A question-statement that airworthiness office hears sometimes from the sales persons: "We have sold the STC on platform A last year, I have now sold the STC on platform B for next month, isn't it great? After all reusing the STC package on a new platform B is just paperwork you can do in a couple of days".
Well, It isn't... and that is also why it does not take "just a couple of days". 🙂
The original STC was approved against a specific certification basis - the applicable requirements, amendment levels, maybe some special conditions tied to that host aircraft's Type Certificate. Move the same physical modification onto a different aircraft, or even the same aircraft, and that basis can shift underneath you - sometimes significantly. Systems interfaces change. Sometimes the applicable CS paragraph itself has been amended significantly between the 2 platforms. Maybe new tests are required.
Copy the compliance matrix across and you've inherited answers to yesterday's questions, not today's. The modification might genuinely still comply - but "might still comply" and "demonstrated compliance" are different things.
The organisations that handle this well don't treat STC reuse as a copy-paste exercise of an excel spreadsheet. They re-run the applicability check against the new host's certification basis as a discrete, documented step, every time - even when they're confident nothing has changed.
And if they have this not just documented properly, but as a properly managed process, they might be able to avoid the same question next time 🙃: because everybody in the organisation could see what the re-validation actually means in project management terms that use language everybody understands: from engineering, to sales.
How do you STC DOAs out there handle this?