ISO 21785:2020

Air cargo unit load devices - Load distribution model

This document defines the industry agreed model for load distribution on air cargo unit load devices (ULD) bases to apply the maximum allowable centre of gravity (C.G.) eccentricity. Its purpose is to establish a common reference load distribution algorithm for: a) comparable and repeatable ULD testing methods, or equivalent numeric simulations; b) aircraft structure and cargo systems design assumptions, consistent with existing airframers practices; and c) definition of operators unit load devices utilization rules and cargo build-up training programs. It applies to all types of unit load devices intended for use on board civil transport aircraft and airworthiness approved in accordance with the performance requirements and testing parameters of either ISO 21100 or, as applicable, ISO 8097. It also applies to non-airworthiness approved (non-certified) containers as defined in ISO 4118, the utilisation of which is controlled by the provisions of the aircraft type's Weight and Balance Manual and other airframe manufacturer's documents.

Unités de charge de fret aérien — Modèle de répartition des charges

General Information

• Status: Published
• Publication Date: 25-Feb-2020
• ICS: 49.100 - Ground service and maintenance equipment
• ICS: 49.120 - Cargo equipment
• Technical Committee: ISO/TC 20/SC 9 - Air cargo and ground equipment
• Drafting Committee: ISO/TC 20/SC 9 - Air cargo and ground equipment
• Current Stage: 9060 - Close of review
• Completion Date: 02-Sep-2030

Overview

ISO 21785:2020 defines a standardized load distribution model for air cargo unit load devices (ULDs) to manage maximum allowable centre of gravity (C.G.) eccentricity. The standard provides an industry‑agreed reference algorithm used for repeatable ULD testing and simulations, consistent aircraft structure and cargo‑system design assumptions, and the definition of operator ULD utilization rules and build‑up training. It applies to certified ULDs (per ISO 21100 or ISO 8097) and to non‑certified containers controlled by an aircraft’s Weight and Balance Manual.

Key topics and technical requirements

• C.G. eccentricity definition: Eccentricity is expressed as a percentage of ULD base length or width (offset between the ULD+C.G. and the geometric centre).
• Reference load distribution: Certified maximum eccentricity is represented by a homogeneous cargo footprint occupying a reduced area of the pallet or container base (quadrant model centred on the load C.G.). This defines the most critical load case for design and testing.
• Numeric limits: The standard specifies allowed eccentricity demonstration at maximum gross mass; typical allowed limits are ±10% of base length and width, with an exception of ±5% length and ±10% width for sizes G, H, J and R (per the document).
• Area load vs. base area load: Distinguishes the maximum allowable distributed area load derived from the model from the maximum ULD base area load defined in ISO 21100/ISO 8097. If the derived area load exceeds the ULD’s base area load, operational eccentricity limits must be reduced.
• Operational C.G. trade‑offs: Enables proportional increases of allowable C.G. offset when actual ULD gross mass is below the zonal limit:
  • One‑direction trade‑off (longitudinal OR lateral) allows larger offsets (example: 50% mass → up to ±30% offset).
  • Two‑direction trade‑off (both axes simultaneously) provides a more conservative increase (example: 50% mass → up to ±20% each).
• Precautions and best practice: Stacking should start at the base centre; partial loads must be centred, avoid placing loads only in a corner, use tie‑downs, nets, shoring and training to prevent in‑flight or ground shifting.

Applications and users

ISO 21785 is practical for:

• ULD manufacturers and test laboratories performing certification and repeatable load testing or simulations.
• Aircraft manufacturers (airframers) and cargo systems engineers for structural design assumptions and compatibility checks.
• Airlines, cargo operators, load planners and ground handling teams for ULD utilization rules, build‑up procedures and staff training.
• Regulators and safety auditors referencing consistent C.G. and load distribution criteria.

Key keywords: ISO 21785, air cargo, unit load devices, ULD, load distribution model, centre of gravity, C.G. eccentricity, weight and balance, pallet, container.

Related standards

• ISO 21100 - ULD performance requirements and test parameters
• ISO 8097 - Minimum airworthiness requirements for certified ULDs
• ISO 4118 - Non‑airworthiness containers vocabulary
• ISO 16412 - Pallet utilization guidelines (shoring and area load methods)

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 21785
First edition
2020-02
Air cargo unit load devices — Load
distribution model
Unités de charge de fret aérien — Modèle de répartition des charges
Reference number
©
ISO 2020
© ISO 2020
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ii © ISO 2020 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Load distribution model . 2
4.1 Principle . 2
4.2 Application . 4
5 Operational C.G. trade-off . 4
5.1 General . 4
5.2 One direction trade-off . 4
5.3 Two directions trade-off . 5
5.4 Required precautions . 5
Bibliography . 7
Foreword
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iv © ISO 2020 – All rights reserved

Introduction
This document specifies a reference model for load distribution on air cargo unit load device (ULD)
bases, to reflect in a standardized manner maximum allowable centre of gravity (C.G.) eccentricity
limitations.
The civil aviation requirements referred to in this document are those concerning certification
of transport aircraft and appliances to be installed aboard them, and constitute the set of design
and operation requirements internationally agreed in application of International Civil Aviation
Organization (ICAO) Annex 8, Airworthiness of aircraft, to the Convention on International Civil
Aviation.
Throughout this document, the minimum essential criteria are identified by use of the key word "shall".
Recommended criteria are identified by use of the key word “should” and, while not mandatory, are
considered to be of primary importance in providing safe air cargo unit load devices. Deviation from
recommended criteria should only occur after careful consideration and thorough service evaluation
have shown alternate methods to provide an equivalent level of safety.
INTERNATIONAL STANDARD ISO 21785:2020(E)
Air cargo unit load devices — Load distribution model
1 Scope
This document defines the industry agreed model for load distribution on air cargo unit load devices
(ULD) bases to apply the maximum allowable centre of gravity (C.G.) eccentricity.
Its purpose is to establish a common reference load distribution algorithm for:
a) comparable and repeatable ULD testing methods, or equivalent numeric simulations;
b) aircraft structure and cargo systems design assumptions, consistent with existing airframers
practices; and
c) definition of operators unit load devices utilization rules and cargo build-up training programs.
It applies to all types of unit load devices intended for use on board civil transport aircraft and
airworthiness approved in accordance with the performance requirements and testing parameters of
either ISO 21100 or, as applicable, ISO 8097.
It also applies to non-airworthiness approved (non-certified) containers as defined in ISO 4118,
the utilisation of which is controlled by the provisions of the aircraft type's Weight and Balance Manual
and other airframe manufacturer's documents.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 8097, Aircraft — Minimum airworthiness requirements and test conditions for certified air cargo unit
load devices
ISO 10254, Air cargo and ground equipment — Vocabulary
ISO 21100, Air cargo unit load devices — Performance requirements and test parameters
3 Terms and definitions
For the purposes of this document, the terms and definitions of ISO 10254 and the following apply:
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
eccentricity
offset
plan view distance between the overall centre of gravity (C.G.) of the unit load
device and its contents and the geometric centre of its base, expressed in percentage of the base length
and width
3.2
homogeneous cargo
cargo of homogeneous density and of such a nature that the whole weight of each piece is evenly
distributed over the whole of that piece's footprint area
4 Load distribution model
4.1 Principle
4.1.1 Airworthiness approval of each type and size of unit load device includes maximum load centre
of gravity (C.G.) eccentricity (offset) allowances at unit maximum gross mass, that are part of its testing
and approval (see ISO 21100 or ISO 8097, as applicable, for unit load device configurations and ultimate
load criteri
...