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Studies for drone regulations accelerated the pace for the assessment of risk for drone operations. A recently published ‘Annual Safety Review 2017’ discusses the aviation accidents in detail containing a chapter specialized for drones. This report by EASA, involves the data from European Central Repository (ECR) experienced by EASA member states.
With the increase in the number of drones and possibly raising consciousness on reporting occurrences, the numbers of non-fatal accidents raised by 470% in 2016 relative to 2011-2015 average, luckily maintaining zero fatalities. Most of the times, it is commercial airliner pilots to report the occurrences, and rarely the UAS pilot.
The prior key risk areas has been investigated and aircraft upsets is by far the most common cause of the occurrences and set as the first key risk to address for safe integration of drones into airspace. 50% of RPAS accidents falls in this case which often results in a damage or destruction of UAS following loss of the control of the drone by the pilot.
Second key risk area is airborne collision although it is rarely encountered due to probable frequency with exponential increase in the number of drones. Obstacle collision is the 3rd risk area which will tend to increase with integration of drones especially in urban areas.
Ref : https://www.easa.europa.eu/system/files/dfu/209735_EASA_ASR_MAIN_REPORT_2017.pdf
The air traffic can be divided into cooperative and non-cooperative traffic. The cooperative traffic is equipped with avionics facilitating its detection. The non-cooperative traffic has no such equipment and detection is solely based on ground or onboard sensors. It is important to note that detecting cooperative traffic is a lot easier and more precise than detecting non-cooperative traffic. This is why many experts advocate for all low altitude traffic to be cooperative, at least in high density airspaces, and a proposed solution is to use ADS-B. This solution seem acceptable considering that a large part of the existing traffic is already required (or will be soon) to carry ADS-B out, the technical solutions exist and they are affordable both in terms of SWaP (Size, Weight and Power) and cost. Now, a crucial questions remains: is it possible to introduce hundreds of ADS-B users in already busy (radio frequency wise) airspaces without disturbing the performances of existing systems, e.g. ATM systems ?
To answer this question, the MITRE conducted a study on the impact of equipping low level drones with Universal Access Transceiver (UAT) ADS-B. Both air-to-air and air-to-ground communications were considered. According to this study the crucial parameters are the traffic density and ADS-B transmission power. The following table, extracted from the study, shows the probability to decode a message depending on drones density and transmission power with values in bold being acceptable for ATM applications. With a density of 5 drones per square kilometer the emission power cannot be higher than 0.01W, which strongly limits the communication range, though experiments to know the precise range depending on the transmit power should be conducted.
Overall, the results of this study show that using ADS-B UAT in high density airspaces will prove difficult has reducing the transmission power of ADS-B is likely to decrease detection ranges and impact safety. For the particular case of UAT, considering the fact that it is only used in the US, principally aimed at General Aviation (GA) and with the current grow in GA traffic, the FAA is unlikely to approve such solution to make the drones cooperative. From a broader perspective, the study showed how quickly a cooperative method can overload a communication mean. Having only cooperative traffic is desirable but this kind of study make it look like an unreachable objective. For now…
A recent report, from the John A. Volpe National Transportation Center, prepared for the FAA, presents an analysis of 220 reports of the Aviation Safety Reporting System (ASRS) related to UAVs. The ones filled by Air Traffic Control Operators (ATCOs) are of particular interest as they are crucial players for the integration of UAS in controlled airspaces.
On top of different statistics concerning the events, the report puts forward seven events particularly surprising for ATCOs:
- An unanticipated appearance of the UAV in the airspace;
- Difficulties to contact the UAS pilot;
- The UAV does not comply with pre-coordinated route;
- The UAV cannot accept (comply with) an instruction issued by the ATCO;
- The behavior of the UAV is unexpected; and
- The required actions for the controller are unknown or unclear.
When reading this report, keep in mind that most of these encounters imply military UAS pilots, which explains the high number of remote pilots disregarding ATC instructions.
This type of study is very useful when designing Real Time Simulations with ATCOs in the loop. Indeed, it allows creating worst case scenarios to experiment with ATCOs workload while representing realistic scenarios.
According to the JARUS operational categorization, drone operations fall into one of three categories: A, B and C (or Open, Specific and Certified in EASA vocabulary). Flying in category A just requires to follow a fixed set of rules (altitude limitation, mandatory equipage, etc) while flying in category C requires a certified drone system. To operate in category B, an operator needs to demonstrate that operational risks are mitigated by available systems; such demonstration requires a risk assessment approach.
However existing safety tools are complex to use and not always suited to drone operation specificities. In order to provide adapted, accessible and universal tools to the drone community, the JARUS WG-6 developed the Specific Operations Risk Assessment (SORA), a somewhat simple yet powerful methodology allowing to perform risk assessment for drone operations. The full description of the SORA methodology has been recently published for external consultation on the JARUS website.
Though it is unlikely that using the SORA will be mandatory (other risk assessment methods will remain an option), its design will probably make it the best choice for operators willing to build a safety case. So if you plan on using drones in category B scenarios, you can start reading this document as it is likely to become your bedside book. For those interested, we will publish a more detailed article on the SORA methodology in the months to come.