By Jeremy R.C. Cox
In the United States alone, over 360 billion dollars worth of goods are shipped via airfreight every year. This figure is going to increase exponentially well into the 21st century, as industry pundits have projected that the worldwide air cargo fleet is going to grow by 85 percent in the next 20 years. In fact, officials at Boeing have reportedly estimated that roughly 70 percent of the aircraft that will join the air cargo fleet in the next 20 years will be modified and converted aircraft; not new OEM-supplied cargo aircraft.
A typical modification candidate is an aircraft that has a fair amount of time in service, usually 10,000 to 12,000 hours for a business jet or 30,000 to 40,000 hours for a commercial airliner. Also, the typical candidate was originally delivered as a passenger aircraft, meaning no cargo door, no emergency control relocation, no additional escape hatches, no cargo liner, and no restraining nets, etc. So what does it take to convert an aircraft into a cargo variant? I will attempt to provide you with a brief insight into this major task.
First, to install a cargo door on a 'narrow door' passenger aircraft, an intense and comprehensive structural substantiation and analysis process must be initiated. The OEM generally designs a semi monocoque, or stressed skin, aircraft structure. This type of fuselage structure consists of an internal skeletal main frame and beam primary structure that the rest of the pressurized metal airframe is built onto and around. The most significant structural loads are fed into this primary structure, while the exterior skin and stringers, known as secondary structure, is designed to carry the remaining loads in a stressed skin design.
Obviously, it will be necessary to breach the load path design of both the primary and secondary structure to enable the installation of an effectively-sized cargo door and crew emergency escape hatch. The remaining structure, after cutting the opening for the door and crew emergency escape hatch, has to be reinforced to carry the imbalanced/ altered loads. Also, it is essential that the new load paths within the modified structure are contained to acceptable safe limits and no stress risers are allowed to develop because of an incomplete stress analysis being done at the design stage of the project. A stress riser is a location at which stressful loads are concentrated and may cause failure of the effected structure.
Next, the maximum payload that can be carried without exceeding the center of gravity and weight limits must be determined. With this basic information group in hand, it will allow the engineers to perform a stress analysis of the floor structure and then provide guidance to how the under floor and floor support structure must be altered to carry these new loads. On virtually all converted aircraft, it is necessary to divide the cargo area (aircraft cabin) up into load zones that have differing weight limits. This is usually required to keep the aircraft within its C of G Range. Also, in some cases, the load bearing capacity of the floor structure varies throughout the compartment. The floor structure in a pressurized aircraft not only has to carry the load of the items that are placed upon it, but the structure has to support the variable internal air pressure loads that are imposed upon it by the pressurization system.
The cargo load zones that are determined for the aircraft will have to be placarded and will most likely be copied into the freight loadmasters load distribution paperwork. By copying this information into a loading form, a loadmaster will find it easier to calculate the load and distribute the freight safely. Some aircraft have certain devices and controls that have to be accessed in flight during normal or emergency situations. Two examples of this may be landing gear extension controls or fuel shut off valve controls. However, sometimes with freight on board, full unobstructed access to these devices and controls by a crewmember is impossible. In this instance, an alternate means of operating these devices or controls must be designed and incorporated into the cargo modification.
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