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Unit Load Devices
From SKYbrary Wiki
Unit Load Devices (ULD) and associated accessories are used to restrain baggage and cargo in the holds of wide body aircraft during flight, ensuring that their contents remain in its allocated location irrespective of flight conditions such as turbulence.
ULD perform a safety critical function during flight. In most passenger aircraft and almost all freighter aircraft, containers, pallets and nets (commonly referred to as ULD) and straps and fittings (accessories) are required by the aircraft operating procedures to secure baggage and cargo securely. ULD do perform many other useful functions, most specifically the unitization of baggage and cargo which facilitates many parts of the baggage and cargo process; however these are generally secondary to the prime function of ULD and ULD accessories, which is to ensure flight safety.
Identification of ULD
Containers and pallets are almost always identified with a unique combination of letters and numbers. The IATA standard system comprises 3 leading letters, 4 or 5 numerals and 2 trailing letters. The 3 leading letters (eg AKE, PMC) define the type of ULD, the 4 or 5 numerals are a unique number allocated by the owner airline and the 2 trailing letters are the ULD owners 2 letter ICAO code which may be for an airline (eg BA) or a ULD leasing company (eg JG). Full details of the coding standards are found in the IATA ULD Technical Manual. ULD may also carry a bar code, which will usually contain the same identifier as the printed/engraved code.
Certification of ULD
Containers, pallets, and nets are in most cases required to be approved for use (“certified”) by the airworthiness authority for the country where the manufacturer is located. Currently the 2 most commonly used certification standards are:
- FAA - Technical Standards Order (TSO) C90c and
- EASA - ETSO C90c,
though other countries such as China, Australia etc have complementary standards for certification of ULD.
In order to obtain approval or certification from the authorities, the designer must submit calculations and test results to show that the ULD is able to withstand the loads required while being restrained in the manner laid out in the TSO standards. These loads are extremely high, in order to prove that the design is capable of restraining the contents of the loaded ULD under extreme flight conditions, for example:
- The upload test on an LD3 container requires that the LD3 structure can survive a load of 9826 lbs in the upwards direction while restrained by just 4 locations at the base for a period of 3 seconds.
- A cargo net for a PMC pallet needs to withstand an upload of over 57000 lbs25,854.765 kg
25.855 tonnes for 3 seconds.
Certified and Non Certified ULD
The great majority of ULD are designed and sold as certified ULD, however there are some exceptions. These can be identified by the first letter of their identification code:
|Non Certified container||D|
Certified ULD are required to be used in all aircraft having holds that have insufficient strength to contain their contents during extreme flight conditions, and instead rely on the ULD being locked to the floor of the hold. Non certified ULD may be used in an aircraft having a hold structure which is designed to withstand the forces from the contents during extreme flight conditions. An example of this is the B767, which not only has holds designed to withstand these forces but also uses a base size for its containers that is unique to the aircraft type (units known as DPE (LD2), DQP (LD4) and DQF (LD8)).
While airlines may choose to make their own definitions as to exactly what types of ULD may or may not be loaded on their aircraft, it should be remembered that many ULD are transferred between aircraft and even other airlines before reaching their destination; therefore, it is usually the safest assumption that any container marked with first letter “A”, any pallet marked with first letter “P” and all cargo nets plus any accessories are performing a safety critical function each time they are placed on board an aircraft. Additionally, while a “non certified ULD” may not have a flight safety function it should still not be used in a damaged condition as it is a hazard to the aircraft, handlers and its contents.
Correct use of ULD
It is the responsibility of the ULD designer to create clear instructions regarding the use of the ULD, and to include them in the operations manual. Additionally, airlines should make operations manual instructions on the correct use of the ULD available to all parties involved in loading and unloading the aircraft. While different manufacturers create different instructions, there are considerable similarities:
- Every ULD should be inspected before use to determine if any damage has occurred which would render the ULD unserviceable.
- Baggage and cargo should be loaded evenly, paying attention to maintaining the centre of gravity of the load with the 10% of the centre of the base.
- After completing the load the door of the container must be securely closed.
- No certified ULD may be used for flight unless its manufacturers plate/marking (also known as TSO plate) is attached and legible. This plate/marking is required by the rules of the certifying authorities (eg FAA, EASA etc) and is a legal requirement.
- Container doors shall always be securely closed or secured in an open position.
A ULD not loaded in compliance with these instructions may very well not be able to carry out its defined purpose of providing restraint to the contents in flight, and should not be allowed on board the aircraft.
Loading of pallets is similar to loading containers; particular attention should be given to the cargo net being used to secure the load to the pallet. Cargo nets are subject to severe wear and tear and are often found to be unserviceable, however there is an all too common tendency to use cargo nets in an unserviceable condition or to attempt some kind of “unauthorized repair”; neither of these actions is acceptable. Indeed, any attempt to carry out an “ad hoc” repair of a damaged ULD using any kind of material or process not specifically defined by the owner airline’s manuals is not acceptable, including but not limited to:
- Using “speedtape” “duct tape” or similar tapes to close over punctures on container panels or doors or to hold a door closed if the proper mechanism has failed
- Using rope to repair damaged parts of a cargo net or to replace the OEM’s lashing line (corner rope)
- Attaching random 2-stud fittings into the edge of a cargo net (to secure it to a pallet) when the correct fitting is damaged.
Certain brands and types of ULD may have OEM approved “temporary repair” solutions; if so, these may be used but only as defined by the owning airlines documentation.
Loading / Unloading of Containers
- Always check the unit before starting loading. Any damage found must be below the required damage limits otherwise the ULD is not airworthy.
- If the cargo is to be protected from possible rain damage by the use of plastic sheets, these should be taped inside the container. The use of plastic sheeting around the outside of the container is subject to restrictions: the material must meet FAA/ EASA burn rate requirements and the manufacturers plate (TSO Plate) must remain visible.
- Cargo or bags should be stacked in an interlocking fashion as far as possible.
- Avoid placing heavy cargo in the outboard section of the container (overhanging section) as this can cause the container to rock or tilt to the outside, causing loading difficulties.
- Ensure that cargo/ baggage in the door side does not extend to the point that the door is not flat when closed.
- Inspect the loaded ULD to ensure it is within the designated contour. It may appear that a container is within its contour by the nature of its structure, however there are extremely small clearances between the container and the cargo hold liner in many aircraft and even a small distortion in the container structure (as may occur due to certain types of damage, or from loading heavy cargo in the overhanging section of the container) can result in the container touching the hold walls, causing considerable damage and grounding the aircraft (hold liners are fire containment devices and if punctured the aircraft cannot take off). Care must be taken that all ULD are within their designated contour before they are loaded to the aircraft.
- Consult airline guidance for any heavy items which may require spreader boards to spread the weight on the base and may require additional tie down using cargo straps.
- When closing the door, do not use hammers or other excessive force to operate latches etc as these will almost certainly not be openable on arrival at destination.
- Never use knives to cut open a container door
Loading/ Unloading of Pallets and Nets
- Carry out a pre loading check to determine that the pallet and net are serviceable before starting loading. Particular attention should be given to the cargo net being used to secure the load to the pallet. Cargo nets are subject to severe wear and tear and are often found to be unserviceable. It is important to determine that the net is serviceable before starting to load the pallet.
- If plastic sheeting is to be used this must be underneath the pallet net.
- Load cargo in an interlocking manner as far as possible. Ensure the load is within the specified contour. Overhanging loads ( as used in lower deck loads) must be built in such a manner that the overhanging sections will not collapse in flight.
- After loading all cargo the net shall be used as follows:
- Place the net over the load, attach the net fittings to the seat track using even spacing along the sides
- Use the “reefing hooks” along each side to take up any excess net body (as found on less than full contour loads)
- Use the corner ropes ( lashing lines) or other corner closing devices as fitted to close and lightly tension the net .
- NOTE: While the general impression in the industry is that nets need to be as tight as humanly possible this is incorrect. Neither the manufacturers, nor the aviation safety authorities nor IATA make any such requirement, and the correct procedure is that the net be tight but not overtight. The purpose of the net is to provide restraint against in flight forces, and it performs that function perfectly well without being pulled tight on installation.
- Over-tensioning a net will not only damage cargo but also may pull up the pallet edge rails making it hard or impossible to lock the pallet into the aircraft.
- Often loads may travel considerable distances by road to the nearest airport and it has become common practice to use the cargo net as a “ securing device” to hold the cargo in place during road transport. The design of the cargo net (a diamond mesh) is not suited to this purpose.
- Where there are doubts as to the stability of the load the loaders may use rope and other materials to stabilize and secure the load BEFORE placing the cargo net over the load. The purpose of the net is to ensure inflight safety NOT to prevent the cargo falling off the truck on its way to the airport.
- Never use a knife to cut a net. This damages an expensive airline asset and makes the net unserviceable.
- Never hammer or in any way force a net fitting into the seat track as it will be impossible to remove at destination. If the fitting will not install easily check the seat track for dirt or damage.
Handling of ULD
Containers are typically lightweight structures comprising a structure of aluminium extrusions, a relatively thick ( 2.5mm - 4mm) aluminium base sheet, aluminium or composite side and roof panels, and a fabric or metal door. By necessity they are made as light as possible, however they may be subjected to extremely severe handling conditions. The explosive growth of aviation and in particular air cargo over the past 10 years has resulted in many ULD being handled in sub standard facilities using unsuitable handling practices. The resulting damage is not only extremely costly to airlines but also creates a severe safety hazard.
The various published industry standards covering handling of ULD all call for ULD to be supported at all times on dollies/roller beds/racking. The standards also state clearly that unless a ULD is specifically designed to be handled by a forklift it should not be forklifted. The use of forklifts to move empty or loaded ULD will almost certainly result in damage to the ULD, possibly destroying it totally or at least rendering it unsafe for further use without repair.
Storage of empty ULD
Empty containers should always be stored in suitable container racking, set up for easy transfer to dollies or other handling equipment without the use of a forklift. Outdoors racking should be equipped to prevent containers blowing away in windy conditions. Empty containers should only be stacked where there exists proper training and controls and where the containers are designed to be lifted by forklift.
With the exception of forkliftable ULD (eg ULD with built in forklift pockets) a ULD, whether full or empty should never be left on the ground.
Empty pallets may be stored in stacks, taking care that:
- Cargo nets, if attached, are not damaged during the stacking/ unstacking process
- Stacks of pallets built up for transfer to another location by ground or air are properly built up on a “base pallet” , with suitable spacing material between the bottom of the pallet stack and the “base pallet”, and secured to the base pallet by tie down straps.
- When removing a number of pallets from the stacks, that this is carried out in manner such that fork tines do not damage the pallet edge rails.
Nets may be permanently attached to a pallet (along one side by locking the fittings in place) or may be completely removable. If removable they should be stored by airline in a dry location where they can easily be accessed for their next use.
Storage of loaded ULD
Loaded ULD must always be stored on a suitable transfer vehicle such as:
- Slave pallet
- Container racking
- Cargo terminal ULD handling and storage system
- Purpose designed road vehicle
and only be transferred using special purpose equipment equipped with rollers/ ball mats or similar support/ transfer devices. Specifications for most of these equipments may be found in the IATA Airport Handling Manual. Chapter 9. Poor handling may well result in a ULD, which was perfectly serviceable when it was loaded, arriving at the aircraft door in an unserviceable state, resulting in off loads, delays , reloads and other inconveniences.
Use of “Accessory Items”
Cargo straps are generally used to secure large/heavy objects that are not suited for restraint by a cargo net. They are also used to secure heavy items loaded inside a container subject to certain conditions. Cargo straps are typically of 2 types:
- 2000 lbs breaking strain straps having webbing about 1” (2.5cm) wide and single stud attachment fittings at each end; and
- 5000 lbs breaking strain straps having webbing about 2” (5.0cm) wide with hooks and/or double stud fittings at each end.
2000 lbs straps are generally used in the bulk holds of aircraft (such as 737 holds) while 5000 lbs straps are used to secure loads to pallets/containers. The use of cargo straps to secure any piece of cargo in lieu of a net requires great care and should only be done in accordance with the carriers specified procedures. General guidance can be found in the IATA ULD Technical Manual Standard Specification 60/2 Appendix A which is a simplified version of ISO standard 16049 Part 2, but this cannot replace specific airline instructions.
Tie down fittings are generally a double stud with a 2” dia.welded stainless steel ring. These may be used to connect a tie down strap terminated only with a hook into the pallet edge rail. They are not to be used as “temporary repair” for cargo nets, nor as some kind of additional lashing process.
Both straps and tie down fittings may only be deemed safe when issued by the owner airline. The use of a strap or fitting picked up from the floor of the terminal is unsafe. At present there is no certification standard for straps as there are for containers, pallets and nets, however a standard is in process and awaiting FAA implementation.
The primary role of ULD is to ensure flight safety. A brand new ULD, properly loaded, will indeed ensure safety. However, the widespread lack of attention/awareness of ULD damage limits and the severe risks associated with using ULD that are not within their damage limits, coupled with a widespread attitude of “it doesn’t matter as long as the cargo/ baggage makes the flight”, and a widespread lack of training in proper ULD operations, creates a serious safety hazard.
Damaged and/or improperly loaded ULD present a hazard to operators, aircraft systems and structure, cargo and baggage and GSE. Used correctly ULD will remain within their safety limits for many years, requiring only minor repairs to wear and tear type damages. Incorrect handling of ULD, particularly the indiscriminate use of forklifts will result in ULD damage rendering them unserviceable for use.
Failure to observe rules and a “make it up as we go along “ approach to ULD operations will result in safety breaches.
The solutions to these safety risks are found in:
- Provision of correct ULD handling infrastructure, guidance for which can be found in the IATA Airport Handling Manual. While such equipment is not cheap it is a necessity when it comes to ULD handling.
- Provision of training to all parties handling ULD to ensure correct procedures are followed. ULD may look simple, indeed a person loading ULD maybe considered less skilled than - say - a driver, however the person loading a ULD is actually performing a flight safety critical function, and can put the lives of all on board the aircraft at risk. “On the Job” training delivered by a worker who likely has had no formal training himself is not a sufficient method to manage to risks associated with ULD operations.
- Sufficient diligence by all parties in the ULD process to ensure that safety breaches are not allowed “just this time” but are caught and rectified each time. Letting an unserviceable or badly loaded ULD go “just this time” is as dangerous as ignoring any other defect on an aircraft and should never be permitted. An unserviceable ULD should never make it to the aircraft door, the defects should be picked up at the pre loading inspection and the ULD rejected at that stage by having suitably trained staff carrying out the loading process.
- A culture of “ask if not sure” needs to be observed. Guess work has no place in flight safety and this applies to ULD operations as much as to any other field of aviation.