User:HuffTheWeevil/sandbox/History of ULDs

463L master pallet
During the late 1950s, the U.S. Air Force started a project, SS-463L, to standardize military air cargo. By the early 1960s, the project produced a finalized standard, the HCU-6/E pallet, more commonly known as the 463L master pallet. The pallets are 88" by 108" and still in use by the military today. Cargo aircraft, such as, the C-130, C-5, C-27, CH-47, KC-10, C-17, and C-9 all accept these standard pallets.

Alongside the SS-463L project, the United States Department of Defense created specifications for intermodal freight transport on trains, ships, and trucks. Trains and trucks were each about 8 feet wide, which influenced the specifications. In the late 1960s, the International Organization for Standardization (ISO) issued standards for intermodal containers based upon the U.S. Department of Defense specifications. The most common dimension is 8 feet wide (96"), 8 feet high, and a length of either 20 or 40 feet. These dimensions would eventually influence the shape of the Boeing 747.

Boeing 707
The original 707 was built for passengers only, without the concern for any heavy cargo lifting. After several modifications were made to the base model, the final 707 variant built was the 707-320C, which had a large fuselage door for cargo. The 88"x108" standard that the US military created fit inside the 707 sideways. But the 707's main deck was just under 11 feet wide, and to utilize every cubic inch of space, a new pallet, 125" wide could be used in place of a 108" pallet. The main deck could fit 13 positions of 88" each, with the last position turned sideways due to the tapered fuselage.

With these two standards in mind, the 707's main deck floor was designed to accept both sizes. The 125"-wide pallets took up the whole width, while the 108"-wide pallets were pushed to one side with a narrow 17"-wide passageway that allowed the flight crew to have access to the rear passenger cabin. The impressive feature of the 707-320C was the C, representing "Convertible". Operators could convert the layout of the main deck as needed. Not only could the aircraft be changed from all-passenger to all-cargo and back, but almost any combination of cargo and passengers was available. This type of aircraft, called a combi aircraft, is still used today by some operators. On the 707-320C, three to ten pallets could be loaded in front of a passenger cabin that would utilize the remaining space.

While the 88"x108" dimension was already in place, the 707 gave birth to 88"x125" dimension. Both would become standards in the air cargo industry for decades. The 727, 737, and 757 all shared the same fuselage cross section as the 707. Because of this, all of their respective freighter and combi versions used the same types of pallets. The lower compartments of these aircraft, only 44" tall, were "bulk loaded" with baggage or cargo.

Douglas DC-8
The DC-8 is the first aircraft to utilize any sort of container system for the belly compartments, an option that was only exercised by United Air Lines on their DC-8-61s. These containers, called "baskets", were loaded from a door on the very bottom of the aircraft and lifted straight up by an electric motor. Then they were moved fore and aft using a hand crank. Like ULDs of today, the baskets were contoured to the shape of the lower compartment, round on the bottom and flat on top. Their depth was remarkably similar to today's standards, as well.

Boeing 727-200
Although it was standard to bulk load the belly of 727s, an option of the 727-200 was to use a very unique container to fit in the lower compartments. The containers were shaped to fit the compartments without wasting space. They were 45.5" wide at the base which contoured up to 92.4" wide about half way up, which continued to the top. The overall height of 41.1" and depth of 43.4" allowed it to fit in the door, which was changed from the -100 to the -200 by designing it to open outward instead of inward to accommodate the height of the containers. The were then pushed, on rollers built into the floor, into position. On the 727-200, seven of these containers could fit in the forward compartment and four in the aft.

Boeing 747
Even before it lost the CX-HLS contract, Boeing was pressed by Juan Trippe, president of Pan Am, one of their most important airline customers, to build a passenger aircraft more than twice the size of the 707. Joe Sutter, transferred from Boeing's 737 development team, initiated a design study with Pan Am and other airlines, to better understand their requirements. At the time, it was widely thought that the 747 would eventually be superseded by supersonic transport aircraft. Boeing responded by designing the 747 so that it could be adapted easily to carry freight and remain in production even if sales of the passenger version declined. In the freighter role, the clear need was to support the containerized shipping methodologies that were being widely introduced at about the same time. Standard containers are 8 ft square at the front and available in 20 and 40 ft lengths. This meant that it would be possible to support a 2-wide 2-high stack of containers two or three ranks deep with a fuselage size similar to the earlier CX-HLS project. Ultimately, the CX-HLS Boeing design was not used for the 747. Concern over evacuation routes and limited cargo-carrying capability caused this idea to be scrapped in early 1966 in favor of a wider single deck design. The cockpit was, therefore, placed on a shortened upper deck so that a freight-loading door could be included in the nose cone.

The resulting cross-section allowed for modest size lower deck, compared to that of previous aircraft such as the DC-8, 707, and 737, which only accept bulk luggage in their lower decks. The 747's lower deck, shaped similar to previous jet aircraft, spanned 10 feet on the floor and over 15 feet from wall to wall, with a contoured edge on either side to maximize space, and a height of 66". The 10-foot floor width and 66" height would become standards on all future widebody aircraft, except the 767 which has a slightly smaller width. Boeing developed containers similar in shape to the optional ones for the 727-200 but split them in half, so that a pair of them would occupy one row in the compartment. The width of the base is 61.5", half of the 125" deck width, and they are contoured up to 92" wide, half of the 186" full width. The containers are 64" tall, leaving a two inch gap above, and 60.4" deep. The 60.4" dimension was determined by the size of the compartments: the forward was 41.5 feet long, the aft was 36.5 feet long, a difference of about 60".

Coincidentally, the width of the 747 lower decks were the same as the width of the main decks of the 707, 727, and 737, which were already carrying pallets, specifically of the 88"x125" size. With the design of the 747, these pallets could be easily transferred to the 747's lower decks if the height of the pallet was 64" or less.

American Airlines specification
In 1966, American Airlines offered a specification to manufacturers for a widebody aircraft smaller than the Boeing 747 but capable of flying similar long-range routes from airports with shorter runways. Douglas Aircraft and Lockheed both designed aircraft with three engines. Airbus, a multi-national consortium started in Europe, also designed an aircraft, but with only two engines.

McDonnell Douglas DC-10
Following an unsuccessful proposal for the U.S. Air Force's CX-HLS in 1965, Douglas Aircraft began design studies based on its CX-HLS design. The DC-10 became McDonnell Douglas's first commercial airliner after the merger between McDonnell Aircraft Corporation and Douglas Aircraft Company in 1967. An early DC-10 design proposal was for a four-engine double-deck wide-body jet airliner with a maximum seating capacity of 550 passengers similar in length of a DC-8. The proposal was shelved in favor of a trijet single-deck wide-body airliner with a maximum seating capacity of 399 passengers, and similar in length to the DC-8 Super 60.

The DC-10 and L-1011 have a slightly smaller cross section then the 747 but were still able to accommodate two 96"-wide pallets/containers with some contouring necessary. The floor of the lower decks were the standard 125". However, the width from wall to wall of the lower decks (that is, the width above the contoured parts) was smaller (160") than the 747's. So this is where the LD3 (and LD6) standard came from, narrowing the overall width to 79" but keeping the base width and depth, to allow for backwards compatibility in the 747. All future widebodies, except the 767, were designed with the LD3 in mind.

Airbus A330/A340
The A330 and A340 would use the front and rear fuselage sections of the A310. Components across the aircraft were modular, and interchangeable with other Airbus aircraft where possible to reduce production, maintenance and operating costs.

Boeing 767
Several years later the 767 was designed to be midway between the 707 and 747. Because the aircraft was smaller than other widebodies, the main deck could only accept two columns of 88"-wides or a single column of 96"-wides with a huge waste of space. The lower deck was not wide enough to accommodate two standard LD3 wide-body unit load devices side-by-side. As a result, a smaller container, the LD2, was created specifically for the 767. The LD2 has a overall width, 61.5", equal to that of the LD3, and a base width of only 47". A "double-wide" version, the LD8, was preferred over two LD2s. The depth of the containers were kept at 60.4", as to allow the 767 to accept LD1s and LD3s with a loss of space because one LD1/LD3 occupies two LD2 spaces.

Airbus A350
The original version of the A350 superficially resembled the A330 due to its common fuselage cross-section and assembly. Airbus faced almost immediate criticism on the A350 project from the heads of their largest customers. They called on Airbus to bring a clean-sheet design to the table, or risk losing most of the market to Boeing.

The proposed new A350 was a new design, including a wider fuselage cross-section. The new XWB fuselage will also have a constant width from door 1 to door 4, unlike previous Airbus aircraft, to provide maximum usable volume. The double-lobe (ovoid) fuselage cross-section will have a maximum outer diameter of 5.97 m, compared to 5.64 m for the A330/A340.