What happens if the design deviates from the supplemental type certificate?

When an aircraft's design deviates from the specifications outlined in its Supplemental Type Certificate (STC), the aircraft is considered unairworthy. This is because the STC is a regulatory approval that provides a clear set of instructions and standards for modifications made to the aircraft. Adhering to these specifications is crucial to ensure the safety and performance of the aircraft. Any deviations from the STC may compromise the integrity, safety, or performance characteristics that were thoroughly evaluated and approved by the aviation authority during the STC issuance process.

If an aircraft does not conform to these approved modifications, it may not meet the necessary safety standards that have been established, leading to an assessment of unairworthiness. In such a case, the aircraft cannot be legally operated until it is returned to compliance with the STC or undergoes the necessary evaluations to establish that it remains safe for flight.