In depth description of central maintenance system of aircraft

1. Central maintenance system /
on- board maintenance system
ATA-45
• What is CMS?
• CMS components
• Details of components

2. • It is electronic / computer based system used to monitor and record
the performance status of systems.
• It monitors the aircraft systems for faults , record and store the fault
data and provide the information about these faults to flight crew and
maintenance personnel.
• The data collected by on board maintenance system can be accessed
both in flight and on ground.
• On board maintenance systems also store procedural data
contained in maintenance manuals, troubleshooting manuals,
flight manuals, and other publications.
What is CMS?

3. Components of CMS:

4. Central maintenance computer :
• The CMC is the main processing unit of the CMS.
• The CMC acquires, processes and outputs data to and from various
airplane systems.
• Typical inputs to a CMC would originate from airplane system built in
test results initiated by the CMS, airplane system operating modes
and commands from flight and maintenance crews.
• The outputs from the CMC are provided for flight deck visual and
aural displays, to initiate airplane system built in test equipment
(BITE), data transfer to other storage media such as remote memory
devices, printers, transmission via airplane and reporting
systems(ACARS).

5. Schematic diagram of CMC:
.

6. CMC:
• The power supply module receives the airplane power supply voltage 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.
• 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.
• 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 CMS components.
• A "crosstalk" bus enables the two CMC's to share and monitor faults within each CMC. 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.
• the central processing unit (CPU) performs all of the arithmetic and control functions within
the CMC. It can generate upto 10000 fault codes

7. Printers
• Aircraft printers receive input from CMCs, the ACARS system, and other
sources by means of data lines.
• 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.

8. Multi functional display unit :
• The MCDUs enable the user to navigate the on board maintenance system
by selecting from various on-screen menus.
• user can view and select current/present faults, previous faults, upload and
download databases, and initiate system BITEs.
• The user can also view component and database information such as, part
numbers, serial numbers and database versions.

9. 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).
• It gather and concentrate data concerning aircraft position, attitude, speed, fuel status, etc,
as well as data from the CMS. is data is then 'packaged' and sent to the airplane operators
ground operation stations where the information may be analysed and the performance of
the airplane be monitored.
• The ACARS data is transmitted to the ground using VHF communications radio, HF radio or
satellite communications (SATCOM).
• In ACARS, there are two modes of operation: Demand Mode and Polled Mode.
1. Demand Mode: In this mode, the aircraft sends data only when it is specifically requested by the
ground station or when certain events occur.
2. Polled Mode: In Polled Mode, the ground station periodically sends requests for specific data to the
aircraft. The aircraft responds by transmitting the requested data at regular intervals, regardless of
whether any specific event or trigger has occurred.

10. Data loading system:
• 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.
• The primary uses for the data loading system are the uploading of program
updates, the uploading of database updates, and the downloading of reports.
• 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.

11. BITE:
Units that are monitored by the CMC may contain circuits known as built in test
equipment (BITE).
• typical BITE functions initiated by the CMC is 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 and complexity.
• 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.
If any fault is detected by the BITE during these tests, an output is generated and
sent to the CMS.

12. FAULT CLASSIFICATION:
• Level A :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.
• Level B: 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.
• Level C: Advisories, displayed as a WHITE visual indication, have no
accompanying aural tone and require general crew awareness and is recorded
by CMS for retrieval by maintenance crews on the ground.