Complex STC proposal might have wide effect

 - February 1, 2007, 9:00 AM

The FAA is proposing a “Policy for Complex Supplemental Type Certificates (STC) 1” that will change FAR 21.31 (type design) and 21.53 (statement of conformity) by introducing a new classification of STC known as “complex STCs.”

If it is implemented, the FAA will give greater scrutiny to applicants proposing complex STCs defined as “an extensive design change to a type-certificated product, or a simple design change that interfaces with an existing complex system and incorrect assumptions about the complex system could have safety implications for the airplane.”

In either case, the installer will be required to have access to both the STC substantiating and type design data because the FAA does not believe the installer can safely make a determination of the compatibility of the approved modification on subsequent aircraft without access to all STC data.

According to the FAA, an extensive design change is broad in scope, encompassing many individual changes and affecting a significant area of aircraft structure. Because of these conditions, the potential exists for critical adverse effects when deviating from the originally approved STC design data. An extensive STC type design data review and approval will be done by both FAA engineering and manufacturing inspection personnel.

Even with a simple design change, the FAA believes the effect of a complex STC might be difficult to determine because it interfaces with highly integrated systems on the aircraft, and all of the possible combinations of initial conditions of the integrated system would be difficult to specify.

Whether an extensive change or a change affecting a highly integrated system, a consequence of the complex STC is an FAA judgment. For further information contact the FAA’s Steve Flanagan at (202) 267-3549 or via e-mail at