congress

Fourth DTSO International Conference on Health and Usage Monitoring

• Cost – can easily be a factor lower. • Future Proof – COTS developments ensure that new technologies and

upgrades can be integrated at low risk to maintain a system free from fears of obsolescence.

• I/O – A wide range of input (analogue - temp, S/G, voltage, ICP, LVDT/RVDT etc, digital, databus - 1553, ARINC 429, RS232/422/485 etc, video, scanner etc.) and output (PCM, ARINC 429 / 717, Ethernet, Firewire, Fibre Channel, Flash Memory etc) are already available.

• Flexibility – COTS systems are expandable and programmable. As the needs or flight profiles change the system configuration can be changed to suit. New developments such as smart sensors can be readily integrated when needed.

• Cost of Ownership – spares holding can be less across all types of aircraft. As production equipment lead times are lower they can be ordered when needed.

• Environmental qualification approvals to published standards such as Mil-Std-810 and Mil-Std-461 are already a baseline. Only additional testing outside the baseline needs to be considered.

• If the same approach to data monitoring and acquisition for Flight Test and OLM is adopted then it becomes an ‘orange box’ installation. In these cases the where data is only ever monitored it need not be deemed to be a flight critical installation therefore not requiring FAA or CCA LRU type approvals. Note that OLM and ODR are already being performed on in service aircraft.

• ARINC 717 output – If selected data can be inserted into an ARINC 717 format, in addition to the mandatory parameters, this data can be stored in the on board ‘Crash’ Recorder.

Standard Interfaces International standards for interfaces mean lower cost and reduced risk. Common aerospace I/O interfaces in use today are shown in the following diagram -

429MIL-STD

1553

SensorsS/G

VoltageScannerL/RVDT

etc

Analog

1553ARINC 429

RS232EthernetFirewireAsync

Discreteetc

Digital

A/CPower

Processed

MPEG4 Video/Audio

Media

RF

Ethernet

CAIS

CDU

CompactFlashMemory

Arinc 717

PCMCIAMemory

Tape,Disk,Solid State

Ethernet, Memory Modules, IRIG PCM,etc

Data Inputs

429429MIL-STD

1553

SensorsS/G

VoltageScannerL/RVDT

etc

Analog

1553ARINC 429

RS232EthernetFirewireAsync

Discreteetc

Digital

A/CPower

Processed

MPEG4 Video/Audio

Media

RF

Ethernet

CAIS

CDU

CompactFlashMemory

Arinc 717

PCMCIAMemory

Tape,Disk,Solid State

Ethernet, Memory Modules, IRIG PCM,etc

Data Inputs

Fig:1 Standard Interfaces

COTS high environmentally qualified data acquisition systems have been used for more than 30 years in the Flight Test, Operational Loads Monitoring ( OLM) and Operational Data Recording (ODR) arenas. The prime difference between these users and HUMS / FDAU users in the past are –

• Number of Parameters o Flight Test – in the thousands. o OLM – in the hundreds o ODR – in the hundreds o FDAU – in the tens o HUMS – in the tens

• Data Rates

o Flight Test - typically ranges from 50K samples per second to more than 2M samples per second.

o OLM - can range from 1 sample per second to a few hundred samples per second.

o ODR – tens of K samples per second o FDAU- rates often similar to OLM. o HUMS - rates currently range from a tens of samples per second to a

few samples per minute. ( If raw vibration data is required this could easily rise to tens of K samples per second for short durations).

• Data Storage

o Flight Test - Data is recorded on board with subsets of the critical data transmitted to the ground via a real time link for processing.

o OLM - Recorded on board for post flight processing. o ODR - Recorded on board for post flight processing. o FDAU - Recorded on board for post flight processing. o HUMS - Recorded on board for later processing. On board processing

often reduces the data to a minimum (exceedences outside pre-set limits are recorded only).

• Data Processing

o Flight Test – Real Time and Post Flight. o OLM – Post Flight o ODR - Post Flight o FDAU – Post Flight o HUMS – on board data reduction and Post Flight

Flight Test is always the most challenging due to the heavy demand for high speed sensor, databus and video data. The same equipment concept is easily adapted to OLM, FDAU, ODR and HUMS. The wide range of COTS input and output interfaces available in today’s flight data acquisition mean that a flexible, low cost FDAU / HUMS airborne hardware system is readily available. High Speed Analogue Data Measurements Low Power A-D conversion, high density gate arrays (FBGAs) and advances in digital filter techniques have enabled high accuracy, small size, multi-channel

analogue input modules for vibration (40KHz bandwidth per channel) measurements. These can now be acquired with slower rate data in a common COTS digital data acquisition system maintaining time synchronisation using synchronous sampling techniques. Advances in Affordable Solid State Storage In the last 5 years the removable solid state storage technology has advanced in leaps and bounds thanks primarily to the digital still and video camera markets. The desire for high resolution full frame rate video is driving up not only memory size but also writing speed to levels that are extremely attractive to the data acquisition industry. The continued demand by the ‘professional’ users also means that the need for what are called industrial versions of the memory modules (-40ºC to +85ºC and specified vibration levels to 1,000G plus) are also available. The aerospace data acquisition market has little influence over the development of this type of memory since usage is only a small fraction of the commercial market, the benefit of employing this COTS technology though are huge. Compact Flash Memory (CFM) is a very rugged mechanical design, which suit the small airborne data acquisition recording application well. PCMCIA is also a very rugged package and but is reliant on the smaller CFM development for its capacity. This means that regardless of the size advantage the PCMCIA memory capacity is rarely larger than the available CFM. Towards the end of 2003 the Compact Flash Memory (as well as other formats such as PCMCIA, Secure Digital etc.) capacities available were typically in the region of 64 to 128MB, before that 4 to 16MB was considered large. A little over a year later the capacity has risen to a staggering 12GB with forecasts of more to come. Although at the time of writing this paper the cost of a 12GB commercial memory module is around $15,000 it is a well known fact that this will reduce fast once full production is under way. 12GB solid state recorders for aerospace use however do cost considerably more than $15,000.

Fig:2 Compact Flash Memory in a Data Acquisition System

As an example a system recording 64 data channels ( 16 bits per data word ) sampled at an average of 100 samples per second per channel will store data at 0.0128 MBytes/sec. Based on a 1GB memory size the data could be continuously stored for more than 20 hours. The following graphs give some idea of the advances in memory capacity, writing speed and cost.

0

5000

10000

15000

20000

25000

30000

35000

2000 2001 2002 2003 2004 2005 2006

Year

MB Capacity

Fig :3 Memory Capacity vs Time

0

5000

10000

15000

20000

25000

2000 2001 2002 2003 2004 2005 2006

Year

MB

/Sec Burst Speed

Cont Speed

Fig :4 Memory Writing Speed vs Time

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

2000 2001 2002 2003 2004 2005 2006

Year

$ $Price/MB

Fig : 5 Memory Cost per MB vs Time

When memory was only a few MB, and in earlier times a few tens of KB, it was common to employ data reduction methods to ensure that enough data could be stored for the many hours of mission flight time. This means that data is processed in flight resulting in loss of the raw data that can be invaluable when anomalies occur and more detailed cross parameter analysis would have been desirable. Storing raw lossless data means that the same parameter, or groups of parameters, can be processed in many different ways post flight. This reduces the possibility of errors that might be due to sensors out of calibration, sensor faults, initial condition miscalculations etc. A significant benefit in storing raw data for post-flight analysis is, in addition to the discovery of anomalies, changes in mission profile and/or aircraft configuration can lead to changes to the aircraft's original fatigue model as well as other maintenance and usage models. If the raw data has been preserved, a full life history can be re-run through the modified models, even many years into the life of an airframe, engine etc. These large, fast access low cost memories also enable high speed data, such as vibration, visual images and flutter data to be sampled and stored at higher rates during periods of exceedence (i.e. events), or regular timed acquisitions, thus providing a more detailed picture of any impending problems. This could typically be of interest when assessing the wear patterns of helicopter gearbox bearings. Video Compression The use of video as an ‘auto observer’ in an aircraft environment is becoming more attractive now that video compression techniques, such as MPEG-4, are producing high quality colour images at data rates down as low as 64 KBPS. In fact in some cases it has become mandatory to store video images, at a slow update rate, on the crash recorder what pilot’s actually see on their instruments. This is proving to be a popular addition by the crash investigation teams as part of incident evidence. Two or more low-speed compressed video channels in the recorder would allow faults in head-up and head down electronic displays, as well as traditional analog instruments, to be investigated. A low-cost external camera (becoming commonplace on very long airframes) would allow landing gear stowage status and wing control surface configuration to be better understood. These benefits might also apply to flight

operations quality assurance, where the cause of anomalous behaviour needs to be investigated. A picture provides so much more information than an open/closed status switch, or a flap angle setting switch. Improvements are continuing and the necessary PC processing tools for this type of video compression are readily available, in many cases distributed through the public domain – hence basically free. Solid state cameras in many shapes and sizes are on the open market many designs being generated by the security industry. Many are small enough to be placed in restricted space areas or even outside the aircraft with little modification to the aircraft structure. Back in the 40s and 50s using a ‘wet’ camera as an aircraft system auto observer in the cockpit, recording the instrument readings periodically, was often the only instrumented information available to the maintenance crews on the ground. It is ironic that it has taken this long for digital techniques to make it feasible again. For MPEG-4 a guide as to the quality of the image expected vs data rate are - Quality Level Data rate Compression Level Uncompressed Colour Video 166 Mbps 0 True Lossless Compression 45 Mbps /4 Visually Lossless 3-10 Mbps /16 to 55 High Quality Videoconferencing 384 Kbps to 1.5 Mbps /110 to 432 Acceptable Quality Videoconferencing 112 Kbps to 256 Kbps /648 to 1482 Videophone 20 Kbps to 64 Kbps /4940 to 8300 It is now clear that the availability of large high speed memories mean that data as well as video can now be stored simultaneously, over many hours of flight, at an affordable cost. Data Processing Software Many COTS data processing software packages exist and have the ability to interface to most database formats via XML ( Extensible Markup Language) and more recently the open standard XidML (Extensible Instrumentation Definition Markup Language). XML is a widely used international standard for data interchange and requires a simple ‘schema’ to be added to allow data interpretation between commercially available and bespoke processing software and get access to data. For data acquisition applications XidML schemas are intended to be published within the public domain. Software systems such as LabView, MatLab, MathCAD, Eurilogic Magali and nCode Glyphworks are commonly in use around the world for aerospace data

processing applications. Programs such as Magali and Glyphworks provide access to many of the algorithms that are commonly used for flight and fatigue data analysis. All these software systems allow the user to define and design data processing functions and displays to suit their own application in an automated manner. Programs such as nCode nSofte and Glyphworks include many features specifically designed for multi channel and multi function processing of fatigue type data –

• Stress-life/strain-life calculations, uniaxial/multiaxial/frequency loadings, access to materials data, fracture mechanics.

• Strain-life (EN) method to predict crack initiation, the stress-life (SN) method for total life prediction, and fracture mechanics for crack growth.

• Constant amplitude loads, product duty cycle or design spectra, and variable time histories from strain gauges.

• Uniaxial and non-proportional multiaxial load cases. • Frequency response analysis (cross-spectrum, gain, phase and coherence),

octave analysis and auto and cross-correlation functions. • Remez filter algorithm for user-defined time domain filtering, and a user-

defined fast Fourier filter. • Time series created from a rainflow histogram or a Markov matrix. • Formula processing, peak counting, advanced statistics, range-mean analysis

and simultaneous maximum/minimum output.

Data collected in the real world will suffer from a variety of problems – such as drop-outs, overloads, drift and so on. It is critical that such anomalies be quickly spotted and corrected when they lie “somewhere” in a pool of data with potentially hundreds of channels and gigabytes of samples. The GlyphWorks Anomaly Detection module uses industry-proven algorithms to automatically hunt down and highlight those sections of data that need further investigation – before they compromise the analysis process and ripple downstream through the organization. The procedure not only identifies the more obvious problems such as drift, flat-lines, spikes, overloads – but also abnormal variances between channels by employing statistical comparisons.

GlyphWorks provides the industry-leading technology needed to calculate fatigue life from measured data. The Stress-Life method uses a nominal stress approach for high-cycle conditions or non-metallic applications; while the Strain-Life method is more appropriate for more severe loading conditions (low-cycle fatigue) where plasticity is significant.

Correction for mean stress and surface finish effects, even back calculate from each data channel to determine a scale or fatigue concentration factor required to achieve a target life. Damage histograms can then be reviewed to determine which load cycles were most damaging, even output damage time histories to show exactly when the damage occurred – the start of a fatigue-sensitive editing process.

All this and more are available as a COTS product that can be easily tailored for a variety of complex data processing of aircraft flight test, load monitoring and usage applications.

Fig :6 Typical Data Processing Flow COTS software is generally highly programmable and configurable to suit a wide and varied market. Like the airborne hardware manufacturers, software suppliers maintain a continuous development plan, using state of the art programming techniques and languages. This ensures that the software is operable on the latest operating systems, remains supportable and future proof. A Practical Example The UK E-3D Sentry Airborne Early Warning System fleet of aircraft are all fitted with a Loads and Environmental Spectra Survey ( L/ESS ) recording system. This system recorded airframe stress and other aircraft operational parameters on every flight. Data from all the aircraft are replayed into a ground processing system where the data is scaled and formatted before being sent to Tinker AFB in the USA. Here the data from many other aircraft in use around the world is collected, correlated and analysed thus forming a unique history of the all the aircraft’s fatigue and usage parameters over the fleet’s operational life. The analysis of such trend data readily indicates if there are some potential fatigue or maintenance problems on the horizon well before they become critical. The original system installed consisted of a Data Acquisition system and recorder manufactured by a company called Moog Esprit. When the company changed its focus this system became obsolete, leading to poor reliability and a replacement was sought. ACRA CONTROL LTD proposed a COTS solution using current hardware (KAM-500) and the UK fleet are all flying with this system today. Not only the

Acquisition Conversion & viewing Manipulation & calibration

Rainflow analysis Fatigue analysis Combine channels

Calibration Offset correction

True to Engineering units etc

Calibration Offset correction

True to Engineering units etc

Damage analysis Results display

g Spectrum

-4 -2 0 2 4 6 8

10

1.00E+00 1.00E+0

1 1.00E+02 Counts per

hr.

g level reached or exceed

g Spectrum

-4 -2 0 2 4 6 8

10

1.00E+00 1.00E+0

1 1.00E+02 Counts per

hr.

g level reached or exceed

Exceedences

Acquisition Conversion & viewing Manipulation & calibration

Rainflow analysis Fatigue analysis Combine channels

Calibration Offset correction

True to Engineering units etc

Calibration Offset correction

True to Engineering units etc

Damage analysis Results display

g Spectrum

-4 -2 0 2 4 6 8

10

1.00E+00 1.00E+0

1 1.00E+02 Counts per

hr.

g level reached or exceed

g Spectrum

-4 -2 0 2 4 6 8

10

1.00E+00 1.00E+0

1 1.00E+02 Counts per

hr.

g level reached or exceed

Exceedences

hardware but also the software was obsolete (based on DOS) meaning that this was also to be upgraded to the latest Windows environment. This also enabled a quicklook analysis of the raw data to be performed that assessed the quality of the data before being re-formatted and sent to the USA for detailed analysis. A simple piece of conversion software was produced to transfer the data stored in flash memory in the KAM-500 to the same format that Tinker AFB were using before.

Fig :7 Original Installation Fig :8 KAM-500 COTS Installation Some significant benefits were –

• Cost - no development, COTS solution • Weight – original Installation 15Kg, KAM-500 2Kg • Power – 50% lower • Space - < one quarter of the volume • Maintainability – standard product • Supportability – spares, new modules and upgrades (e.g. to removable

compact flash memory ) • Reliability – COTS tend to be well proven • Future Proof – continuous development of compatible hardware and software • Commonality – many other aircraft in the UK (C130-J, TriStar, E-3DOLM

etc) use the same basic system building blocks. Conclusion There is indeed a fine line to be drawn between Flight Test, OLM, FDAU, ODR and HUMS in terms of the hardware and software used to collect, store and analyse the data. In the majority of cases the building blocks are identical, only the application is different. Use of common configurable COTS hardware and software technologies available today can drastically reduce the cost of purchase, cost of ownership, support, reliability and maintainability of airborne data acquisition requirements for all types of applications.

Original Moog Esprit SystemOriginal Moog Esprit System

KAM-500 system mounted on the

same equipment tray

KAM-500 system mounted on the

same equipment tray