Introduction to Remotely Piloted Aircraft Systems (RPAS)

Introduction to Remotely Piloted Aircraft Systems (RPAS)


This article introduces several enhancements to SKYbrary’s compilation of articles and source references that primarily discuss remotely piloted aircraft systems (RPA/RPAS), a subset of the broader term unmanned aircraft system (UA/UAS) favored by the International Civil Aviation Organisation (ICAO). The term drone serves as an informal, popular and generic substitute for an RPA or a UA.

Most of the information in SKYbrary pertains to small RPAS because many civilian aviation industry stakeholders are involved in the integration of small RPAS or drones — unlike larger RPAS — into national airspace systems any may have some responsibility for addressing small-RPAS operational safety issues likely to impact manned aviation.

The stakeholders primarily comprise the world’s national aviation authorities (NAAs, including the European Aviation Safety Agency (EASA) and the U.S. Federal Aviation Administration (FAA)); air navigation service providers (ANSPs, including EUROCONTROL); airlines; RPAS manufacturers and suppliers; and small RPAS operators.

In contrast, the larger RPASs — whether operated by/for government research, military/intelligence or corporate missions — share similar histories, levels of advancement and strong track records in safety. Their mature level of integration into controlled, uncontrolled and restricted airspace at any altitude means that accidents involving large RPAs rarely affect national airspace systems. Therefore, improving safety performance of larger RPAS temporarily may receive lower priority among these stakeholders.

Priorities change over time among these stakeholders. One “snapshot” of such priorities by a U.S. research team said in 2017: “Associated with the proliferation of civil applications for sUAS [small UAS] is a paradigm shift from single-UAS visual operations in restricted airspace to multi-UAS beyond visual line of sight operations with increasing use of autonomous systems and operations under increasing levels of urban development and airspace usage. … This is challenging for sUAS operations due to insufficient mishap (accident and incident) reporting for sUAS and the rapid growth of new sUAS applications (or use cases) that have not yet been implemented.”


  • Remotely piloted aircraft (RPA)— ICAO explains this term in Annex 2, Rules of the Air, as: “An unmanned aircraft which is piloted from a remote pilot station:
    • expected to be integrated into the air traffic management system equally as manned aircraft [and,]
    • real-time piloting control is provided by a licensed remote pilot.”
  • Remotely piloted aircraft (RPA) — ICAO defines this term in ICAO Cir 328, Unmanned Aircraft Systems, as “An aircraft where the flying pilot is not on board the aircraft. (Note: This is a subcategory of unmanned aircraft.)”
  • Remotely-piloted aircraft system (RPAS) — ICAO defines this term in ICAO Cir 328 as “A set of configurable elements consisting of a remotely-piloted aircraft, its associated remote pilot station(s), the required command and control links and any other system elements as may be required, at any point during flight operation.”
  • Unmanned aircraft (UA) — ICAO explains this term on The ICAO UAS Toolkit website as “Any aircraft intended to be flown without a pilot on board is an unmanned aircraft. They can be remotely and fully controlled from another place (ground, another aircraft, space) or pre-programmed to conduct its flight without intervention.”
  • Unmanned aircraft system — ICAO defines this term in ICAO Cir 328 as “An aircraft and its associated elements, which are operated with no pilot on board.”

In practical terms, a small RPAS can be visualized as having these configurable elements and characteristics:

  • An RPA;
  • Human-operated remote pilot station(s) usually located on the ground or on a ship, but which may be aboard another airborne platform;
  • A command and control system — sometimes called a communication, command and control system — that includes data links and other system elements (e.g., instruments and transponders on satellites and/or terrestrial cable networks) that connect the remote pilot station(s) to the RPA.

Below are links to other glossaries and explanations of terms related to RPAS:

Civilian Applications of RPAS

Current civilian applications of small RPAS have antecedents in government-operated and military-operated aircraft from the early 1990s — then popularly called unmanned aerial vehicles (UAVs), drones and other names.

Small RPAs (small UAs) in the United States as of March 2019 must weigh less than 55 pounds (25 kilogrammes), operate to a maximum altitude of 400 feet above ground level (AGL) and fly at maximum ground speeds of 100 miles per hour (87 knots). Flying higher than 100 feet AGL is permitted if the RPA remains within 400 feet of a structure.

The remote pilot-in-command (PIC) and the person manipulating the controls of the small RA must operate with a direct visual line of sight (VLOS) to the small RA; alternatively, the small RA must remain within the VLOS of the PIC’s designated visual observer.

Operators and remote pilots frequently apply for and some obtain FAA waivers of several rules, such as waivers that enable nighttime flight, flights above 400 ft AGL, flight beyond VLOS and operation over people. Air traffic control (ATC) accepts from remote pilots/RPAS operators requests for authorization to fly in controlled airspace near airports. To grant an authorization, ATC requires full compliance with Part 107 rules.

Examples of RPAS operators’ missions are:

  • Security surveillance;
  • Emergency response, including involvement in search and rescue (SAR), and delivery of medications and automated external defibrillators (AEDs);
  • Enabling data communication and broadcast of information in remote areas;
  • Small package and bulk cargo transport;
  • Visual, spectral and thermal examination of structures;
  • Monitoring of linear network infrastructure such as railway tracks, power lines and pipelines;
  • Photography, videography, cinematography and cartographic survey;
  • Agricultural fertiliser and chemical application;
  • Aircraft external maintenance inspection and airport infrastructure inspection; and,
  • Atmospheric research and documentation of global warming effects.

Operational Risks to Manned Aviation

To enable routinely flying missions mentioned in the preceding section, regulatory authorities had to mitigate risks of collision with other aircraft (manned and unmanned) and mitigate the risks of a small RA striking people on the ground. (Privacy-protection issues are beyond the scope of this article.) Legacy rules enabled small RPAS flights only in permanently or temporarily segregated airspace, with access by manned aircraft excluded or strictly controlled. The need for sense-and-avoid technology was delayed under older rules.

Current concepts of operation presume that to operate in non-segregated airspace, small RPAS will operate at least at an equivalent level of risk to that which manned aircraft experience. In the United Kingdom, U.K. CAA-approved remote-pilot training requirements and schools that conduct approved courses have enabled people with no prior experience in piloting manned aircraft to qualify to fly RAs. Several other countries began adopting this qualification system in 2015.

Safety Regulation and Guidance

Research scientists have performed a comparison of RPAS-related philosophies and regulations across 56 nations. Their stated purpose was to inform subject matter experts’ perspectives of the global state of RPAS risk management and to benefit NAAs and other stakeholders.

Over time, SKYbrary anticipates adding articles to the RPAS Category, reporting on regulatory and guidance philosophies implemented by NAAs in several states and regions. At present, such viewpoints are briefly described in UAS Rules and Guidance - EU and UAS Rules and Guidance - USA.

For example, FAA in mid-2018 emphasized in public conferences the degree of risk analysis and management required in safety cases presented by RPAS operators seeking operational waivers as described in the above “Civil Applications of RPAS” section. FAA also explained its focus on promoting rapid development of the small-UAS industry (especially small RPAS) through close government-industry collaboration, while requiring every operator-applicant for waivers and/or airspace authorizations to prove its safety case with strong evidence of effective mitigations and logical arguments.

Meanwhile, in the European Union, the Joint Authorities for Rulemaking on Unmanned Systems (JARUS) received European Commission approval of proposed changes to safety regulatory oversight of RPAS and airworthiness of RAs.

JARUS since 2007 has pursued “a single set of technical, safety and operational requirements for the certification and safe integration of UAS into airspace and at aerodromes” — with cross-border harmonisation and minimal duplication of NAA effort. Currently, 35 states, including several non-European states such as China, the Russian Federation and the United States, are involved in JARUS initiatives.

JARUS also has influenced international research into the next-generation airborne collision avoidance technology known as ACAS X.

RPAS Operational Safety Issues

The principal operational safety issues raised during the evolution of small RPAS have revolved around:

  • The risk and potential consequences of a Mid-Air Collision with another RA or a manned aircraft — For example, the U.S. Government Accountability Office (GAO) said the FAA has acknowledged that “the pilot of the small UAS, who is physically separated from it during flight, may not have the ability to see manned aircraft in the air in time to prevent a mid-air collision;”
  • The risk of Loss of Control of an RA — For example, the GAO said the FAA has acknowledged that “the pilot of the small UAS could lose control of it due to a failure of the communications link between the small UAS and the pilot’s handset for controlling the UAS;”
  • The risk of intentional misuse of an RPA — This could be use as a weapon for terrorism or as a tool to invade personal/corporate privacy or to commit crimes; and,
  • Whether the mission of the RPAS operator involves certain classifications of flights — Military/government, commercial, leisure/recreational and hobby purposes each have unique risk profiles.

Disparate factors like these have led to inconsistent priorities among states as they strive independently to achieve their own “most appropriate balance.” This sometimes adversely affects effort in writing and promulgating safety-focused regulatory requirements versus effort issuing safety guidance — which often succeeds or fails based on RPAS operators’ and RA pilots’ voluntary acceptance and implementation.

In some cases, NAAs prohibit RPAS operations as the default response to the ambitions of this aviation-industry segment. At another extreme, NAAs can oversimplify their rules based on the intended use of RPAS or on the classification of an RPAS operator, pending NAA officials’ completion of a comprehensive risk-based approach.

EUROCONTROL also has found that harmonising state regulations to cover the full range of RPAS-aircraft sizes initially appeared to highly desirable, but the current arbitrary split among categories is based on RPAS-aircraft weight. This differentiation — aircraft weight of “below 150 kg” versus aircraft weight of “150 kg or more” — serves to distinguish between the regulatory competence (i.e., jurisdiction) of a national aviation authority (NAA) versus that of EASA regulatory competence.

EUROCONTROL sees this as an arbitrary distinction unsupported by evidence, and a distinction not necessarily significant for the safety issues raised by RPAS operations. In particular, the third-party risks of RPAS operation are not necessarily proportional to either the weight or size of the RPA.

As to key RPAS operational safety issues for operators and pilots, the FAA and EASA have made similar inroads working with their small-RPAS communities to instill a strong RPAS safety culture and pilot professionalism. They expect remote pilots to see themselves as the safety counterparts of airline pilots. They also encourage airline pilots and air traffic controllers to respect and to regard every RPAS pilot-in-command as a fellow professional who influences the overall safety of the national airspace system.

Near-Future Challenges

Until recent years, the general public — including many pilots of manned aircraft and air traffic controllers — tended to regard all operators and pilots of small RPAs as potential threats. During the introduction of small UAS aircraft affordable to worldwide mass markets, even the organized groups of self-regulated, model-aircraft enthusiasts — long respected for safely flying their aircraft “by the book” (i.e., only VLOS operation and no violations of voluntarily accepted limitations) — temporarily fell under suspicion.

In 2018, government officials and safety researchers speaking at U.S. conferences offered reasons why typical operators and pilots of small RPAs had begun to gain greater public acceptance — as well as acceptance among manned-aviation professionals. Rare exceptions, such as cases of near-midair collisions and collisions involving pilots of RPAs (at fault) and manned aircraft, still harm the reputation of the RPAS segment of aviation

Experts attribute growing acceptance to pilot certificates (licences) reflecting prescribed training, privileges and limitations — and to progress in building a robust RPAS safety culture. As a result, many professional pilots of manned aircraft in the United States, for example, have qualified for separate certificates (licences) to also exercise the privileges of small-RPA pilots. All now share accountability for operating and maintaining registered RPA, for flying strictly according to the regulatory regime and applying official safety guidance.

In Europe, the evolution of small RPAS has been similar to that of the United States and other nations, although details of risk mitigations through regulatory oversight and safety guidance differed in some respects. The article cited the United Kingdom, where even the smallest RPAS — originally operated commercially under a permit or an exemption granted by the Civil Aviation Authority (U.K. CAA) — rapidly came under a regulatory regime that required RPAS operator training and standardised competency certification for the pilot based on the specific RPA. These are the Basic National UAS Certificate (BNUC) or the Basic National UAS Certificate for Small Unmanned Aircraft (BNUC-S). Training organisations approved by the U.K. CAA issue these documents and countries outside the United Kingdom adopted the same certification system in the absence of their own comparable alternative.

Looking forward a few more years, one research team in the Netherlands warns that small RPAS still deserve very close attention to “how risk control is distributed between the pilot and the automated functions of drones.” The researchers see this as an RPAS version of performance-based classification of manned-aircraft capabilities.

Some attempts to apply hazard-analysis methods and risk assessment methods to RPA operations have been based on probabilistic and deterministic approaches like those used for manned aircraft operations. In 2016, they reported that some threats to RPAS safety, from a technical analysis perspective, have not fit neatly into existing standardized paradigms of commercial manned aviation and manned general aviation. They theorized that significant reasons include:

  • Regulators and operators involved with RPAS — unlike their counterparts responsible for manned-aircraft safety analysis— typically do not have a universally accepted, risk-assessment framework. Such frameworks enable, for example, data that identifying failures and accident precursors in routine flight operations. Such data tend to be too difficult to collect, de-identify and analyze at the individual-operator level or aggregated multi-operator levels. Flight data monitoring, non-punitive voluntary safety-issue reporting systems, etc. are not yet common in some RPAS subcategories.
  • The team assessed that industry assumptions about RPAS pilots’ reliability (i.e., consistent performance and reslience to handle unfamiliar situations) are “mostly invalid.”
  • In some states and RPAS-industry segments, some RPAS pilots and operators have been a “heterogeneous and unmonitored population” that — unlike the norm in manned aviation — handle all maintenance duties and flight duties to operate their RPAS.
  • In the largest RPAS-pilot segments, the pilots’ main purpose and scope of drone flight often is personal diversion without serious acceptance of responsibility to others (i.e., “no connection of the end-user with social responsibility, job security, etc.”).
  • RPAS pilots without a civil aviation background or RPAS-pilot training and certification, often “lacked knowledge, experience and training in human-performance limitations.”
  • Findings of inadequate education, combined with over-reliance on fully automated flight modes of RPAS, led these researchers to note that RPAS pilots studied, in many cases, lacked “detailed technical knowledge of how drones function [and how] to react successfully to unforeseen events.” Nevertheless, they acknowledged that RPAS automation and autonomy (e.g., geo-fencing airspace to maintain rule-based spatial limits, and automatic return to a designated landing area in response to power failure or other failures) support RPAS end-users in important ways.
  • Intense focus on assigning primary safety responsibility (e.g., observing rules and limits during flight) to the RPAS operator and the RPAS pilot — largely ignoring the role and responsibility of aircraft manufacturers and regulators — adversely affect safety. Specifically, wide variability was observed in the extent to which manufacturers of some small RPAS met safety requirements of regulators. Moreover, variability among countries in their required mitigations for key RPAS risk factors “might confuse users.”

Accidents & Incidents

On 3 August 2009, control of a rotary UAV being operated by an agricultural cooperative for routine crop spraying in the south western part of South Korea was lost and the remote pilot was fatally injured when it then collided with him. The Investigation found that an inappropriately set pitch trim switch went unnoticed and the consequentially unexpected trajectory was not recognised and corrected. The context was assessed as inadequacies in the operator’s safety management arrangements and the content of the applicable UAV Operations Manual as well as lack of recurrent training for the operators’ qualified UAV remote pilots.

On 15 January 2021, the pilot of a DJI Inspire 2 UAV being operated on a contracted aerial work task under a conditional permit lost control of it and, after it exitied the approved operating area, the UAV collided with the window of a hotel guest room causing consequential minor injuries to the occupant. The Investigation found that the loss of control was attributable to “strong magnetic interference” almost immediately after takeoff which caused the compass to feed unreliable data to the Internal Management Unit which destabilised its accelerometer and led to the loss of directional control which resulted in the collision.

On 19 November 2020, the police operator of a DJI Matrice M210 UA lost control of it over Poole when it drifted beyond Visual Line Of Sight (VLOS) and communication ceased. It was subsequently damaged when colliding with a house in autoland mode. The Investigation found that a partial power failure had followed battery disconnection with its consequences not adequately communicated to the pilot. It faulted both the applicable UA User Manual content and the absence of sufficient UA status and detected wind information to the pilot. A failure to properly define VLOS was identified but not considered directly causal.

On 4 July 2019, the operator of an Alauda Airspeeder UAV lost control of it and it climbed to 8000 feet into controlled airspace at a designated holding pattern for London Gatwick before falling at 5000 fpm and impacting the ground close to housing. The Investigation was unable to establish the cause of the loss of control but noted that the system to immediately terminate a flight in such circumstances had also failed, thereby compromising public safety. The approval for operation of the UAV was found to been poorly performed and lacking any assessment of the airworthiness of the UAS.

On 29 February 2016, control of a 50 kg, 3.8 metre wingspan UAV was lost during a flight test being conducted in a Temporary Segregated Area in northern Belgium. The UAV then climbed to 4,000 feet and took up a south south-westerly track across Belgium and into northern France where it crash-landed after the engine stopped. The Investigation found that control communications had been interrupted because of an incorrectly manufactured co-axial cable assembly and a separate autopilot software design flaw not previously identified. This then prevented the default recovery process from working. A loss of prescribed traffic separation was recorded.

On April 25, 2006 a Predator B an unmanned aerial vehicle (UAV), collided with the terrain following a loss of engine power approximately 10 nautical miles northwest of the Nogales International Airport, Nogales, Arizona.

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