Aerospace and Electronic Systems Magazine March 2017 - 5

or fuzzy logic. To arrive at this conclusion we first review aviation
severity levels and fault trees. Then we discuss the challenges associated with bringing the two together.

FAA SYSTEM SAFETY DEFINITIONS
To quantify aviation risk in the United States, the FAA has defined
five severity levels [7]. Similar definitions have been established
by other civil aviation authorities around the globe. The five severity levels are no failure, minor, major, hazardous, and catastrophic.
As loosely summarized below, each severity level describes the
consequences of an aviation accident.
C
C

C

C
C

No failure: An accident with no safety effect.

outcome (e.g., loss of aircraft) is at the top level of the tree (its
root). The branches of the tree are the sequences of events that result in the undesired outcome. A representative fault-tree diagram
is illustrated in Figure 1.
A common process for constructing a fault tree is to leverage
an FMEA. The FMEA is a systematic brainstorming and analysis
effort that leverages expert knowledge to catalog possible failure
scenarios and their severities [3]. The FMEA traces cause and effect relationships, ultimately mapping component level failure
events to high-level consequences and their associated risks. The
fault tree effectively catalogs all identified failure events, connecting them to the consequence at the root of the tree. As a result
of its rigorous structure the fault tree can be used as a functional
mapping to quantify how low-level risks associated with each fail-

Minor: An accident that reduces safety margin but not in a
significant way, such that effects do not impede operational
functions even though they may increase workload (e.g. for
air traffic control), cause minor property damage, or impact
the environment in a noticeable but minor way.
Major: An accident that reduces safety margin in a significant way, reducing the effectiveness of operational functions
to result in increased workload, major property damage, or
damage to the environment.
Hazardous: An accident that causes one or more fatalities.
Catastrophic: An accident that causes many fatalities and
complete loss of the aircraft.

These severity levels serve as a basis for defining safety requirements for new aviation technologies. The notion is that safety
risk is the product of severity and frequency. If the potential outcome of an accident is more severe, it must be guaranteed to happen with lower frequency. For the purposes of certification, the
FAA quantifies allowed frequency for each severity category in
terms of worst case probability values, as tabulated in Table 1.

A representative fault tree.

Table 1.

FAA Aviation Severity Levels and Allowed Risk [7]
Severity

FAULT TREES
Fault trees are used in civil aviation as a graphical and quantitative
tool for assessing system safety. Fault trees are commonly used
to catalog all fault modes and associated risks in the process of
certifying new aviation technologies. The structure of a fault tree is
defined to show the relationships among fault events that may lead
to an undesired outcome. In this graphical structure, the undesired
MARCH 2017

Figure 1.

No failure

Maximum Allowable Probability
1

Minor

1

Major

10−5

Hazardous

10−7

Catastrophic

10−9

IEEE A&E SYSTEMS MAGAZINE

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