Megan: Agreed. My point is that the average turbo-prop isn't going to get high enough or fast enough for Mach to be a factor. We took the C-130 up to the low thirty thousands, and used airspeed limits.There was no way that aircraft would approach a critical Mach No. I've been up higher in jets, which of course are generally faster machines, and on all three types of jet I've operated, the transition from IAS to Mach was around 28,000'.