Flutter responds to TAS.
Interesting point. I've looked up the design requirements and the FAA flight test guide, and they do not specify which speed (IAS/TAS) to use for these tests. The flight test guide does mention "consideration of calibrated airspeed...", but no mention of TAS.
I'll be test flying with a Transport Canada Test Pilot, and Flight Test Engineer in the next month, including dive tests, so I'm going to discuss this with them. Thanks for introducing the thought Megan!