3.2. Risk Identification and Network Building
The BN model topology depends on the number of variables considered in a risk scenario and how they are linked to each other. The number of variables directly determine the number of nodes used in a BN model. By using Bayes’ theorem, one or more variables can be updated based on the evidence stated in one or more variables. The rule of updating variables is determined by the way that one and other variables are linked.
The risk scenario of a drone crash during operation in the 3T Region of Indonesia depends on many external and internal factors. The external factors consist of but are not limited to, interference, environmental conditions, obstacles, navigational environment, air traffic environment, electrical environment, communication, and the human factor. The internal factors can arise from mechanical, thermal, electronic, algorithmic, technical factor, software, and hardware. The number of variables involved in the model should be enough to represent the significant phenomena to build a proper crash risk model of drone operation, but not so many that cause excessive and unnecessary calculation.
The number of variables in the BN model is determined based on the available data and subjective view of the model developer as an expert in a specific field. This method is applied because the BN model not only considers relative frequency as a statistic element but also consider causality between variables. That is why in the BN model subjective view is inevitable to model a proper risk scenario.
In fact, many data are hard to obtain from measurement such as reliability of drone system, reliability of each drone components, probability of disasters, etc. Therefore, this research only focuses on two factors to be included in the Bayesian Network model. The weather effect is comprised of wind speed and rain/precipitation probability. Those two variables are specific for each place in the world, including the 3T Region of Indonesia. The wind speed and precipitation probability model in this research represent the condition of Indonesia low altitude atmosphere (< 120 m) in Papua, Indonesia. The internal system component failure or malfunction (SCFM) probability model represents the probability of a non-critical onboard system to be failed or malfunction during drone operation.
As can be seen that to build a BN model, data is important. The more complete data, the more accurate the BN model will be to predict the probability of an event. It is difficult to collect the related drone data in Indonesia because drone application is still in its early development phase and that the regulation for drone operation data recording requirement has not been established. Without established drone operational regulation, the operators may have different method of data recording and only consider data that important for their own purposes.
In this research, the BN model’s variables are determined by considering the experience of drone operators in Indonesia. We have conducted interviews with three different popular drone operators in Indonesia. Three managers from different Operator (1, 2, and 3) are interviewed to qualitatively assess risk factor of UAV operation. As a background, Operator 1 is a multinational company specializing in aerial mapping and inspection, with most of the company's client are oil and gas company. Second company (Operator 2) is from technology startup which provides wide range of UAV applications, including aerial mapping, tower inspection, and pesticide spraying. Lastly, Operator 3 is from non-government organization specializes in disaster mitigation and damage assessment using UAVs, and already deployed in several events, such as Garut flood in 2016 and Palu flood ad earthquake in 2018. Even though all the three interview resources have different business products, all of them have experiences operating in remote areas around Indonesia.
In the intervie, three drone operators in Indonesia gave us information about the challenge in operating drone in Indonesia based on the company experience. Genuine experiences of the drone operators in Indonesia give more accurate judgment to develop proper risk model of drone operation in Indonesia. Using BN approach, this testimony will become valuable data for the model. The result of the interviews is summarized in
Table 1.
From the Drone Operators’ explanation, factors that can cause drone to crash during operation in Indonesia are high wind speed, storm, inaccurate pilot judgement, miscalculated of natural obstacles (tree), artificial obstacle (electrical wire, pole, etc.), inaccurate guidance system and wild bird attack.
Based on the interview results, all the interviews confirm that there are several environmental factors that have a significant effect, which are wind speed, terrain topography, predatory birds and trees, while others have less effect on the drone operation, such as rain (to some degree). The result of the interview can be seen in
Table 1, indicating the significant factors that caused drone incidents. From this fact, the wind speed will contribute higher to drone crash probability, while rain and SCFM will have less effect on drone crash probability.
Crash risk probability is modelled using three parent nodes (Rain, Wind, SCFM), one intermediate node (Weather Effect), and one child node (Crash Risk) that are connected, as shown in
Figure 4. Each parent node has its own probability distribution assigned to represent the probability of occurrence. Wind speed is classified as low, medium, and high based on its effect on a drone in flight. As can be seen in
Table 2, low speed wind is 0-4 m/s, medium speed wind is 4-8 m/s and highspeed wind is 8-10 m/s based on Operator 1’s explanation about wind effect on drone operation. Low-speed wind does not have any effect on drone operation, while medium speed can cause deviation from the desired flight direction, and high-speed can cause loss-of-control on drone operation.
The effect of rain intensity (precipitation rate) is not significant to the operation. It is important to note that drone operators in Indonesia usually avoid flying while raining to reduce the risk of uncontrolled weather suddenly turning into a storm. The rain classification by BMKG (Meteorological, Climatological, and Geophysical Agency) as shown in
Figure 5 states that light rain is less than 100 mm/month, medium rain from 100-300 mm/month, heavy rain for 300-500 mm/month, and very heavy rain for those higher than 500 mm/month.30 The effect of each class of rain intensity is minor to UAV flight, as stated by one of the operators. Thus, the causality of rain to drone flight will not be specified in detail, but it can be assured that the rain affects in reducing visual of the ground pilot that may lead to increased crash risk.
Rain and Wind nodes are connected to Weather Effect as an intermediate node to simplify the process of modelling the crash risk’s conditional probability table (CPT). The size of Crash Risk’s CPT is determined by the number of its parent node. By using weather effect as intermediate node, it reduces three parent nodes (Wind, Rain and SCFM) into two parent nodes (Weather Effect and SCFM). The Weather effect node has three states that represent the intensity of weather effect to the crash risk, which are light, medium, and heavy. The CPT of the weather effect node is built based on each factor’s contribution to the drone crash. The CPT for weather effect is shown in
Table 3.
The SFCM node represents the flight data of UAVs system component failure or malfunction occurrence in an annual average, which was adopted from Reference [
24]. However, there are different definitions of SCFM in the data collected by Reference [
24]. For example, the data obtained from NASA says that SCFM is one of the significant factors that can cause UAVs to crash, which is estimated that cause 60% of UAV crash cases. As for other resources than NASA, SCFM probability is in the range of 6-32%. This probably happened because NASA and other resources have different definitions of SCFM. SCFM with a low-risk percentage is selected because this research assumed that SCFM only include a minor component that, if it fails, will not cause a major effect on drone flight.
The crash risk node is the child node of the BN model. It is required to have a fully defined CPT. One of the challenging steps in modelling probability using the BN approach is constructing a proper CPT table to represent the real conditional probability of occurrence based on all the contributing factors. For the crash risk’s CPT, a table is created to map every combination of weather effect states and SCFM values, as shown in
Table 4,
The crash risk node has four states, which are adopted from risk severity ranking or category in ISO 12100. The severity of a hazard is measured based on its consequences to people, operations, drones, and the environment. The four categories of severity are catastrophic, critical, marginal, and negligible. Each severity category is adopted from Reference [
24]. They are,
Catastrophic: the hazard causes harm or serious injuries or deaths to humans. The severity of such hazards is the highest considering that it affects human safety and thus must be carefully addressed and removed to avoid fatal situations.
Critical: the event influences third parties except people, for example damaging buildings or assets in general.
Marginal: the event causes damages to the drone system itself.
Negligible: the event does not affect the operational capability of the drone (safe operation), or the drone doesn’t get crashed at all.
Using Operator 1’s drone operations annual data in 2021.
Statistical data of rain probability (Rain node), wind speed probability (Wind node) and system component failure or malfunction (SCFM node) are put in each associated parent node. Wind and Rain nodes are connected to the Weather Effect node. This requires a definition of CPT of the weather effect node. Then the weather effect node and SCFM are connected to the Crash Risk node as shown
Table 2. All this data and relationship definition between nodes are implemented in AgenaRisk26.
AgenaRisk is a Bayesian Network software designed for Risk Assessment by utilizing the Bayesian Network Method. AgenaRisk provides the user with a graphical user interface (GUI) to create the Bayesian network model. Thus, the user does not have to create the subroutine manually. AgenaRisk can also be used for diagnostic inference, causal inference, scenario analysis, and sensitivity analysis. Users can choose from GUI what analysis is required to be conducted.
AgenaRisk offers severall options to model each node of a probabilistic event. For example, users can choose to model a node as a Boolean, ranked, continuous interval, integer interval, etc. Each node type requires user to assign probability of the event. Each node can be connected to create a link based on the user requirement. If a node is determined as a child or intermediate node, then the user has to set a CPT that govern the contribution of its parent node to the child or the intermediate node probability of occurrence.