Requirements for UAS in specific category

18.06.2021 – Marius Schröder (Third Element Aviation GmbH)

Requirements in the „Special Condition Light UAS“  by EASA and their implications for operating in u-space airspaces in the context of the research project UDVeo

With the Special Condition Light UAS, the European Union Aviation Safety Agency (EASA) created a first legal framework for requirements for unmanned aircraft systems (UAS), focusing medium risk range in the specific category. This rang also covers applications with higher operational risks and small UAS (with a size/wingspan of max 3 meters and a maximum takeoff mass of 25kg and below). A good example is the use of highly automated flights in urban spaces.

The Special Condition Light UAS defines the scope of application to UAS, which:

  1. are not intended for the transport of persons
  2. ensure the possibility of manual intervention by the pilot
  3. have a maximum take-off mass (MTOM) of 600 kg and
  4. are operated in the specific category in a medium risk range (SAIL III and IV)

In that regard, typical u-space operations are also in the scope of application.

For the scope of application, airworthiness requirements are defined in various subcategories in correlating capers (subparts) of the document. In this document, these subparts are introduced and summarized, with a focus on requirements that are of relevance to u-space applications.

For manufacturers and operators, the special condition is to facilitate the use of UAS. Manufacturers can declare conformity with these requirements and operators can use such declarations in their specific risk assessments to make the risk evaluation easier. EASA further declares that the special condition is to be converted into a certification specification in the medium run. This will allow certification bodies to assess and certify UAS in relation to these requirements.

It is to be noted, that the document contains very general requirements, which typically are a basis for more concrete norming and standardization efforts, with then also contain quantitative requirements, parameters and testing methods.

Subpart B – Flight

Subpart B – Flight defines general requirements for flight operations within the scope of the document. As a key concept, flight envelopes need to be defined. These are to be defined as normal, operational and limit envelopes. External influences, especially wind and weather, need to be taken into account. From a technical perspective, the UAS needs to provide a sufficient performance for these envelopes.

Further, subpart B states that controlability must not rely on special piloting skills.  

Subpart C – Structure

Subpart C – Structure defines general requirements towards the structural integrity of the UAS, especially towards the airframe. Among other requirements, like suitable dimension and acceptable deforming of the airframe, subpart C also states that these dimensions already need to be defined and regarded during design of the UAS, following the principles of safety by design.

Subpart D – Design and Construction

In Subpart D – Design and Construction, further and more specific requirements are defined, that need to be regarded during the design and construction process of the UAS. Examples are suitable fixation of external parts like landing gears of payloads, protection from fire (to possibly protect persons on board) of protection from lighting strike, if such events are not excluded in the operation conditions.

The operation limits derived from the envelopes in Subpart B, have to be taken into account durin the design and construction process and the design has to be developed in accordance with these limits, just as in Subpart C.

Subpart E – Lift/Thrust/Power Systems

In Subpart D – Design and Construction, further and more specific requirements are defined, that need to be regarded during the design and construction process of the UAS. Examples are suitable fixation of external parts like landing gears of payloads, protection from fire (to possibly protect persons on board) of protection from lighting strike, if such events are not excluded in the operation conditions.

The operation limits derived from the envelopes in Subpart B, have to be taken into account durin the design and construction process and the design has to be developed in accordance with these limits, just as in Subpart C.

Subpart F – Systems and Equipment

Subpart F – Systems and Equipment addresses requirements for a systemic point of view. This includes that airworthiness requirements need to be considered both for the aircraft system as a whole but also for all mission critical components. This includes suitable dimensions, also for operations in the limit flight envelope, and in SAIL IV operations (the higher medium risk class, see introduction), the provision of alarms for the remote crew in case of failure of a critical component.

Probable failures must not lead to operation outside operational area. These failures are for example technical faults in individual components or software faults, that cause an uncontrolled operation (so called fly away) or an uncontrolled crash. The aircraft itself need to be able to stay inside the operational area and the flight envelope.

If the ground risk outside the operational area is higher than inside – a typical scenario fo urban u-space operations – the systems must not contain any single points of failure that are critical for the operation. Also hardware and software need to be designed and developed in accordance with accepted standards (which are not defined inside the document). This is also a critical requirement for UAS operations in the u space airspace.

For signaling lights requirements are defined that are similar to the requirements of manned aviation. These requirements are not consistent with requirements from the open category and need to be harmonized.

Subpart G – Remote Crew Interface

In Subpart G – Remote Crew Interface, requirements are defined for the command unit for the operator on the ground. Throughout the document the term remote crew is used and not the established term remote pilot. This implies that possibly several persons may do the task of commanding and controlling the aircraft. For any operation the minimum crew needs to be defined.

Generally, as in all current regulations, the command unit is considered an integral part of the unmanned aircraft system. All necessary information needs to be visualized and continuously transmitted to the command unit. All safety relevant information must be provided in a way that it is easily accessible and clearly marked.

Subpart H – C2-Link

In Subpart H – C2-Link defines requirements for the command and control link (C2). The importance of the link as an essential part of the UAS definition as an unmanned aircraft system is explicitly outlined here again.

Technically the link must be adequately dimensioned. The power and quality of the connection must be monitorable from the ground if it is critical for the operation. Besides these safety requirement, also general security requirements are defined. Specifically, the link must be designed in a way that the aircraft can permanently verify the authenticity of the signal.

Summary

The special condition is a first tangible and clear illustration of requirements for unmanned Systems that are operated in the specific category in medium risk classes. For manufacturers and operators this is a first help and orientation for design requirements and hence an important support document.

Typically for regulation these requirements are formulated for different aspects and are illustrated with qualitative requirements. Later quantitative requirements and further requirements as well as testing and verification methods need to be derived from this.

In the future, manufacturer declarations on fulfilling the special condition requirements are to make the risk assessment in the specific category easier for operators.

Sources and references:

EASA: Special Condition for Light Unmanned Systems

EASA: Guidelines for design verification of drones operated in the specific category

EASA: Easy Access Rules for Unmanned Aircraft Systems