BEagle
The best a Hawk T1 can do in terms of IAS is 550 knots.
Sea level, Mach one is approximately 660 knots.
Could an extended air brake at high IAS really cause the airflow to exceed the Mach? Besides which the extent of the airbrake extension would be limited by the high IAS.
I’m not saying you’re wrong it’s just something I’ve never heard before. Surely, if true, it would have been part of the infernal A2 groundschool I was forced to endure?!
BV