ISO 21384-4:2020

Unmanned aircraft systems - Part 4: Vocabulary

This document defines terms and definitions relating to unmanned aircraft systems that are widely used in science and technology.

Aéronefs sans pilote — Partie 4: Vocabulaire

General Information

• Status: Published
• Publication Date: 25-May-2020
• ICS: 01.040.49 - Aircraft and space vehicle engineering (Vocabularies)
• ICS: 49.020 - Aircraft and space vehicles in general
• Technical Committee: ISO/TC 20/SC 16 - Unmanned aircraft systems
• Drafting Committee: ISO/TC 20/SC 16/WG 1 - General
• Current Stage: 9599 - Withdrawal of International Standard
• Start Date: 08-Jul-2025
• Completion Date: 13-Dec-2025

Relations

• Revised: ISO 21384-4:2025 - Uncrewed aircraft systems - Part 4: Vocabulary
• Effective Date: 06-Jun-2022

Overview

ISO 21384-4:2020 - "Unmanned aircraft systems - Part 4: Vocabulary" is an ISO standard that defines commonly used terms and definitions for unmanned aircraft systems (UAS). Its primary purpose is to establish a consistent, internationally accepted vocabulary for the UAS community-covering drones, remotely‑piloted aircraft, remote pilot roles, control links, airspace concepts and related operational terms. This vocabulary supports clear communication across regulation, design, operations, safety cases and research.

Key topics

This part of the ISO 21384 series focuses on terminology rather than prescriptive technical requirements. Key subject areas and illustrative terms included are:

• Aircraft types and categories: aeroplane, airship, manned aircraft, unmanned aircraft (UA/RPA), optionally piloted aircraft (OPA)
• Operational concepts: aerodrome pilot, flight duty period, operational volume, geo-limitation, aerodrome/ droneport
• Pilot and crew roles: remote pilot, remote pilot-in-command (remote PIC), remote crew member, RPA observer
• Flight modes and ranges: visual line‑of‑sight (VLOS), extended VLOS (EVLOS), beyond visual line‑of‑sight (BVLOS)
• Communication and control: command and control (C2) link, ATS communication link, payload link, radio line‑of‑sight (RLOS), down-link, up-link
• Safety functions and equipment: detect-and-avoid, airborne collision avoidance system (ACAS), collision boundary/volume, collision avoidance threshold, flight termination system, flight recorder
• Operational documentation and oversight: operations manual, operating manual, maintenance programme, qualified entity (QE)

Practical applications

Although ISO 21384-4 does not set performance or design requirements, it provides essential value by:

• Enabling consistent terminology in regulatory texts, certification paperwork and airspace integration policy for UAS
• Standardizing language used in manufacturer manuals, operations manuals, and maintenance programmes
• Supporting interoperability and safety analysis by ensuring engineers, operators and ATM (air traffic management) stakeholders use the same definitions for C2 links, BVLOS, detect-and-avoid, etc.
• Helping training organizations, legal teams, insurers and researchers draft clear contracts, safety cases, incident reports and academic papers
• Assisting international collaboration by aligning terms across national authorities and standards bodies

Who uses it

Key users include national aviation authorities, UAS manufacturers, operators (commercial and research), air navigation service providers, standards developers, training organizations, and technical writers preparing manuals and regulatory submissions.

Related standards

ISO 21384-4 is part of the ISO 21384 series prepared by ISO/TC 20/SC 16 (Unmanned aircraft systems). For related terminology resources see the ISO Online Browsing Platform and IEC Electropedia.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 21384-4
First edition
2020-05
Unmanned aircraft systems —
Part 4:
Vocabulary
Aéronefs sans pilote —
Partie 4: Vocabulaire
Reference number
©
ISO 2020
© ISO 2020
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
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Phone: [phone]
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Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
Bibliography .10
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles,
Subcommittee SC 16, Unmanned aircraft systems.
A list of all parts in the ISO 21384 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2020 – All rights reserved

INTERNATIONAL STANDARD ISO 21384-4:2020(E)
Unmanned aircraft systems —
Part 4:
Vocabulary
1 Scope
This document defines terms and definitions relating to unmanned aircraft systems that are widely
used in science and technology.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at http:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
aerial work
aircraft (3.6) operation in which an aircraft is used for specialized services such as agriculture,
construction, photography, surveying, observation and patrol, search and rescue, aerial
advertisement, etc.
Note 1 to entry: The definition is adapted from Reference [7].
3.2
aerodrome pilot
remote pilot (3.63) familiar with a defined aerodrome or landing site, which transfers responsibility to
another pilot a few minutes after take-off or accepts responsibility for approach, landing and possibly
taxing and parking
3.3
aeroplane
power-driven heavier-than-air aircraft (3.6), deriving its lift in flight chiefly from aerodynamic
reactions on surfaces which remain fixed under given conditions of flight
Note 1 to entry: The definition is adapted from Reference [6].
3.4
air traffic service
ATS
generic term that can refer to flight information service, alerting service, air traffic advisory service,
air traffic control service (area control service, approach control service or aerodrome control service)
Note 1 to entry: The definition is adapted from Reference [6].
3.5
airborne collision avoidance system
ACAS
aircraft (3.6) system based on secondary surveillance radar (SSR) transponder signals which operates
independently of ground-based equipment to provide advice to the pilot on potential conflicting
aircrafts that are equipped with SSR transponders
Note 1 to entry: The definition is adapted from Reference [6].
3.6
aircraft
machine that can derive support in the atmosphere from the reactions of the air other than the reactions
of the air against the Earth’s surface
Note 1 to entry: The definition is adapted from Reference [10].
3.7
aircraft category
classification of aircraft (3.6) according to specified basic characteristics
EXAMPLE Aeroplane (3.3), helicopter, glider, free balloon.
Note 1 to entry: The definition is adapted from Reference [12].
3.8
airship
power-driven lighter-than-air aircraft (3.6)
Note 1 to entry: The definition is adapted from Reference [9].
3.9
airspace management
ASM
planning function with the primary objective of maximizing the utilization of available airspace by
dynamic time-sharing and, at times, the segregation of airspace among various categories of users
based on short-term needs, while securing aviation safety
3.10
air traffic management
ATM
dynamic, integrated management of air traffic and airspace including air traffic services (3.4), airspace
management (3.9) and air traffic flow management, safely, economically and efficiently, through the
provision of facilities and seamless services in collaboration with all parties and involving airborne and
ground-based functions
Note 1 to entry: The definition is adapted from Reference [13].
3.11
altitude
vertical distance of a level, a point or an object considered as a point, measured from mean sea level (MSL)
Note 1 to entry: The definition is adapted from Reference [6].
3.12
ATS communication link
digital or analogue communication link to transfer voice or data between remote crew members (3.61),
ATS (3.4), airspace users and other airspace users
Note 1 to entry: It includes air-ground, air-to-air and ground-ground links.
2 © ISO 2020 – All rights reserved

3.13
autonomous system
system that, perceiving its environment and determining if this affects its goals, takes action to ensure
as far as practicable that its goals will be safely achieved
3.14
beyond visual line-of-sight
BVLOS
operation of a UAS (3.80) other than VLOS (3.85) or EVLOS (3.26)
3.15
collision avoidance threshold
boundary around the UA (3.79) at which the collision avoidance function declares that action is
necessary to avoid a collision, by preventing the threat from penetrating the collision volume (3.17)
Note 1 to entry: The definition is adapted from Reference [14].
3.16
collision boundary
closest point of approach or minimum distance to be achieved between two aircrafts (3.6) to ensure
that a collision is avoided taking account of any inaccuracies in the system
3.17
collision volume
cylindrical volume of airspace centred on the UA (3.79) with a horizontal radius and vertical height
(3.36) within which a collision is most likely and avoidance of a collision can only be considered a
matter of chance
3.18
command and control link
C2 link
data link between the remotely-piloted aircraft (3.67) and the remote pilot station (3.64) for the purposes
of managing the flight
Note 1 to entry: The definition is adapted from Reference [6].
3.19
commercial air transport operation
CAT
aircraft (3.6) operation involving the transport of passengers, cargo or mail for remuneration or hire
Note 1 to entry: The definition is adapted from Reference [7].
3.20
control station
defined location containing one or more controls
[SOURCE: ISO 12643-1:2009, 3.12]
3.21
detect and avoid
capability to see, sense or detect conflicting traffic or other hazards and take the appropriate action
Note 1 to entry: The definition is adapted from Reference [6].
3.22
down-link
direct or indirect data link from the unmanned aircraft (3.79) to one or more peers
3.23
drone
unmanned system which is remotely or autonomously operated
3.24
droneport
any aerodrome including vertiports (3.83) exclusively dedicated to landing, ground-handling and take-
off of unmanned aircrafts (3.79)
3.25
dual instruction time
time during which a person is receiving instruction from a properly authorized remote pilot (3.63) at
the controls of the remote pilot station (3.64)
3.26
extended visual line-of-sight
EVLOS
operation beyond the unaided visual range of the remote pilot (3.63), but where the remote pilot is
supported by vision systems or one or more visual observers
3.27
flight duty period
period which commences when the first remote crew member (3.61) reports for duty that includes a
flight or a series of flights and which finishes when the last remote crew member’s duty ends
3.28
flight level
surface of constant atmospheric pressure which is related to a specific pressure datum, 1 013,2 hPa,
and is separated from other such surfaces by specific pressure intervals
Note 1 to entry: The definition is adapted from Reference [6].
3.29
flight manual
manual, acceptable by the local aviation authority, containing the order of actions in normal, abnormal
and emergency procedures, checklists, limitations, performance information, details of the aircraft
(3.6) systems
3.30
flight recorder
any type of recorder installed in the aircraft (3.6) for the purpose of complementing accident/incident
investigation
Note 1 to entry: In the case of unmanned aircraft (3.79), it also includes any type of recorder installed in a remote
pilot station (3.64) for the purpose of complementing accident/incident investigation.
Note 2 to entry: The definition is adapted from Reference [12].
3.31
flight termination system
means and/or procedure triggered manually or automatically to initiate a pre-programmed action or a
set of actions designed to terminate UA (3.79) flight, minimising risks to third parties
3.32
flight time
total time from the moment the on-board sys
...