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analysis technique is to uncover latent (sneak) circuits and conditions that inhibit desired functions or cause
undesired functions to occur, without a component having failed. The process is convert schematic
diagrams to topographical drawings and search for sneak circuits. This is a labor intensive process best
performed by special purpose software. Figure 9-2 shows an automobile circuit that contains a sneak
circuit. The sneak path is through the directional switch and flasher, the brake light switch, and the radio.
Figure 9-2: A Sneak Circuit
FAA System Safety Handbook, Chapter 9: Analysis Techniques
December 30, 2000
9 - 9
The latent nature of sneak circuits and the realization that they are found in all types of electrical/electronic
systems suggests that the application of SCA to any system that is required to operate with a high
reliability is valuable. This process is quite expensive and is often limited to highly critical (from the safety
viewpoint) systems. Applications include many systems outside the FAA such as nuclear plant safety
subsystems, ordnance handling systems, and space craft. Consideration should be given to utilizing this
tool for FAA applications that eliminate human control such as an autopilot.
The fact that the circuits can be broken down into the patterns shown allows a series of clues to be applied
for recognition of possible sneak circuit conditions. These clues help to identify combinations of controls
and loads that are involved in all types of sneak circuits. Analysis of the node-topographs for sneak circuit
conditions is done systematically with the application of sneak circuit clues to one node at a time. When all
of the clues that apply to a particular pattern have been considered, it is assured that all possible sneak
circuits that could result from that portion of the circuit have been identified. The clues help the analyst to
determine the different ways a given circuit pattern can produce a "sneak." Figure 9-3 is a node topograph
equivalent of Figure 9-2
Power
Directional
Switch
Flasher
Lights Brake
Light
Switch
Radio
Figure 9-3: Topical Node Representation of Sneak Circuit
There are four basic categories of sneak circuits that will be found.
Sneak Paths - allow current to flow along an
unsuspected route
Sneak Timing - causes functions to be
inhibited or to occur unexpectedly
Sneak Labels - cause incorrect stimuli to be
initiated
Sneak Indicators - cause ambiguous or false
displays
FAA System Safety Handbook, Chapter 9: Analysis Techniques
December 30, 2000
9 - 10
In addition to the identification of sneak circuits, results include disclosure of data errors and areas of
design concern. Data errors are identified and reported incrementally on Drawing Error Reports from the
time of data receipt through the analysis period. These errors generally consist of lack of agreement
between or within input documents. Conditions of design concern are primarily identified during the
network tree analysis. Design concern conditions include:
Unsuppressed or improperly suppressed
inductive loads
Excess or unnecessary components
Lack of redundancy
Failure points.
The three resultant products of SCA (sneak circuit, design concern, and drawing error conditions) are
reported with an explanation of the condition found, illustrated as required, and accompanied with a
recommendation for correction.
9.6 Energy Trace
This hazard analysis approach addresses all sources of uncontrolled and controlled energy that have the
potential to cause an accident. Examples include utility electrical power and aircraft fuel. Sources of
energy causing accidents can be associated with the product or process (e.g., flammability or electrical
shock), the resource if different than the product/process (e.g., smoking near flammable fluids), and the
items/conditions surrounding the system or resource of concern (e.g., vehicles or taxing aircraft). A large
number of hazardous situations are related to uncontrolled energy associated with the product or the
resource being protected (e.g., human error). Some hazards are passive in nature (e.g., sharp edges and
corners are a hazard to a maintenance technician working in a confined area).
The purpose of energy trace analysis is to ensure that all hazards and their immediate causes are identified.
Once the hazards and their causes are identified, they can be used as top events in a fault tree or used to
verify the completeness of a fault hazard analysis. Consequently, the energy trace analysis method
complements but does not replace other analyses, such as fault trees, sneak circuit analyses, event trees,
　
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