WIRING CHECKS USING TSM "NFF POLICY"

WIRING CHECKS USING TSM
"NFF POLICY"

GENERAL

A system is composed of LRUs which can be: computers, sensors, actuators. Probes, etc. With the new technology, most of these Line Replaceable Units (LRUs) are controlled by digital computers. For safety reasons, these LRUs are permanently monitored, they can be tested and troubleshooting can be performed. In each system, a part of a computer is dedicated to these functions: it is called Built-In Test Equipment. Sometimes, in multi-computer systems, one computer is used to concentrate the BITE (Built-In Test Equipment) data of the system.

BITE

During normal operation, the system is permanently monitored: internal monitoring, inputs/outputs monitoring, link monitoring between LRUs within the system.

FAULT DETECTION

If a failure occurs, it can be permanent (consolidated) or intermittent.

ISOLATION

After failure detection, the BITE is able to identify the possible failed LRUs and can give a snapshot of the system environment when the failure occurred.

MEMORIZATION

All the information necessary for maintenance and trouble shooting is memorized in a Non-Volatile Memory.

 

TEST

The test function can be divided into 4 groups

1. POWER UP TEST

The power up test is first a safety test. The purpose of a safety test is to ensure compliance with safety objectives. It is executed only on ground after long power cuts (more than 200ms) Its duration is function of the system that is not operation during the power up test.

If the aircraft is airborne, the power up test is limited to few items to enable a quick return to operation of the system. The typical tasks of a power up test are: test of microprocessor, test of memories, test of ARINC 429 and various I/O circuits, configuration test.

 

2. CYCLIC TESTS

These tests are carried out permanently. They do not disturb system operation. The typical tasks of a cyclic test (also called IN OPERATION TEST) are: Watchdog test (a watchdog is a device capable of restarting the microprocessor if the software fails), RAM test, Permanent monitoring is performed by the operational program (e.g.: ARINC 429 message validity).

 

3. SYSTEM TEST

The purpose of this test is to offer to the maintenance staff the possibility to test the system for trouble shooting purposes. This test can be performed after the replacement of an LRU in order to check the integrity of the system or sub-system. It is similar to the POWER UP TEST but is more complete. It is performed with all peripherals supplied.

 

4. SPECIFIC TESTS

For some systems, specific tests are available. The purpose of these tests is to generate stimuli to various command devices such as actuators or valves. They can have a major effect on the aircraft (automatic moving of the slats or flaps, engine dry cranking).

 


 

 

NEW CONCEPT:

The BITE information stored in the system BITE memories is sent to a centralized maintenance device. The manual tests (SYSTEM TEST and SPECIFIC TESTS) can be initiated via this centralized maintenance device. Its main advantages are:

- single interface location (cockpit)

- easy fault identification

- reduction of the trouble shooting duration

- simplification of the technical documentation

- standardization of the equipment.

 

 



Lets take this quiz and check your score.


 

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