ON BOARD MAINTENANCE SYSTEM (OBMS)

GENERAL

On board maintenance systems (also known as Central Maintenance Systems (CMS) are electronic/ computer based systems used to monitor and record the performance status of systems on-board modern aircraft.

The structure and configuration of on board maintenance systems vary from one aircraft type to another.

However, the core functions of these systems are broadly similar.

They

• monitor the aircraft for faults

• record

• store the fault data,

• provide information

• about these faults to flight crews and maintenance personnel.

The data collected by on board maintenance systems can be accessed both in flight and on the ground.

• In flight, the system advises the flight crew of faults that may affect aircraft operation.

• On the ground, maintenance crews use the system for testing and troubleshooting purposes.

In some applications, the aircraft can relay fault information to the ground whilst in flight via ACARS.

The systems allow maintenance personnel to access these publications without having to carry books and papers to the aircraft. On board maintenance systems allow such technical data to be uploaded, downloaded, viewed, and printed by maintenance personnel. The systems are used for both line and base maintenance. For the most part, the information provided here is general. When aircraft specific information is given, it should be noted that system details and terminology differ between aircraft manufacturers.

CENTRAL MAINTENANCE SYSTEMS (CMS)

The primary function of a CMS is to initiate airplane system tests, record and store test results, monitor airplane systems status, recall test results from previous flight legs, upload/download data and software updates, isolate and identify faults. Furthermore, a CMS may interface with other onboard computers to provide data for display to flight crew, initiate warnings and cautions, and to enable or disable some CMS functions depending on the flight phase or configuration of the airplane.

CMS STRUCTURE

A typical CMS consists of

• Single or dual central maintenance computers connected to a printer,

• Data loader,

• Data reporting/transmission system (ACARS)

• Multifunction control display units (MCDU).

The central maintenance computer (CMC) is the main processing unit of the CMS.

The CMC acquires, processes and outputs data to and from various airplane systems. These inputs and outputs may be

• digital,

• analogue,

• in a particular format (such as ARINC 429)

and

• discrete (of a specific value such as a voltage, current, pressure, etc).

Typical inputs to a CMC would originate from airplane system built in test results initiated by the CMS, airplane system operating modes and configuration status, airplane flight modes, and commands from flight and maintenance crews.

An input from the airplane’s landing gear ‘weight on wheels’ switch would indicate to the CMC that the airplane is either airborne or on the ground. This information is stored to record the flight phase of the airplane and is used to restrict or inhibit particular tests that may be requested via the CMC.

Outputs from the CMC are provided for flight deck visual and aural displays (EFIS, EICAS/ECAM), to initiate airplane system built in test equipment (BITE), data transfer to other storage media such as remote memory devices, printers, transmission via airplane condition and reporting systems (ACARS) and data communication systems (VHF, HF or SATCOM).

In order to perform these functions a CMC consists of a number of modules or circuits which are typical in any computer system.

The power supply (PSU) module receives the airplane power supply voltage (usually 115V AC) and provides the appropriate regulated DC voltages to the logic circuits in the CMC. Operating software and fault history are stored in the non-volatile memory (NVM).

Approximately 500 faults can be stored in the NVM for later retrieval by maintenance crews.

The random access memory is used to store active faults during the current flight leg.

Discrete inputs and outputs are used to convey

• BITE status to the CMC

or

• to initiate a BITE sequence in an airplane system.

In each case it may be necessary to convert the discrete signals from analogue to digital (A/D conversion) formats to allow the CPU to process information or from digital to analogue (D/A conversion) to activate a system test sequence or discrete indicator such as a caption warning I,ED. Data receivers multiplex data inputs from the airplane systems and convert data streams from serial to parallel. The transmitters convert processed data from the CPU into a serial output for transmission to airplane systems and other S components.

In modern airplane systems ARINC 429 is the most common data bus used to interconnect airplane systems to the CMC. It is common, particularly on large commercial airplanes, to have a dual CMC installation in the CMS. With dual CMCs, all data inputs are available to both units.

One CMC will be “active” at any given time with the other CMC on “standby.” The active CMC is the one providing outputs. If the active CMC should fail, the standby can be substituted for it either automatically or manually by the crew.

AUTO SELECTION

“Crosstalk” bus enables the two CMC’s to share and monitor faults within each CMC.

A fault monitoring circuit within each CMC controls a “data switch” which, upon detecting a fault in its own CMC, isolates the defective CMC and allows the other CMC’s data to be passed directly through the faulty unit.

A discrete input to the CMC from a manually operated switch in the flight deck enables the crew to manually select the other CMC.

The central processing unit (CPU) performs all of the arithmetic and control functions within the CMC.

By comparing outputs from airplane systems with expected values based on known inputs and the airplanes configuration, attitude and flight phase, the CMC will generate a fault message/code which is then notified to the crew, when necessary, and can generate up to 10,000 different fault codes.

The CMC can be a self-contained line replaceable unit (LRU) installed in its own mounting rack in an avionics compartment as found in Airbus or Boeing airplanes or as a removable printed circuit board (PCB)/card unit installed in a card rack such as the integrated avionics processing system (IAPS) in the Rockwell Collins Proline system.

FAULTS

Faults occurring in aircraft systems either on ground or in flight may, or may not, affect the performance and/ or capability of the aircraft.

Some faults would require immediate action by the crew whereas others may require crew awareness or no crew involvement at all. Clearly, some faults need to be brought to the attention of the crew more rapidly than others and therefore must be classified in order of priority, displayed and recorded as such.

A system of warnings, cautions and advisories is adopted on most monitoring and display systems.

Warnings are the highest priority fault classification and are usually displayed as a RED visual indication accompanied by a continuous aural warning to the crew. These indicate that the fault requires an immediate action from the crew and will continue to be displayed whilst the fault remains.

Cautions are usually displayed as an AMBER visual display, accompanied by a single aural tone indicate that the fault requires immediate crew awareness but no immediate action.

Advisories, displayed as a WHITE visual indication, have no accompanying aural tone and require general crew awareness.

A CMS records these faults in accordance with the classification of

BOEING

• level A (Warnings),

• Level B (Cautions)

• Level C (Advisories)

AIRBUS

Airbus Industries adopt a similar classification system for the Airbus range of aircraft where warnings and cautions are a

• Class 1 fault,

• Class 2 is an advisory or system status notification

• Class 3 fault is not displayed to the crew but is recorded by the CMS for retrieval by maintenance crews on the ground.

BUILT IN TEST EQUIPMENT (BITE)

Units that are monitored by the CMC may contain circuits known as built in test equipment (BITE).

BITE is installed in many systems throughout the aircraft including navigation systems, flight control systems, environmental control systems, and others. Within each system, the BITE circuitry tests numerous individual parameters to determine whether the system is functioning properly.

The individual system BITE circuits are connected to the CMC by a digital data bus.

ARINC 429 buses are used for this purpose in many aircraft.

Other data buses, such as ARINC 629, may also be used.

Typical BITE functions initiated by the CMC serve to detect the fault, isolate the faulty unit and record the fault parameters. The level of test performed will depend on the

• Flight phase of the aircraft,

• System configuration

• Complexity.

Generally, tests fall under a number of categories the most common of which are;

• Initial/Power up Test-Performed immediately at system switch on or following a power interrupt. The BITE will check the functionality of system circuits (power supplies, processors, input/output ports, etc). System software may also be reset or loaded during this test. If a power interrupt were to occur in flight this test may only be restricted to essential checks only.

  
  

• Cyclic/Continuous Test -These tests may occur frequently (once or twice a second) whilst the system is in operation. These tests do not affect the performance of the system and are primarily a monitoring function

  
  

• Interruptive/Maintenance Test-These are extensive tests performed on the ground to aid troubleshooting. These usually involve the simulation of an input signal to a system and or component and monitoring the output for its accuracy or appropriate operation. This may require moving surfaces to be actuated to various positions during the course of the test.

If any fault is detected by the BITE during these tests, an output is generated and sent to the CMS.

If anything that is being monitored fails, BITE will alert the CMC automatically. Crews can initiate the BITE power-up check for a given system from the CMS at any time. This capability is provided as a CMS menu item on the MCDU. This function can be useful when troubleshooting the system. Some LRUs containing BITE have indicator lights that indicate the status of the LRU.

COLOURS IN LRU FOR BITE

• Green lights indicate a normal condition,

• Red lights indicate that the BITE detected a fault in the LRU

• Amber lights indicate that an input, from an external source, to the LRU has been lost or is corrupted.

BITE systems also have the capability of storing fault history. The history is kept in non-volatile memory. Non-volatile memory retains stored data even after the system has been powered off.

In order that the BITE does not adversely affect the operation of the aircraft at any given flight phase the CMS receives inputs from flight guidance computers (FGC) and other system modes/configurations such as, landing gear up/down and aircraft weight on wheels switching. These inputs to the CMC determine the level of test initiated by the CMC and performed by BITE.

Some tests are completely disabled during flight and others are limited to non-interruptive tests.

MULTIPURPOSE CONTROL DISPLAY UNIT (MCDU)

CMCs are accessed via control units in the flight deck, multifunction control display units (MCDU) are found on most medium to large airplanes in service today. The number of MCDUs fitted may vary due to the size and complexity of the airplane. Smaller airplanes may have a single MCDU for pilot use, whereas larger airplanes would have two (pilot and co-pilot) or three with one acting as a standby or maintenance use.

The MCDUs enable the user to navigate the on board maintenance system by selecting from various onscreen menus.

Navigating the menus allow the user to view and select

• current/present faults,

• previous faults,

• upload and download databases,

• Initiate system BITEs.

The user can also view

• component and database information such as,

  • part numbers,

  • serial numbers

  • database versions.

Current faults are, of course, important for determining the aircraft’s current status prior to dispatch or when troubleshooting. Fault history is used to monitor fault trends, such as recurring failures of a particular component.

In addition, a CMC menu permits the user to check the current status of individual systems, even if no fault condition is present. Menus are displayed on the screen and the user uses the line keys on the left and/or right to select the menu item.

The menus allow the user to access specific information about each fault that was sensed, such as the date and time the fault occurred. In addition to the cockpit-mounted control units, some on board maintenance systems allow for the connection of a remote device such as a laptop computer. When the laptop computer is connected, it can be used to access the data stored in the CMC. Present fault reports and previous fault reports can be downloaded to the remote device.

AIRCRAFT COMMUNICATION ADDRESSING AND REPORTING SYSTEM (ACARS)

Data from the CMS can be relayed to ground stations via an on-board data management system such as an

• aircraft communications and reporting system (ACARS) or an

• air traffic information management system (ATIMS).

These systems utilize a central management unit (CMU) to gather and concentrate data concerning aircraft position, attitude, speed, fuel status, etc., as well as data from the CMS. This data is then ‘packaged’ and sent to the airplane operators ground operation stations where the information may be analyzed and the performance of the airplane be monitored.

Airplane fault data relayed to the ground station can be used to prepare the maintenance crews with sufficient information to allow them to make a diagnosis before the airplane arrives at its destination where tooling and spare components will have been made ready to fix the airplane.

The ACARS data is transmitted to the ground using

• VHF communications radio,

• HF radio or

• satellite communications (SATCOM).

Worldwide, a network of ground stations is able to communicate digitally with aircraft using the system.

ACARS is a stand-alone system and is used to transmit data from any number of systems on board the airplane. It can be used by the flight crew to send messages manually, and automatically send reports.

This mode of operation is referred to as ‘Demand’ mode.

Data can also be requested from the ground stations where an ACARS data transmission may be ‘triggered’ by a signal from the ground.

This mode of operation is referred to as ‘Polled’ mode.

When in demand mode ACARS will automatically send messages containing data relating to any condition that affects the flight safety of the airplane.

DATA LOADING

An aircraft’s data loading system provides a means to upload data to, and download data from various airplane systems, including the CMS. The data loading system can be used with any digital system that requires data uploads and downloads while installed in the aircraft.

Early data loading systems used floppy disks as the data storage medium.

An example of this is the multipurpose disk drive unit (MDDU) used on many Airbus Industries airplanes. The MDDU uses 3.5 inch floppy disks for uploading, downloading, and data storage.

In the Airbus system, a Data Loader Selector (DLS) is used to select the system where data should be loaded to or from. This is enabled by a data router installed in the system, the data router ensures that data is transferred to the correct computer from the MDDU as selected by the DLS.

A DLS may be found on the flight deck overhead panel or on the center pedestal.

An MDDU may be installed on the center pedestal or on the co-pilots side panel

B777

On the Boeing 777, data loading is accomplished through a Maintenance Access Terminal (MAT) on the flight deck. Data loading systems also allow for the use of other forms of storage media. Newer systems can be connected to a laptop computer through a USB (universal serial bus) cable.

A CD-ROM disk, or a USB memory stick or “flash drive” may also be used.

In some aircraft, there are multiple locations to connect external devices to the data loading system.

For example, the B777 has two laptop maintenance access terminal interfaces.

One is located on the flight deck, and one is located in the main equipment center below the flight deck. The primary uses for the data loading system are the uploading of program updates, the uploading of database updates, and the downloading of reports.

An example of a unit requiring program updates is the central maintenance computer, which contains an operating program that is upgraded from time to time. The program upgrades to the CMC are input through the data loading system. The same is true for other aircraft systems with internal programming. The number of systems that require program updating varies from aircraft to aircraft.

An example of a database that requires updating is the navigation database which forms a part of the flight management system (FMS). The navigation database contains a great deal of information used by the flight crew. This includes the locations of airports, airways, waypoints, and intersections, the locations and frequencies of radio navigation aids, and other information needed to create and follow a flight plan. Because changes to this information occur from time to time, the navigation database requires periodic updates. These updates are uploaded through the data loading system. The standard frequency for navigation database updates is every 28 days.

The data loading system can also be used to download reports from the aircraft. An example of this is the report of faults stored within the central maintenance computer. Reports on both current faults and fault history can be downloaded.

ELECTRONIC LIBRARY SYSTEM

An electronic library system (ELS) consists of databases containing information used by flight crews and maintenance personnel. These databases can include maintenance manuals, illustrated parts catalogs, wiring diagram manuals, troubleshooting manuals, flight manuals, service bulletins, and many other kinds of documentation from the manufacturer or the aircraft operator. The ELS takes the place of paper manuals. This results in a weight savings, and can make accessing the information in the manuals quicker and easier. The databases in an electronic library system can be accessed through an on-board display terminal and keyboard. They can also be accessed by an external personal computer, or through another digital device such as an iPad or tablet.

The laptop or other external device is typically connected to the system using a serial bus cable. The databases in an ELS must be updated periodically as revisions are made to the technical data contained in the manuals. These revisions can be input through the data loading system.

PRINTER

Many aircraft have capability to print out paper copies of reports from the on board maintenance system, as well as other documents.

Aircraft printers typically conform to ARINC Standard 744A, which gives technical requirements for such printers.

These printers are able to print high-resolution alphanumeric text, as well as graphical images. The printers can print on paper up to 8.5 inches wide.

The speeds of aircraft printers vary, depending on the specific model of printer, and on what is being printed. Text generally prints faster, and images take longer. Some printers can print a page of text in as little as 5 seconds, while others are slower.

Print resolution also varies. A standard resolution is 300 dots per inch (dpi), but some printers are capable of greater resolution. The paper supply for aircraft printers comes in the form of rolls. The paper rolls are typically 150 feet long, and may be perforated or non-perforated.

Inside the printer, an electric motor is used to advance or ‘Slew’ the paper. The printer uses a thermal print head, and the paper is heat sensitive.

For this reason, care must be taken to keep the paper away from heat sources and out of direct sunlight while it is being stored. Exposure to heat can darken the paper, making it unusable for printing. Aircraft printers receive input from CMCs, the ACARS system, and other sources by means of data lines, which may be ARINC 429 buses or Ethernet cables. Some printers are capable of receiving input wirelessly, and operated as part of a wireless LAN (local area network).

A typical aircraft printer is equipped with an indicator light to show whether the power is on or off. It will also give an alert when the paper supply is running low. Some printers perform a self-test on power-up, and will provide an indication if a fault is found during the test.

STRUCTURE MONITORING

Structure monitoring, also known as damage tolerance monitoring, has been recognized as an important function in aircraft maintenance. As aircraft age, their structures becomes more susceptible to damage caused by fatigue.

Repeated cabin pressurization cycles cause fatigue. Repairs and alterations can change the structural characteristics of an aircraft, introducing different stresses than were present with the original design. Corrosion can seriously weaken an aircraft’s structure. Also, events such as hard landings can lead to structural damage which may be difficult to detect.

Certification regulations require aircraft manufacturers to identify critical areas of the aircraft’s structure. These areas are known as fatigue critical structures (FCS).

These critical structures are identified by performing fatigue testing on test articles, which are subjected to repeated load cycles until they fail. The results of this testing are analyzed to determine the FCS for the aircraft.

Aircraft operators are required to monitor all FCS on their aircraft. This monitoring is intended to detect cracks and other structural deformations before they reach critical proportions, resulting in catastrophic failure. The FCS monitoring process is accomplished by performing damage tolerance inspections (DTis).

DTIs are inspections focused specifically on fatigue critical structures. The aircraft’s DTI program will state when and where to inspect, how to inspect, and how often to repeat the required inspections. DTI inspections may be accomplished using visual inspection, eddy current, penetrant, X-ray, or other methods. However, drawbacks to these methods are that the inspections are a ‘snapshot’ of the airplanes structural condition at the time that the inspection was performed and, although trends can be determined, it is not a real-time monitor of the airplane. These inspections usually form part of a planned maintenance schedule.

A condition based maintenance (CBM) strategy ensures that maintenance is carried out on an airplane only when the maintenance is needed. A fully integrated vehicle health monitoring (IVHM) system supports a CBM programme by providing a constant source of airplane system performance and integrity data to allow maintenance analysts to schedule maintenance as and when it is needed. An on-board structure health monitoring (SHM) system utilises sensors deployed at key points on the airplane structure. Strain sensors are bonded to a critical point on the structure. If the structure at that point becomes deformed, the strain sensor also becomes deformed.

This deformation changes the electrical characteristics (typically the resistance) of the sensor indicating that damage, stress or strain is evident in the structure. The sensors are controlled and monitored by central sensor controllers which are linked to the airplane onboard maintenance computers. This data is then recorded for download and analysis by ground maintenance staff. Data can also be relayed to ground stations whilst the airplane is in flight. A number of technologies have been or are being developed for structure and systems monitoring. Ultrasonic, comparative vacuum monitoring and smart aircraft structure systems have come some way to realise the effectiveness and viability of integrated SHM systems. The ultrasonic method uses a series of ultrasonic radiating elements and receiver sensors attached to the airplane structure. The waves radiating from the ‘transmitting’ elements are altered by damage to the structure. When the received waves are compared by analysts with waves from an undamaged structure, the level and extent of damage can be determined.

COMPARATIVE VACUUM MONITORING (CVM)

This method uses sensors containing a vacuum manifold and an air manifold. These manifolds are referred to as ‘galleries’ and are inter-meshed in the sensor material. The sensor is bonded to a point on the aircraft structure.

A vacuum source is used to create a low vacuum in the vacuum ‘gallery’, the air gallery is pressurised to atmosphere. Under normal conditions a differential pressure exists between the two galleries which, is measured by a pressure monitor. If a crack forms in the structure across the sensor galleries air will be allowed to leak from the atmospheric gallery to the vacuum gallery reducing the differential pressure between them. This pressure reduction is detected by the pressure monitor. The sensor controller processes the data from each sensor to determine the location and size of the crack.

Very small cracks can be detected using this system as the air and vacuum manifolds in each sensor are placed between 100 to 300 micro-meters apart. Increased use of composite materials in airplane manufacture has introduced new methods and strategies for maintenance and repair of airplane structures. Composite materials are more brittle and susceptible to damage than aluminum alloys traditionally used in airplane structures.

Where lower electrical conduction qualities in composite materials pose a challenge with regard to airplane electrical bonding, lightning strike and grounding, developments in newer composite materials known as ‘nano-composites’ possess semi-conductor properties which can be used in the manufacture of ‘smart aircraft structure systems’.

Components manufactured from ‘nano-composites’ could be monitored by on-board systems where any change in the components semi-conductive properties is measured to determine if any damage has occurred. Effective structure monitoring is crucial for preventing accidents caused by structural failure. For this reason, all data gathered during damage tolerance inspections must be recorded and carefully evaluated to ensure that the aircraft remains structurally sound