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).
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RVSM installation requirements.