What's a ULD

A Unit Load Device (ULD) is used to transport cargo by aeroplane. ULDs can convey any type of cargo, from passenger suitcases and perishable goods to horses and Formula-1 cars. ULDs are divided into two main categories: aircraft containers and aircraft pallet/net combinations.

The difference between a container and a pallet

Both pallets and containers allow a large quantity of cargo to be bundled into a single unit, which can be used on wide-body aircraft and certain narrow-body aircraft.

Cargo on pallets are secured by a net, which is attached to the pallet’s rim. Containers, also known as cans and pods, are typically lightweight structures comprising a base, a frame with side and roof panels, and a fabric or solid door. There are many different types of containers. Some are collapsible, some are insulated, others are ventilated or refrigerated and so on.

ULD terminology

Terms and definitions which are commonly used in the ULD industry (Source: IATA ULD Regulations)

Certified Unit Load Devices (ULDs)

Unit Load Devices (ULDs) are nearly always required to be approved for use, that is to say, certified for continued airworthiness by the relevant Civil Aviation Authority (CAA). That’s because, just like any other aircraft component, ULDs have to be robust enough to cope with all likely and extreme circumstances during flight (e.g. when the aircraft accelerates and decelerates, banks, climbs and descends or hits air pockets or turbulence). Under such severe circumstances, a certified ULD ensures the cargo does not move around the hold, endangering the aircraft’s structure and systems, because it is locked to the hold’s floor.

It must be noted that the certification process is conducted solely between the ULD manufacturer and the applicable regulatory authorities such as FAA (Federal Aviation Administration in the USA) and EASA (European Aviation Safety Aviation in Europe).

Identifying a Unit Load Device (ULD)

Each Aircraft Unit Load Device (ULD) is identified by its ULD code. This code is a unique combination of letters and numbers, starting with a three-letter prefix that indentifies the type of ULD. This prefix is followed by a unique 4 or 5-digit serial number to distinguish it from others of the same type. The last two or three characters designate the owner of the ULD (e.g. the airline).

Many different parties handle ULDs as they pass between airlines and airports around the world, so a system was needed to identify easily and quickly each ULD. Therefore, the International Air Transport Association (IATA) introduced a global standard system of identification. Accordingly, each ULD is assigned a unique ULD code, which is clearly visible on the relevant unit. By standardising the system, all manufacturers, cargo handlers and airlines can now identify the ULD’s classification and the owner at a glance.


The first three letters of a ULD code are perhaps the most important. They are used to identify the type, size and shape of the ULD. This information is vital in determining not only the type of cargo it can contain but also the aircraft it is compatible with.

The three-letter prefix works as follows:

  1. The first letter represents the type of ULD
  2. The second letter represents the base size of the ULD
  3. The third letter represents the container’s contour or the pallet’s restraint system

Maximising an aircraft’s cargo space

When designing an Aircraft Unit Load Device (ULD), careful thought is given to making the most of the aircraft’s cargo space. Unfortunately, aircraft holds come in many shapes and sizes, which means that ULD’s also have to come in many shapes and sizes.

Since ULDs are often transferred between different types of aircraft, this gives rise to three types of fit: optimal fit, non-optimal fit and no fit:  

  • Optimal fit: the ULD’s contour is the same as the aircraft’s hold;
  • Non-optimal fit: the ULD’s contour does not exactly match the hold’s contour;
  • No fit: the ULD does not fit the aircraft’s hold.

Clearly, a ULD that fits the contour of the hold maximises the aircraft’s cargo space. In addition, a ULD can be an optimal fit for one or more than one type of aircraft. That doesn’t mean this ULD can’t be used in another type of aircraft if it’s not an optimal fit. In fact, it may be compatible with several different aircraft. It simply won’t be making the most of every hold’s space.

Main deck and lower deck examples of an optimal fit (on the left), a combination of optimal and non-optimal fit (middle) and non-optimal fit (right).

How do you decide which is the most suitable contour?

The contour you select depends on a few factors:

  • Which airline do you use?
  • Which aircraft does the airline use?
  • Is the ULD also going to be used for land/sea transport?
  • What type of cargo is being transported? 
  • Do you need to use more than one aircraft or airline (interlining)?

To help you select the best contour for you, VRR has developed an ULD – Aircraft compatibility tool.

Conversion table: ATA / IATA Designation

Each Aircraft Unit Load Device (ULD) is identified by its ULD code. This code is a unique combination of letters and numbers. The codes such as LD2, LD3, LD9, M1 etc. were developed under ATA many years back, while the codes such as AKE, AAP, AMA, RKN, VGA are part of the IATA ULD identification system. In general the use of IATA Identification system codes is preferable.

ATA: Air Transport Association of America
IATA: International Air Travel Association

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