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26 M.O. ŞERİFOĞLU, T. HIZARCI and H. AKYILDIZ

responses to faults in its range. Active fault tolerant systems depend on an assistant FDI system.
AFTC system has a complex structure and it responds to failures actively. AFTC can answer a
wider variety of faults and can improve itself over them. AFTC has many sub-method and also
has a wider variety of application. It also has research possibilities and many areas to improve.
But it is complicated and because of its high mathematical requirements, it is expensive and hard
to apply. Fault Detection and Isolation system or FDI is one of the most important parts of AFTC.
Its function is to detect the failure and the find its location. Reliability of this system plays a great
role in reliability of the AFTC in general.

4.3 Application of FMEA Method

FMEA will be a guide to supply necessary information for FTC method later. To apply FMEA,
we need to first determine possible failure modes, second, assign severity, probability and
detection values and third, calculate the risk priority numbers (RPN) for each failure mode. For
this work, experience, technical sheet information of individual parts of the system and some
experimental data are used rather than statistical failure data as there was not enough data. For
severity, probability and detection values general tables and ranking are used. In the, Table 2, 3
and 4 below, the used parameters can be seen for these values.

Effect Table 2: Severity Table Ranking
Severity
Hazardous without warning 10
Failure mode affects safe system operation without
Hazardous with warning warning 9
Very high Failure mode affects safe system operation with warning 8
High System inoperable with destructive failure 7
Moderate System inoperable with equipment damage 6
Low System inoperable with minor damage 5
System inoperable without damage
Very low System operable with significant degradation of 4
performance
Minor System operable with some degradation of performance 3
Very minor System operable with minimal interference 2
None No effect 1

Table 3: Probability table

Probability of Failure Probability Ranking
10
Very High >1/2 9
8
1/3 7
6
High 1/8 5
4
1/20 3
2
Moderate 1/80 1

1/400

1/2000

Low 1/15000

1/150000

Remote <1/1500000

GiDB|DERGi Sayı 8, 2017