Unit load device
A Unit Load Device (ULD) is either an aircraft pallet combined with a net or an aircraft container, designed to group, restrain, and protect cargo, mail, and baggage during air transport on commercial aircraft.[1] These devices function as removable aircraft parts, subject to rigorous civil aviation authority standards for design, testing, production, operations, repair, and maintenance to safeguard flight safety and structural integrity.[1] Introduced in the late 1950s, ULDs revolutionized air cargo handling by standardizing load units, thereby enhancing loading efficiency, reducing turnaround times, and minimizing damage risks for airlines worldwide.[2] Today, approximately 1 million ULDs are in active service, holding a collective replacement value of over USD 1 billion, with annual repair and loss costs reaching USD 330 million; effective management of these assets can yield savings of up to USD 475 million per year through optimized operations and reduced incidents.[1] Governed by the International Air Transport Association (IATA) ULD Regulations, which align with International Civil Aviation Organization (ICAO) guidelines, national authorities, and standards from bodies like the International Organization for Standardization (ISO) and the Society of Automotive Engineers (SAE), ULDs play a pivotal role in the global air freight ecosystem despite challenges such as outsourced operations and shipper-built units that can compromise compliance.[3][1] ULDs are broadly classified into two categories: pallets and containers, selected based on cargo dimensions, weight, and aircraft configuration.[1] Pallets consist of flat platforms secured by nets, ideal for bulky or irregularly shaped items, and are often used on main decks of wide-body aircraft.[4] Containers, by contrast, are fully enclosed for secure transport of smaller or sensitive goods and are prevalent in lower cargo holds.[4] Among the most common container types are the LD-3 (also known as AKE), which accommodates up to 1,588 kg and 4.3 m³ internally and fits lower decks of aircraft like the Boeing 747 and Airbus A300, and the LD-9 (AAP), capable of handling up to 4,625 kg and 9.8 m³ for broader wide-body compatibility.[5] These standardized configurations, identified by IATA codes, ensure seamless interoperability across fleets, streamlining global logistics while adhering to weight limits and contour specifications tailored to specific aircraft models.[5]Overview
Definition and Purpose
A unit load device (ULD) is a specialized apparatus used in aviation to consolidate, secure, and transport cargo, mail, and baggage on aircraft. It typically consists of either an aircraft container or a combination of an aircraft pallet and a net, designed to function as a removable part of the airplane structure. This standardization allows for the efficient grouping and restraint of multiple items into a single unit, facilitating seamless transfer between ground handling equipment and aircraft cargo holds.[1][6] The primary purpose of ULDs is to enhance the safety and efficiency of air cargo operations by protecting both the aircraft and its contents during flight. By restraining loads against forces such as turbulence, acceleration, and deceleration, ULDs prevent cargo shifting that could damage aircraft systems or compromise flight stability. They also streamline loading and unloading processes, reducing turnaround times for aircraft and enabling airlines to optimize space utilization in cargo compartments, which is critical for revenue generation in the global air freight industry. Approximately one million ULDs are in active service worldwide, underscoring their integral role in modern air transport logistics.[1][2] ULDs are subject to rigorous regulatory standards to ensure structural integrity and operational reliability. They must comply with international guidelines, including those from the International Air Transport Association (IATA) and the International Civil Aviation Organization (ICAO), as well as national aviation authorities like the Federal Aviation Administration (FAA) in the United States. Certification requirements, such as FAA Technical Standard Order (TSO) C90, mandate testing for load-bearing capacity and compatibility with specific aircraft types, with serviceability inspections required prior to each use to mitigate risks like improper handling, which can result in significant financial losses for the industry. These measures collectively prioritize flight safety while supporting the scalable movement of diverse cargo types, from perishables to high-value goods.[1][6]Historical Development
The concept of unit load devices (ULDs) emerged in the 1950s as the airline industry sought to standardize cargo handling amid the introduction of jet aircraft like the Douglas DC-8 and Boeing 707, which offered higher speeds (around 550 mph) and payloads up to 40 metric tonnes but required efficient loading methods to address the lack of uniformity in airfreight packaging.[7] Early ULDs were developed as interchangeable containers to streamline ground and flight operations, reducing loading times and damage risks compared to loose cargo.[8] By the late 1960s, efforts toward ULD standardization intensified to enable compatibility across airlines and aircraft types, culminating in the debut of the Boeing 747 in January 1970, whose first commercial flight from New York to London utilized ULDs for both passengers and freight, marking a pivotal shift toward containerized air cargo on wide-body jets.[9][7] In the early 1970s, airlines and manufacturers began formalizing ULD designs, with the International Air Transport Association (IATA) forming the Interline ULD User Group (IULDUG) to manage shared ULD inventories through manuals and early IT systems, while initial models featured heavy bases (up to 120 kg) designed for forklift handling but prone to high maintenance.[9] The 1980s saw expanded ULD adoption with the proliferation of wide-body aircraft and outsourcing of ground handling, alongside innovations like the Airbus A320's introduction in 1988, the first single-aisle jet optimized for 45-inch AKH ULDs, and the rise of lightweight courier containers.[9][7] Safety concerns escalated in the 1990s following the 1997 Fine Air Flight 101 crash, which exposed issues with cargo shifting and restraints, prompting the U.S. Federal Aviation Administration (FAA) to issue Advisory Circular AC 120-85 in 2005 for improved compliance in cargo operations.[9][7] The 2000s brought lighter composite ULDs (around 80 kg with fabric doors) and web-based tracking via IULDUG, while the 2011 rebranding to ULD CARE as an independent entity from IATA, following their separation in 2010, emphasized maintenance and safety standards.[9] Further milestones included IATA's first ULD Regulations (ULDR) in 2013, standardizing testing and usage globally, and responses to incidents like the 2013 National AirCargo Flight 102 crash, leading to AC 120-85A in 2015 for enhanced restraint guidelines.[9][7] In recent years, ULD CARE has advanced digital solutions, such as blockchain for tracking (explored since 2018) and augmented reality tools, reflecting ongoing evolution driven by global trade growth, with the ULD market valued at approximately 357.6 million USD in 2022.[9][7] Since then, the market has grown to an estimated USD 2.3 billion as of 2024, with innovations including over 30 new ULD variants launched in 2023 (such as fire-resistant and sustainable designs) and preparations for the 14th edition of IATA ULDR in 2026, focusing on enhanced interoperability and environmental standards.[10][11][3]Types
Pallets
Pallets are a fundamental type of unit load device (ULD) consisting of a flat platform combined with a securing net, designed to consolidate and restrain cargo for air transport without enclosing it. Unlike containers, pallets allow for open loading, making them suitable for oversized, irregularly shaped, or heavy items that exceed container dimensions. They must comply with International Air Transport Association (IATA) standards to ensure structural integrity, load distribution, and compatibility with aircraft cargo holds.[1][12] The design of ULD pallets emphasizes lightweight construction while supporting substantial payloads, typically featuring a rigid frame and base to interface with aircraft restraint systems. Materials commonly include aluminum sheets and profiles for the frame to minimize weight, with bases made from plywood, honeycomb composites, or reinforced plastics to provide a stable loading surface. These pallets are contoured to fit specific aircraft cross-sections, particularly in lower holds, and are tested for impact resistance, fire safety, and load-bearing capacity under IATA ULD Regulations (ULDR), which align with International Civil Aviation Organization (ICAO) and International Organization for Standardization (ISO) guidelines.[3][12][13] ULD pallets are categorized by size and application, with IATA codes denoting their base dimensions and contours. Lower deck pallets, such as the PAG (88 x 125 inches or 223 x 318 cm) and PMC (96 x 125 inches or 244 x 318 cm), are used in wide-body aircraft like the Boeing 777 and Airbus A330 for bulk cargo in the underfloor holds. These have usable areas of approximately 3 x 2.1 m and 3 x 2.3 m, respectively, with maximum gross weights up to 4,536 kg. Main deck pallets, including the 20-foot PGE (96 x 238.5 inches or 244 x 605 cm) for freighter aircraft like the Boeing 747-400F, accommodate longer or heavier loads, such as vehicles or machinery, with usable areas around 5.9 x 2.3 m and gross weights exceeding 13,600 kg. Smaller variants, like the P1P (88 x 125 inches or 223 x 318 cm), are versatile for both passenger and freighter aircraft.[12][13][14] In practice, pallets enhance loading efficiency by allowing forklift access and rapid securing with nets that distribute forces evenly to prevent shifting during flight. They are particularly advantageous for perishable goods requiring ventilation or items needing custom dunnage, reducing labor costs and turnaround times compared to loose cargo handling. However, their open design necessitates careful netting to comply with weight-and-balance requirements and avoid damage from turbulence. Approximately 1 million ULDs, including pallets, are in global service, valued at more than USD 1 billion, underscoring their role in the USD 150 billion air cargo industry.[1][5][12]| IATA Code | Base Dimensions (cm) | Usable Area (cm) | Max Gross Weight (kg) | Typical Aircraft Compatibility |
|---|---|---|---|---|
| PAG/PAJ | 223 x 318 | 210 x 300 | 4,536 | B767, B777, A330 (lower deck) |
| PMC/PQP | 244 x 318 | 230 x 300 | 4,536 | B747, A300 (lower deck) |
| PGE | 244 x 605 | 230 x 590 | 13,608 | B747F, MD-11F (main deck) |
| P1P | 223 x 318 | 210 x 300 | 4,626 | Wide-body freighters |
Containers
Unit load devices encompass both pallets and containers, with the latter serving as fully enclosed structures designed to protect cargo from environmental factors and facilitate efficient loading into aircraft holds. Containers, often referred to as air cargo containers, are rigid or semi-rigid units that provide secure containment for baggage, freight, and mail, optimizing space utilization and reducing handling time compared to loose cargo. Unlike pallets, which are open platforms typically covered by nets, containers feature sidewalls, doors, and roofs, enabling them to withstand stacking and protect contents during transit. Container codes typically start with A (certified by aviation authorities) or D (non-certified but compliant with IATA guidelines).[16] The primary advantage of ULD containers lies in their ability to standardize cargo handling across different aircraft types, ensuring compatibility with lower deck compartments of wide-body and some narrow-body aircraft. They are constructed from lightweight materials such as aluminum or composites to minimize tare weight while meeting structural integrity requirements for flight safety. Containers must be inspected prior to use to verify door security, structural damage, and legibility of certification plates, as any compromise can lead to load shifts or aircraft system interference during flight.[16][17] Standards for ULD containers are governed by international bodies including the International Air Transport Association (IATA) through its ULD Regulations, which outline specifications for design, testing, production, and maintenance. In the United States, the Federal Aviation Administration (FAA) certifies containers under Technical Standard Order (TSO) C90, requiring them to endure extreme dynamic loads, such as 9g forward and 3g vertical forces, as demonstrated in upload tests for models like the LD-3. European equivalents follow the European Union Aviation Safety Agency (EASA) ETSO C90d. Non-certified containers, often used for shipper-built units, must still comply with IATA guidelines for load restraint and center-of-gravity limits, typically within 10% of the base center to prevent instability. Active containers, equipped with temperature-control systems for perishables, require additional approvals under FAA Part 21.[1][16][17] Various container types exist to match aircraft cross-sections and cargo volumes, identified by IATA codes starting with letters like A (certified) or D (non-certified), followed by descriptors for shape and size. Common examples include the LD-3 (AKE), a versatile lower-deck container with an internal volume of approximately 4.3 cubic meters and a maximum gross weight of 1,588 kg, compatible with aircraft such as the Boeing 747, 767, and Airbus A300. The LD-1 (AKC), suited for main-deck use on the Boeing 747, offers 4.7 cubic meters internally and supports up to 1,588 kg. Larger variants like the LD-6 (ALF) provide 8.9 cubic meters for lower decks on multiple wide-body models, with a 3,175 kg capacity. These dimensions ensure precise fit through cargo doors while maximizing payload efficiency.[5][18]| Type | IATA Code Example | Internal Volume (m³) | Max Gross Weight (kg) | Compatible Aircraft Examples |
|---|---|---|---|---|
| LD-1 | AKC, RKC | 4.7 | 1,588 | Boeing 747 (main deck) |
| LD-2 | APE, RPE | 3.4 | 1,225 | Boeing 767 (lower deck) |
| LD-3 | AKE, DKE | 4.3 | 1,588 | Boeing 747, 767, Airbus A300 |
| LD-6 | ALF, RLF | 8.9 | 3,175 | Boeing 747, 777 (lower deck) |
| LD-8 | AQF, RQF | 7.2 | 2,449 | Boeing 767 (lower deck) |
Compatibility and Usage
Aircraft Compatibility
Unit load devices (ULDs) are designed with specific contours and dimensions to ensure compatibility with the cargo holds of various aircraft, primarily wide-body models, while limited types suit narrow-body aircraft. Compatibility is governed by International Air Transport Association (IATA) standards, which define ULD shapes to match aircraft cross-sections, allowing for secure loading, weight distribution, and fire safety through smoke circulation. These standards prevent damage to aircraft structures and optimize space utilization across airlines.[1] Wide-body aircraft, such as those from Boeing and Airbus families, accommodate the majority of ULD types on both lower decks and main decks (for freighters). For instance, the Boeing 747 supports a range of lower-deck containers including LD-1 (IATA code AKC), LD-2 (DPE), LD-3 (AKE), and LD-6 (ALF), as well as pallets like LD-7 (P1P) and main-deck containers such as M-1 (AMA). Similarly, the Boeing 777 is compatible with LD-1, LD-2, LD-3, LD-4 (ALP), LD-6, LD-7, and M-1, enabling versatile cargo configurations up to 6,804 kg per M-1 unit. The Boeing 787 shares compatibilities with LD-1, LD-2, LD-3, LD-4, LD-6, and LD-7, while the Boeing 767 accepts LD-1, LD-2, LD-3, LD-4, LD-7, and LD-8 (DQF). Airbus wide-bodies follow analogous patterns; the A330, A340, A350, and A380 fit LD-3, LD-6, and LD-11 containers on lower decks, with LD-3 (1,588 kg max gross weight) being a common choice across A300 to A380 models for its half-width design. Main-deck ULDs like M-1 are used on Airbus freighters such as the A330F.[13][5][19] Narrow-body aircraft have more restricted compatibility due to smaller fuselage dimensions (typically 3-3.7 m width), limiting ULDs to bulk loading or specific reduced-size containers on lower decks, with pallets primarily on main decks for freighter variants. The Boeing 737 freighter (737F) accommodates LD-7 pallets on the main deck and demi-containers, but standard passenger versions rely on bulk cargo without full ULDs. The Airbus A320 family supports reduced-height LD3-45 containers (AKH) on the lower deck, while the Boeing 757 freighter uses similar pallet-based systems. LD-7 pallets (4,626 kg max) provide cross-compatibility for narrow-body freighters like the 737F and 757F, as well as wide-body lower holds.[13][20] The following table summarizes key ULD compatibilities for representative aircraft models, focusing on lower-deck options unless noted:| Aircraft Model | Compatible ULD Types (IATA Codes) | Notes |
|---|---|---|
| Boeing 747 | LD-1 (AKC), LD-2 (DPE), LD-3 (AKE), LD-6 (ALF), LD-7 (P1P), LD-29, M-1 (AMA, main deck) | Supports full-width and half-width; up to 3,175 kg for LD-6.[13][5] |
| Boeing 777 | LD-1 (AKC), LD-2 (DPE), LD-3 (AKE), LD-4 (ALP), LD-6 (ALF), LD-7 (P1P), M-1 (AMA, main deck) | Versatile for mixed passenger-freight; LD-3 common at 1,588 kg.[13][5] |
| Boeing 787 | LD-1 (AKC), LD-2 (DPE), LD-3 (AKE), LD-4 (ALP), LD-6 (ALF), LD-7 (P1P) | Optimized for efficiency; excludes some older full-width types.[13][5] |
| Airbus A330/A340 | LD-3 (AKE), LD-6 (ALF), LD-11, LD-7 (P1P) | Lower deck focus; A330F adds M-1 main deck.[5][19] |
| Boeing 737 (freighter) | LD-7 (P1P, main deck), Demi-containers | Bulk primary for passengers; pallets for cargo variants.[13] |
| Airbus A320 | LD3-45 (AKH, reduced height) | Limited to smaller containers; no standard LD-3.[20] |
Capacity and Dimensions
Unit load devices (ULDs) adhere to standardized dimensions and capacities established by the International Air Transport Association (IATA) to facilitate compatibility with aircraft cargo compartments and optimize load efficiency. These specifications encompass external and internal dimensions, maximum gross weights, tare weights, and volumes, which vary by ULD type to accommodate diverse cargo densities and aircraft configurations. The standards ensure that ULDs can be loaded through specific door openings while respecting floor strength limits and center-of-gravity constraints in the aircraft.[1] Containers, the enclosed variant of ULDs, are designed with contoured bases to conform to the curved interiors of lower-deck cargo holds in wide-body aircraft. A representative example is the LD-3 container (IATA code AKE), which has external base dimensions of 153 cm × 156 cm and a height of 163 cm, providing an internal volume of 4.5 m³ and supporting a maximum gross weight of 1,588 kg with a tare weight of approximately 82 kg. This allows for door openings of 147 cm × 155 cm, making it suitable for general cargo on aircraft like the Boeing 747 and Airbus A380. Larger containers, such as the LD-7 (P1P), offer an internal volume of 10.5 m³ and a maximum gross weight of 4,626 kg, with external dimensions around 317 cm × 224 cm × 163 cm, enabling higher-capacity loads in compatible holds.[22][19][13] Pallets, consisting of flat platforms secured with nets, provide flexibility for oversized or irregularly shaped cargo and are often used on main decks or in lower holds. The PMC pallet (P6P), a standard lower-hold type, features base dimensions of 318 cm × 243 cm and can handle a maximum gross weight of 6,804 kg, with an effective volume under netting of up to 17.5 m³ when loaded to standard heights. In contrast, the PAG pallet (LD-6 equivalent) has smaller dimensions of 317 cm × 229 cm, supporting 3,175 kg and volumes around 8.9 m³, ideal for narrower compartments in aircraft such as the Boeing 767. These pallets typically have tare weights of 100–350 kg, depending on material and reinforcements, and their contours ensure secure latching to aircraft restraint systems.[5][23] The following table summarizes key specifications for selected common ULD types, highlighting variations in capacity:| ULD Type | IATA Code | Base Dimensions (cm) | Height (cm) | Internal Volume (m³) | Max Gross Weight (kg) | Tare Weight (kg) | Typical Compatibility |
|---|---|---|---|---|---|---|---|
| LD-3 Container | AKE | 153 × 156 | 163 | 4.5 | 1,588 | 82 | B747, A380, B777 lower deck |
| LD-7 Container | P1P | 317 × 224 | 163 | 10.5 | 4,626 | 105 | B747, A330 lower deck |
| PMC Pallet | P6P | 318 × 243 | Variable (net) | 17.5 | 6,804 | 350 | B747F, A330F main/lower deck |
| PAG Pallet | LD-6 | 317 × 229 | Variable (net) | 8.9 | 3,175 | 230 | B767, A340 lower deck |
Identification
Coding System
The coding system for unit load devices (ULDs) is a standardized alphanumeric identifier established by the International Air Transport Association (IATA) to uniquely denote each ULD's type, serial number, and ownership, facilitating efficient tracking, handling, and compatibility verification in air cargo operations.[3] This system is detailed in the IATA ULD Regulations (ULDR), particularly in Section 4, which outlines the identification requirements to ensure interoperability across airlines and ground handlers.[3] The code typically consists of 9 or 10 characters, enabling quick recognition without ambiguity.[16] The ULD identification code is structured into three main components: a three-letter type code (positions 1-3), a serial number (positions 4-8, either 4 or 5 digits), and a two-letter owner code (positions 9-10).[24] The type code specifies the ULD's physical characteristics, including its certification status, base dimensions, and contour compatibility with aircraft cargo holds. Position 1 indicates the category: for example, "A" denotes a certified aircraft container, "P" a certified pallet, "D" a non-certified container, or "M" a pallet/net combination for main-deck use.[5] Position 2 represents the base size, such as "K" for 1.534 m x 1.562 m or "P" for 2.44 m x 3.18 m. Position 3 describes the contour or shape, like "E" for lower-deck compatibility or "F" for a flat base.[16] Examples of type codes include AKE (a certified LD-3 container with 60.4-inch x 61.5-inch base and contoured shape for lower-deck use) and PMC (a certified pallet with 96-inch x 125-inch base and contoured netting for main-deck).[24] The serial number, comprising 4 or 5 consecutive digits (4 digits for ULDs manufactured before October 1, 1993, and 4 or 5 thereafter), is assigned sequentially by the manufacturer or owner to distinguish individual units within the same type code, ensuring no duplicates per owner.[16] For instance, in the code AKE12345BA, "12345" uniquely identifies that specific container. The owner code, the final two alphanumeric characters, is an IATA-assigned identifier for the ULD's proprietor, such as "BA" for British Airways or "AA" for American Airlines; leasing companies may use codes like "JG."[24] These codes are registered and managed through the IATA ULD Board to maintain global consistency.[1] To enhance readability and automation, ULD codes are often represented in bar code format (per IATA standards) and increasingly via RFID tags for real-time scanning during loading and inventory processes.[16] The full code must be prominently displayed on the ULD, with new type codes approved only through formal application to the IATA ULD Board, as outlined in ULDR Attachment A.[3] This system not only prevents mishandling but also supports regulatory compliance, such as weight and balance calculations.[5]| Component | Positions | Description | Example |
|---|---|---|---|
| Type Code | 1-3 | Defines category, base size, and contour | AKE (certified container, 60.4" x 61.5" base, LD-3 contour) |
| Serial Number | 4-8 | Unique identifier (4 or 5 digits) | 12345 |
| Owner Code | 9-10 | IATA-assigned owner identifier | BA (British Airways) |
Owner and Serial Identification
Unit load devices (ULDs) are identified through a standardized alphanumeric code that incorporates details on ownership and uniqueness, ensuring traceability throughout the air cargo supply chain. The owner code consists of the final two characters in the ULD identification code, which denote the entity responsible for the ULD, such as an airline or leasing company, using an IATA-assigned two-character code (e.g., "BA" for British Airways or "JG" for a ULD leasing firm).[16] This code is crucial for distinguishing ULDs with identical serial numbers owned by different entities, facilitating accountability for maintenance and serviceability.[24] The serial number forms the middle portion of the ULD code, typically comprising 4 or 5 numerical digits that provide a unique identifier for each individual ULD within the owner's inventory.[2] Assigned sequentially by the owner upon manufacture or acquisition, this number enables precise tracking, inventory management, and damage reporting across global operations (4 digits for ULDs built before October 1, 1993, and 4 or 5 digits for later ones).[16] For instance, an older ULD might use a 4-digit serial, while newer ones employ 5 digits to accommodate expanded fleets.[2] These identification elements are governed by the International Air Transport Association (IATA) ULD Regulations, which standardize the overall 9- or 10-character code structure: a 3-letter type prefix followed by the serial number and ending with the owner code.[2] Compliance ensures interoperability among airlines and handlers, with the owner bearing responsibility for the ULD's condition and any required repairs.[2] Physical markings, including barcodes replicating the full code, are affixed to the ULD for scanning and verification during loading and unloading.[16]Main-Deck ULDs
Design Characteristics
Main-deck unit load devices (ULDs) are engineered to optimize space utilization in the curved fuselage of cargo aircraft upper decks, featuring contoured upper profiles that conform to the aircraft's structural contours for secure and efficient loading. These ULDs typically consist of a standardized aluminum base with integrated lock fittings that engage with the aircraft's cargo loading system (CLS) rails and locks, ensuring positive restraint during flight. The base is often constructed from lightweight, high-strength aluminum alloys to minimize tare weight while supporting substantial payloads, with peripheral edges reinforced for handling and stacking. Enclosed containers incorporate rigid or semi-rigid sidewalls and roofs made from fiberglass-reinforced panels or aluminum sheets, providing protection against environmental factors and facilitating rapid loading via hinged or roll-up doors.[25][26] Key design elements prioritize structural integrity under extreme aerodynamic loads, adhering to International Air Transport Association (IATA) standards and certifications such as FAA Technical Standard Order (TSO)-C90, European Technical Standard Order (ETSO)-C90, and specifications in NAS 3610 and AS36100. For instance, restraint systems include integrated nets or straps capable of withstanding 9g forward, 1.5g aft, 3g lateral, and 3g vertical accelerations, distributing forces evenly to prevent cargo shift. Pallet-based ULDs, such as the PAG (88 x 125 in / 223 x 318 cm base, tare ~110 kg / 243 lb, max gross main deck ~6,000 kg / 13,228 lb) or P6P (96 x 125 in / 244 x 317 cm base, tare ~120 kg / 265 lb, max gross 6,804 kg / 15,000 lb) types, feature flat or slightly raised aluminum platforms with foldable perimeter lips and attached cargo nets, ideal for oversized or irregularly shaped freight like vehicles or machinery. Containers like the AMA or PMC, built on similar bases, add vertical walls reaching up to 96 inches (244 cm) in height, offering internal volumes of approximately 18-20 m³ while maintaining a low profile to fit within door openings and fuselage curvature.[17][27][13][28] Safety and compatibility are integral to the design, with features like fire-resistant materials in panel construction and pressure relief valves in some enclosed types to manage internal pressurization differences. All main-deck ULDs must be listed in the aircraft's weight and balance manual (WBM) or supplemental type certificate (STC) for approval, ensuring dimensional compatibility with specific models such as the Boeing 777 or Airbus A330 freighters. For example, the PMC container, with its contoured roof and canvas side doors equipped with built-in netting, exemplifies a versatile design for high-density cargo, certified under IATA Type 2 classification for main-deck use. These characteristics enable main-deck ULDs to handle a significant portion (often 70-90% in wide-body freighters) of an aircraft's total cargo capacity.[3][29][16][13]| ULD Type | Base Dimensions (in/cm) | Max Height (in/cm) | Tare Weight (kg/lb) | Max Gross Weight (kg/lb) | Typical Volume (m³/ft³) | Primary Material | Example Aircraft Compatibility |
|---|---|---|---|---|---|---|---|
| PAG Pallet | 88 x 125 / 223 x 318 | N/A (flat) | 110 / 243 | 6,000 / 13,228 | 11.9 / 420 (with net) | Aluminum base, nylon net | Boeing 747, 777 |
| AMA Container | 96 x 125 / 244 x 317 | 96 / 244 | 350 / 772 | 6,804 / 15,000 | 18.5 / 653 | Aluminum frame, fiberglass panels | Airbus A380, Boeing 747 |
| PMC Container | 96 x 125 / 244 x 317 | 64 / 163 (contoured) | 250 / 551 | 6,804 / 15,000 | 17.5 / 618 | Aluminum, canvas doors | Boeing 777, MD-11 |