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)