A Risk-Based Approach for the Classification of UAS

By William Rauch posted 30-06-2014 15:35

  
In a series of papers by Clothier, et al., a risk-based approach for the classification of UAS (Table 1) for determining airworthiness in Australia is proposed.  The “type” columns in the matrix, in order of increasing magnitude of loss, are grouped based on the maximum degree of loss the UAS could cause, independent of where it crashes.  Accordingly, each type column represents a grouping of UAS by some predefined loss criteria irrespective of operational environment.  The row “category” represents the varying operational environments over which a UAS may fly, in order of increasing magnitude for loss, independent of the UAS type.  Consequently, each cell of the matrix represents a unique combination of UAS type and operational environment that defines the level of risk.  As shown in Table 1, the cells, representing the operational environment and associated UAS type category, in the lower right quadrant of the matrix represents the highest levels of risk whereas the cells in the upper left quadrant of the matrix represent the lowest levels of risk.  Unlike air worthiness regulations for manned aircraft, the type category does not define air worthiness for UAS.  Thus, a given UAS may be classified in more than one cell depending on the operational environment flown.   

 

Table 1. Clothier, et al. Classification


 

The basis of this risk matrix is the paradigm that exists between manned and unmanned air systems.  That is, air worthiness for manned systems is primarily based on protecting the people on board irrespective of the operational environment or the purpose of the flight.  Whereas with UAS, air worthiness is based on protecting people and property on the ground (e.g., operational environment) and not on the UAS itself.  Thus, the primary safety concern of interest is not the UAS itself but the potential of the UAS for causing damage to people and property on the ground.  Since UAS may be operated over a wide range of areas, the associated risk varies.  An advantage of the risk matrix is that the operational area is taken into consideration in assessing the overall risk of a UAS flight. 

 

This matrix provides a systematic and flexible approach for regulating the airworthiness of a UAS based on risk.  The type category, based on a predefined maximum level of loss and independent of where it crashes, provides a viable means for classifying the large and variable systems and operations among UAS.  Next, by assigning airworthiness categories based on the operational environment and thus, risk, the proposed classification system satisfies the fundamental differences between manned and UAS risk paradigms.  

     

One of the challenges to the classification risk matrix is determining the appropriate number of airworthiness certification categories (cells).  The minimum number of categories is one, which is the undesirable situation in which all UAS operational environments are regulated under the same standard without consideration of the varying levels of risk.  The maximum number of airworthiness certification categories (cells) is the number of rows times the number of columns in the matrix.  

 

Since the primary safety concern of flying UAS is the potential harm to people on the ground, four type categories have been proposed based on the energy required to injure or kill a person hit by a UAS (Table 2).


 


Table 2. Type Categories Based on Ability to Cause Harm

Category

Description of Loss Outcome

Energy Limit (J)

Description of Energy Limit

1

UAS capable of causing a non-fatal injury to one or more exposed people

KEMax < 42

< 5% probability of causing a fatal injury to an individual standing in the open

2

UAS capable of causing a fatal injury to one or more exposed people

> 42 KEMax < 1,356

> 5% probability of causing a fatal injury to an individual standing in the open

3

UAS capable of causing a fatal injury to one or more people within a typical residential structure

< 1,356 KEMax < 13,560

Capable of penetrating a corrugated-iron roof house

4

UAS capable of causing a fatal injury to one or more people within a typical commercial structure

KEMAX > 13,560

Capable of penetrating a reinforced concrete structure

 

Maximum takeoff weight (MTOW) is a common variable used in classifying UAS.    Mapping almost 400 fixed-wing UAS MTOW to the four categories in Table 2 resulted in an ambiguous classification, indicating that MTOW alone is not sufficient to describe the potential harm to people on the ground.  It was observed that UAS with a MTOW greater than about 20 kg have sufficient energy to be classified as category 4.  Thus, category 4 includes a disproportionate number of UAS types.   

 

Adding maximum impact area assists in differentiating type categories based on the premise that the larger the impact area, the higher the probability of more than one person being harmed in a UAS crash.  Using standard cluster analysis techniques, subdividing category 4 into two sub-categories resulted in a total of five UAS type categories (Table 3).

 

 

Table 3. UAS Type Categorization Scheme

Type Category

Boundary Conditions

1

KEMax < 42 J

2

42 J < KEMax < 1,356 J

3

1,356 J < KEMax < 13,560 J

4

13,560 J < KEMax IArea < 345 m2

5

347 m2 < IArea


This simplified model (see the references for the assumptions) is only intended to illustrate an objective approach for specifying UAS type categories.

 

In summary, the proposed risk-based approach for the classification of UAS for determining airworthiness in Australia differentiates the paradigm between manned aircraft and UAS.  Airworthiness for manned systems is primarily based on protecting the people on board irrespective of the operational environment or the purpose of the flight.  Whereas with this risk-based approach, airworthiness is based on protecting people and property on the ground.  Another advantage is it allows for the grouping of UAS type based on a predefined maximum level of loss and independent of where it crashes.  Thus, it provides for a viable grouping of UAS among large and highly variable systems and operations.   

        

Sources:

Clothier, Reece A., Palmer, Jennifer L., Walker, Rodney A., and Fulton, Neal L.  (2010) Definition of airworthiness categories for civil Unmanned Aircraft Systems (UAS).  In:  27th International Congress of the Aeronautical Sciences, 19-24 September 2010, Acropolis Conference Centre, Nice, France.

Clothier, Reece A., Palmer, Jennifer L., Walker, Rodney A., and Fulton, Neale L. (2011) Definition of an airworthiness certification framework for civil unmanned aircraft systems. Safety Science, 2011.

Wu, Paul and Clothier, Reece A. (2012)  The Development of Ground Impact Models for the Analysis of the Risks Associated with Unmanned Aircraft Operations Over Inhabited Areas.  In: Eleventh Probabilistic Safety Assessment and Management Conference (PSAM11) and the Annual European Safety and Reliability Conference (ESREL 2012), 25-29 June 2012. Helsinki, Finland.

Clothier, Reece A., Williams, Brendan, de Lamberterie, Pierre.  (2012) Evaluation of Robust Autonomy and Implications on UAS Certification and Design.  In:  28th International Congress of the Aeronautical Sciences, 23-28 September 2012, Brisbane, Australia. 

 

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