Containerisation, and why airport ULDs are a different problem

Published 2026-09-10 · Last reviewed 2026-09-10 · Next review due 2026-12-10

This guide is general information written for people working around UK ports. It is not legal or customs advice, and it does not certify that any consignment complies with anything. Requirements change and depend on the specific goods, materials, route and dates involved. Always check the official sources linked below, and take professional advice where the consequences of getting it wrong are significant.

Containerisation at sea

The ISO shipping container standardised international freight around a rectangular steel box with fixed external dimensions and corner castings, allowing the same unit to move between ship, rail and road without the cargo being rehandled. That is the entire economic point: the cargo is handled once at each end rather than at every mode change.

For packaging, the constraints inside a sea container are floor dimensions, door aperture, internal height, permitted payload and the need to restrain the load against sea motion. Pallet footprint choice is largely a floor-utilisation calculation: how many of a given footprint fit without wasted space, and whether the resulting stack is stable.

Blocking, bracing and dunnage

A container at sea moves in three axes. Cargo that is not restrained shifts, and shifted cargo damages itself, damages the container and creates a hazard at the door. Dunnage — timber bracing, airbags, void fill, bearers — is what stops that. Timber dunnage is wood packaging material, and if the container is crossing a plant health boundary, that dunnage is in scope of the destination's requirements just as much as the pallets are.

Air cargo unit load devices

Aircraft unit load devices — ULDs — are a fundamentally different object. They are aircraft equipment, not intermodal boxes. A ULD is either a container or a pallet-and-net combination designed to fit a specific aircraft hold contour, secured into the aircraft's own locking system.

Why the differences matter to packaging

• Shape — aircraft hold cross-sections are curved, so many ULDs have angled or contoured sides. A rectangular pallet stack that fits a sea container may not fit a ULD profile at all.
• Height — build-up heights are constrained by the hold and by the ULD type, far more tightly than in a sea container.
• Weight — every kilogram is a direct operating cost in air freight, and there are hard limits per ULD position. Heavy timber pallets are a real penalty.
• Restraint — ULD cargo is netted and strapped to the ULD base, not braced against walls with timber dunnage.
• Ownership — ULDs are airline-controlled equipment with their own handling, tracking and damage regimes. They are not consumable packaging.

Air cargo and wood packaging rules

Mode does not create an exemption. Timber packaging flown into a country is wood packaging material entering that country, and phytosanitary requirements apply on the same basis as they would by sea. Air freight's speed does not shorten a plant health inspection.

Sources

Each link was checked on the date shown. Where a site blocked our automated check we say so rather than implying a clean result.