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How to mitigate the Ground Risk?
In June 2022, EASA initially proposed Means of Compliance (MoC) for mitigating SORA in M2 for medium and high robustness. After receiving significant attention and feedback, a working group consisting of EASA and some NAAs was formed to further develop the subject. Subsequently, in November 2022, this working group joined the UAS Technical Body’s Airworthiness Task Force (AW TF), which prioritised the development of two deliverables, namely D1 and D2.
D1 focuses on addressing mitigation means for medium robustness, utilising specific technical means associated with the UA. Meanwhile, D2 aims to offer guidance on how to evaluate the UA’s critical area to select the appropriate UA size in SORA step #2, but without using specific technical means associated with the UA.
In this article, we will discuss the document D1 about the development of the Means of Compliance (MoC) for SORA mitigation means M2.
This document applies to all levels of SAIL indexes, and is aimed at the following communities:
- Operators who are not designers of the UAS or of the mitigation means
- They apply for an Operational Authorisation
- Operators who have also designed the UAS and/or the mitigation means.
- They may apply for Operational Authorisation or;
- They may apply for a Design Verification
- Designers who have designed the UAS and/or the mitigation means and do not operate the UAS.
- They may apply for a Design Verification
Nominal target for M2 mitigation with medium integrity
When it comes to drone safety, M2 mitigations are crucial for reducing the impact of a potential ground collision once the control of the operation is lost. There are three types of M2 mitigations:
Type 1: Reduction of the size of the expected critical area
To achieve a ground risk reduction (GRr) of approximately 90% through type 1 M2 mitigation with medium integrity, the critical area should be reduced by following a specific process:
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- Determine in which GRC column the aircraft to be flown is located according to the SORA methodology.
- Determine the value of the Nominal Critical Area (CAn) according to the table below:
Max characteristic dimension (m) ≤1 ≤3 ≤8 ≤20 Nominal critical areas (m2) 0.8 8 135 1350 13500 - Finally, to achieve a 90% reduction in critical area, the claimed critical area (CAc) must be equal to or less than the nominal critical area of the adjacent column to the left of the CAn.
Type 2: Reduction of probability of lethality of a UA impact
“Lethality” is defined as the probability of causing a fatal injury by the UA upon impacting a person, having applied M2.
If lethality is less than or equal to 0.1, one point less of ground risk classification (GRC) can be claimed for a ground risk reduction of approximately 90%.
Type 3: M2 mitigation combines both type 1 and type 2 methods.
To obtain a ground risk reduction (GRr) of approximately 90% by means of both methods we have to options:
- If the CAn is not 135, the equation is: Lethality * CAc/CAn ≤ 0.1
- If the CAn is 135, a correction factor needs to be applied with the equation: Lethality * [(0.9*CAc/127) + 0.043] ≤ 0.1.
While these values and equations are provided for nominal reference, it is acceptable to approximately reach the target safety gain for medium robustness mitigations. By implementing these M2 mitigations with medium integrity, drone operators can significantly reduce the risk of ground impact in case of loss of control.
General Means of Compliance for M2 medium robustness
As a drone pilot or operator, it’s important to be aware of the means of compliance for M2 medium robustness.
Here are the guidelines for complying with M2 mitigation and supporting evidence:
- The mitigation means reduce the effect of ground impact
- The mitigation means works with sufficient reliability in the event of a loss of control
- The mitigation means does not introduce additional risk
However, it’s important to support these declarations with evidence, which can include documentation of appropriate testing, analysis, simulation, inspection, design review or operational experience. Evidence from operational experience should be supported by operation records and flight data.
The evidence should include the description of the mitigation means and how this reduces the effect of ground impact in case of loss of control. When the mitigation means require activation, its functioning should also be described. For example, the Flight Termination System.
The points to follow to comply with the requirements of the Means of Compliance are described below:
Description of the mitigation and the involved systems
To comply with M2 mitigation, you need to provide a description of the mitigation and the involved systems. This includes:
- Describing the physical elements of the mitigation means.
- Describing the functional architecture of the mitigation means, including identifying the mitigation means’ functions and chain of events that lead to the activation of the mitigation means, clarifying the interfaces between the UAS and the mitigation means, and highlighting all UAS’s functions necessary for the operation of the means.
- Documenting the required operational procedures for the utilisation and maintenance of the mitigation means, including supplementing with the recommended training and instructions for the personnel responsible for these tasks. A training syllabus supplement for the operation of the mitigation means should also be available. The operator should provide competency-based, theoretical, and practical training.
Provide evidence that the mitigation means reduces the effect of ground impact
To comply with M2 mitigation, you also need to provide evidence that the mitigation means reduces the effect of ground impact. This includes:
- Describing the principle on which the mitigation means works (Type 1, Type 2, or Type 3).
- Compiling all calculations, test evidence, and other possible evidence into the report, showing that the mitigation means achieves the necessary performance target.
- At least one representative flight test should provide the evidence of the claimed impact characteristics. Parameters to be assessed after activation of the mitigation would be, for example, descent speed, descent angle, evidence of parts detachment, impulse, transfer energy (where applicable).
- Demonstration by simulation should be limited to cases in which testing would be highly impractical or dangerous.
Provide evidence that the mitigation means works with sufficient reliability in the event of a loss of control
This criterion requires that the mitigation means be operational in the event of a malfunction resulting in a loss of control of the drone.
- It is necessary to identify all possible malfunctions that may cause the UA to crash, and provide justifications for how the mitigation means can be successfully activated in those situations.
- Compiling all relevant test evidence and other supporting data into the report to demonstrate that the mitigation means achieves a reliability of at least 90% after activation is crucial.
Provide evidence that the mitigation means does not introduce additional risk for people
- It is necessary to explain how inadvertent activation of the mitigation means does not have a negative impact on the expected loss of control rate for an operation.
- It is crucial to demonstrate that a failure or malfunction of the mitigation means does not lead to an adverse increase in the loss of control rate.
2 Responses
So Question on SORA – SORA always states an explosion when a UAS hits the ground.
Over 20 years of RC Flying and Multi rotor UAS I have not seen this even with Petrol RC planes. The chance of an explosion is a non starter.
Next Flight Termination systems Are Flawed – They can be triggered by mistake causing an issue and these systems employ some serious dangerous ways of terminating flight such as prongs that interfere with the props to destroy them to make the aircraft come down. They also trigger if you go out of the flight terminations ystem radio range but the UAS control range is fine. Whoever thinks thats a good idea is no UAS expert. Problem is understanding what causes a so called fly away best way to deal with it. For example GPS failure is not a fly away or loss of control – you just fly the aircraft for example.
then what science is the falling out of the sky and hurting someone based on ?
ASSURE in the USA has done testing that shows upto a 6kg drone, is extremely unlikely to kill anyone due to the way drones are made and have built in crumple zones. What Testing on human impact if any has been done on this in the EU ? or is it all based off solid weight falling on someone ?
And the final question is; why are UAS rules so complicated vs strapping a paramotor to someones back and just flying it.
Hello!
We fully understand your point of view.
Unlike traditional aviation, drones are a technology that is advancing at a very high speed, and regulators are forced to determine systems to minimise potential harm for aircraft that are not certified and cannot be trusted, unlike their big brothers.
Next year will see the most important changes, as a large proportion of drones will have to be certified and, thanks to the confidence that can be placed in them, the requirements to ensure flight safety will become more permissive over time.