Unit Load Device (ULD) is the correct terminology used by the air transport industry for containers and loading units that are used for the carriage of cargo by air. Pallets with nettings as well as rigid containers are all referred to as ULD.

Modern aircraft carry many tons of cargo and baggage in the aircraft holds. During flight the aircraft accelerates and decelerates, banks, climbs and descends, and from time to time experiences severe vertical acceleration/deceleration as the aircraft hits air pockets/ turbulence.

Without ULD the cargo would move around the aircraft holds causing damage to structure and systems, and possibly allowing a change in the centre of gravity, so endangering the aircraft safety. Certified ULD are strong enough to hold the cargo securely under all these conditions.

An aviation authority certifies a container once they are satisfied that the container has been designed, tested, and manufactured to the approved standards. Any organization wishing to design and manufacture ULD must first obtain the necessary company approvals from their national authority and then each and every ULD design must be submitted to the authority with design calculations and actual test reports. If the authority is satisfied they will issue an approval for the ULD, which can then be called a "certified ULD".

Certain aircraft (particularly Boeing 767) have holds that are sufficiently strong to contain the cargo. Containers used on these aircraft types do not have to be certified

Any aircraft, and all the components that make up the aircraft or fit into the aircraft (such as seats, galleys, ULD) must be designed and manufactured under the control of an airworthiness authority, e.g. FAA for USA, EASA for Europe. An airline may not fit any item to its aircraft unless the manufacturer of that item can provide acceptable certification for that item to the airline and the airline’s Quality control department has accepted that certification.

Not all aircraft require certified ULD, but as a general rule:
• The upper deck of all aircraft requires the use of certified ULD
• All pallets and nets must be certified where ever they are fitted
• All aircraft manufactured by Airbus require the use of certified ULD into the lower holds
• Douglas and Boeing aircraft have different requirements depending on aircraft and hold location
• Boeing 767 aircraft lower holds do not require certified containers (but do require that pallets and nets are certified)

The final determination of whether a ULD can be used on any particular aircraft in any particular airline rests with the airline operating the aircraft.

The seven differences between Certified and non-certified ULDS are depicted below:

1. The ULDs have different ID-Codes.
    According to IATA ULD Technical Manual (latest edition) the first letter of the type codes are as follows:
    Certified container: A
    Non-certified container: D
    Certified pallets: P
    Non-certified pallets: F
2. To get a certification from a local airworthiness authority (FAA; CAA; LBA) the ULD has to be tested according to
    the relevant requirements, described in NAS 3610, e.g. a LD3 container has to be tested acc. to NAS 3610-2K2C.
3. ULDs that are not covered by NAS 3610 cannot get an official certification, like TSO-C90c.
4. All ULDs (LD2, LD4, LD8 and all pallets with the same base sizes), especially designed for Boeing B767 aircrafts are not
    covered by NAS 3610.
5. Aircrafts with a certified cargo compartment are not requiring certified ULDs. These aircrafts can carry non-certified ULDs.
6. Aircraft with a non-certified cargo compartment or with a certified restraint system (only Airbus Wide-Body aircrafts),
    are requiring certified ULDs, except the ULD was tested and approved acc. to the airframe manufacturers specifications.
7. Operational airworthiness criteria, as damage limitations are more restrictive for certified ULDs than for non-certified ULDs.

The only limits for non-certified ULDs are:
- They should keep the load inside the ULD, which means
   • no damaged door locking mechanism
   • no broken frame extrusions
   • no bigger holes in panel sheets and doors
   • no deflection which would affect a proper loading/restraining inside the aircraft
- There should be no damages that could cause any damages to the baggage, to the aircraft and it should not harm any person.

The airworthiness inspection criteria for certified containers can be found in the CMMs under paragraph Inspection, describe the limits under which the relevant ULD is still airworthy and can be used for flight operation. ULDs that exceed these limits are no longer airworthy and have to be taken out of service. The inspection of a ULD has to be carried out before the ULD is loaded on board of an aircraft.

All certified containers have to meet the requirements listed in a document called NAS3610, soon to be replaced with the new AS36100.

A Technical Standards Order (TSO) is a document issued by the FAA. The current issue is TSO C90 (c). Each ULD must carry a TSO marking (a plate or a tag for containers, or engraved for pallets) that contains the manufactures name, the date of manufacturing, a part number, and reference to the TSO under which the ULD was certified. At the moment an equivalent European TSO is issued by the EASA (ETSO C90 c).

When the FAA updates the TSO regulations they do not require that older units are scrapped or upgraded, so these TSO references are still valid.

There is no limit in general. Airlines can continue to use a ULD as long as it can be maintained in an airworthy manner.

Containers that are damaged must be repaired in an approved ULD repair shop.

Maintenance on any aircraft, or the components that are fitted to the aircraft, must be carried out by a company that has been approved by the relevant authority to perform aircraft maintenance work. ULDs must only be repaired in a repair shop that holds a valid approval.

Containers have to be as light as possible to save weight on the aircraft, yet they are often handled very roughly in the cargo terminals and forwarders warehouses.

For each ULD a manual is issued. A manual consists of a Component Maintenance Manual (CMM) and an Illustrated Part List (IPL). Depending on the layout, the CMM and the IPL can be combined as a single document or shown as 2 separate documents. For a legal operation, the owner of the ULD should make these documents available for each operator (ramp staff) of his ULDs, as well as for each ULD repair shop.

Different airlines have different specifications. Metal door containers provide additional protection against weather and other damage to cargo. Flexible door containers are generally lighter. Flexible doors can have different type of closing mechanisms such as straps with Velcro or a bar locking mechanism.

Every container has a maximum gross weight (MGW) limit. However the aircraft itself may also have a lower limit in which case that must be followed.

A system of rollers, drive units and container and pallet locks (called a cargo loading system, CLS) is installed in the aircraft to handle the ULD.

For certain operations where the airlines can accept the higher weight and lower internal volume of a forkliftable container these are preferred over standard base containers as they are less likely to be damaged in operations.

Garment On Hangers (GOH) is a provision in the roof structure of a ULD from which coat hangers can be hung. These GOH can be round tubes, T-shape extrusions with holes in them, or welded constructions.

Pallets have a seat track in the aluminium edge extrusions. In this seat track circular pieces are milled out which allow studs or locks to be installed into the seat track which then allow a container to be fitted on the pallet or a net to be fastened. If the millings are present over the entire length of the pallet extrusions it is called a continuous seat track. If there are only several millings over the entire length of the pallet extrusions it is called a non-continuous seat track. The configuration of the millings can be found in the NAS3610. To which part of the NAS3610 the pallet should comply can be found in the IATA manual if the IATA code is known. The customer should supply the IATA code of the ULD they require.

Yes, see the table with ATA and IATA codes in the pdf-file below.