The 3 Types of Modification Installation Approvals
1. Aircraft Type Certificate
In many cases, a modification installation, such as floats or skis, will be made in cooperation with the manufacturer of the airplane. The manufacturer does the engineering, conducts flight tests and sees to everything necessary to insure that the installation is airworthy. Once deemed airworthy, the airplane manufacturer may add the installation to the Aircraft Approved Equipment List. The modification is then like any other optional equipment and is approved as part of the Aircraft Type Certificate.
2. Supplemental Type Certificate
A manufacturer of an approved modification can obtain approval for a particular airplane through a Supplemental Type Certificate. The modification manufacturer completes a prototype installation and demonstrates the plane is airworthy and conforming to regulations through engineering data and flight tests. The “STC” installation may then be used on all airplanes of that specific model, as long as there is no conflict with previously installed “STC” equipment. A qualified mechanic can install the modification guided by approved instructions and drawings.
3."One Time" or "Field" Approval
Sometimes a modification manufacturer does not, or cannot obtain approval for his modification on a given airplane, perhaps because of low market potential or because the modified plane would not comply with some part of Federal Aviation regulations. The owner of such a plane may apply for a “One Time” approval. They must demonstrate that the modified plane will be safe for the intended operations and the FAA may grant approval to install and use the modification on that one plane, within special limitations. Before undertaking such a project, an owner should consult with the nearest FAA representative.