You are an auto mechanic by profession and own an airplane that is certified under a standard category airworthiness certificate.
A standard airworthiness certificate (FAA form 8100-2 displayed in the aircraft) is the FAA's official authorization allowing for the operation of type certificated aircraft in the following categories:
- Normal
- Utility
- Acrobatic
- Commuter
- Transport
- Manned free balloons
- Special classes
A standard airworthiness certificate remains valid as long as the aircraft meets its approved type design, is in a condition for safe operation and maintenance, preventative maintenance, and alterations are performed in accordance with 14 CFR parts 21, 43, and 91.
You desire to install an auxiliary fuel tank in the aircraft to extend the aircraft's range but find your FBO's quoted price to install it under an approved FAA 337 form too expensive. You have a non-approved fuel tank that will fit your aircraft and you know enough about fuel systems to install it and make it work in your aircraft.
Is this legal under the existing FAA regulations? Is it a good idea to perform these kind of modifications even if your have the technical competence?