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.
After the primary/secondary fuselage structure and floor support structure modification stress issues have been addressed, along with any required control relocation modifications, consideration must be given to what type of fire detection and fire suppression/extinguishing systems that are going to be installed. The FAA established a cargo and baggage compartment classification system for transport category aircraft in 1946. Since then, the requirements have been tweaked after several in-flight catastrophes have occurred; the most recent was the Valujet/Sabretech disaster.
The classification system includes Class A, Class B, Class C, Class D, and Class E. In simplified terms, the Class A compartment does not require any additional fire detection or suppression/ extinguishing systems to be installed. The Class B compartment must have a fire or smoke detection system installed. The Class C compartment must have a fire detection and a fire extinguishing system along with a compartment liner installed. The Class D compartment does not require any additional fire detection or suppression/extinguishing systems to be installed; however, a compartment liner must be installed. Finally, the Class E compartment (most common on all dedicated cargo aircraft) must have a fire or smoke detection system along with a compartment liner installed.
After all of this, there are two major issues left to deal with to complete the modification design. These are: the crew smoke barrier bulkhead and the cargo net installation. As part of the compartment classification requirements, some compartments require that you have a means of controlling the ventilation into the cargo compartment during a fire situation. With this in mind, it will also be necessary to separate the crew from any smoke or fumes that are generated by a fire in the cargo compartment.
This separation can be accomplished by designing and building a sealed metal bulkhead, with a working door, between the cockpit and the cargo compartment. Ultimately, this arrangement will protect the crew's normal working environment and allow them to concentrate on flying the aircraft when a fire emergency occurs. If the pallet hold-down straps failed and the cargo shifted forward, the payload must be prevented from moving forward into the crew compartment area.
This crew protection is normally accomplished by the installation of a solid bulkhead or cargo restraining net. The cargo restraining net or bulkhead has to be able to withstand a 9g load without failing. If a net is installed, it must have enough room allowed ahead for it to flex and deflect while arresting the load.
By Jeremy R.C. Cox
An actual conversion
Now that we have briefly touched on the design requirements, let's discuss an example of an actual conversion program that Avtec has designed for the Falcon 20. The Avtec modification process is broken down into 14 separate and manageable processes.
•Incoming Inspection and Component Removal
• Door Opening and Surrounding Structure, including Cargo Restraining Net Installation
• Build Door - Latching Mechanism
• Cargo Floor and Floor Support Structure
• Emergency Control Relocation
• Window Plug Installation
• Relocation of Oxygen System and Rate Gyros
• Installation of all Cargo Door Control Wiring
• Installation of all Cargo Compartment Lighting Systems
• Installation of all Cargo Compartment Smoke Detection Systems
• Crew Escape Hatch
• Aft and Forward Bulkheads
• Cargo Compartment Liner
• Aircraft Delivery
Before the aircraft arrives for the modification, a deposit has been paid and a complete modification parts kit is fabricated for the aircraft. When the aircraft arrives at the Avtec facility, it is put through a comprehensive incoming inspection. Particular attention is given to the correct function and operation of the avionics and pressurization systems. All interior items, equipment, and loose items are inventoried, documented, and removed from the aircraft. All of the original plastic cabin windows are removed. Usually, the auxiliary power unit (APU) and the thrust reversers (T/R) are removed at this stage of the process, to gain a weight saving.
Most cargo operators who use the Falcon 20 dispense with these units as they consider them as deadweight. The main entrance door is removed and then the outline of the cargo door opening and the escape hatch opening is measured and drawn on the outside skin of the aircraft.
After the relevant fuselage structure has been removed from the inside of the airframe, a metal saw is used to cut the door and hatch openings. New intercostals, stringers (beams), frames, and several external skin doublers are installed.
The existing cabin floor is modified into a flat cargo floor surface, i.e., new floor beams are placed over the original center floor aisle and the forward and aft sections of the floor are extended. The location of the main oxygen system bottle and the rate gyros are changed to accommodate the new cargo structure that is added. The main landing gear emergency extension rings, the main wing fuel tank emergency shut off valve mechanism, the emergency temperature and emergency pressurization controls are all modified to work with a new control extension mechanism. The control mechanisms for these extended controls are routed through the space that is left where the original passenger aisle was located.
Solid metal window plugs are installed in place of the plastic cabin windows. This eliminates the need to inspect and maintain the laminated plastic units, and it also eliminates any possibility of the cargo puncturing or damaging the windows during the loading process. A self-contained door hydraulic system is mounted and installed in a fluid containment box, which is itself, mounted under the door entryway floor, ahead of the cargo net. The 9g cargo net attachment structure and hardware is installed in the forward door-opening zone. The net attaches to various points located on the compartment floor, ceiling, wall, and cargo door.
A forward crew smoke barrier door and bulkhead is installed at the most forward end of the cargo compartment, while a solid structural bulkhead is installed at the most aft end of the cargo compartment. A smoke detection and cargo lighting system is installed, while the entire cargo compartment is covered with a liner material. This material is extremely fire resistant and is finished in a light-reflecting white finish. The liner is also used for cargo load zone identification as the zones are painted directly onto the liner.
Now let's look at certification. Obviously, a cargo conversion of an aircraft will be considered a major alteration; therefore, it will have to be certified and signed off accordingly with an FAA form 337.
After expending so much time, effort, and money, it will be natural for you to want to obtain a supplemental type certificate for your modification to enable you to sell the modification to other people. With this in mind, let's revisit the STC application process.
First, you have to complete FAA form 8110-12 Application for Type Certificate, Production Certificate, or Supplemental Type Certificate (STC). Along with this completed application form, you should include a letter that explains the nature of your proposed STC, how it should be certified and tested (your opinion), including references to all of the relevant FARs, and a proposed certification/test schedule.
After you have sent this letter and application to the manager of the Aircraft Certification Office (ACO) for your region, it will be reviewed and a project number and FAA representative/project engineer will be assigned to you for this project. After many discussions with the ACO and with several changes made to your plan (if necessary), your certification/test plan should be approved.
Next, you will have to get your drawings conformed by a representative of the FAA. This can either be the assigned FAA project engineer or a DAR. When your drawings have been conformed against your modification, the inspecting official will issue a form 8110-3 and send that to the ACO. After careful review, the ACO will issue a type inspection authorization (TIA) to you. Now you are ready to conduct your tests.
It will be necessary for a representative from the FAA to be present to observe these tests; however, a DER can do this on behalf of the FAA. Generally, in the case of a cargo modification, you will be required to conduct a pressurization test (this test verifies that your modification structure will withstand two times the maximum pressure differential of the aircraft); a smoke test (this test will verify that while in flight with a fire emergency, no smoke will enter the crew compartment thus endangering the operation and control of the aircraft); and an emergency control operations test (this test will verify that all required emergency controls fully operate unhindered as normal).
With these tests performed and the results recorded and sent to the ACO, if accepted, the ACO should now issue an STC to you. Once you have the STC issued to you, you can go to the next step and ask for a PMA letter to be issued from the ACO to allow you to approach your MIDO for your region to obtain a PMA. Normally, you would do this if you intended to sell the rights to the use of your STC to other entities, while you produce and supply kit parts for the modification. FAR 21 and its subparts cover the PMA process.
Now that you have an STC, your final signoff for return to service after incorporation of your cargo conversion will require a completed FAA form 337 that references your STC. As you can see, this article provides a very brief overview of a highly complex subject. To successfully design and certify a full cargo conversion, you will probably end up spending several million dollars on research, development, testing, certification, and tooling. For more information on receiving STC approval from the FAA, I suggest that you read all of the following reference material: FAR 21, 23, 25, 27, 29, 31, 33, 34, 35, and 36. Also FAA Orders 8110.4A and 8100.5.