Because it can fly at high altitudes, and therfor can easily eceed the maximum IAS in a descend.I only fly the B200 and its certified ceiling is 35000feet, ido not know the ceiling in a B1900 but if this is much lower than the B200 its probably only for practical purposes in the logistics dep.
Forget about mach as I told U its just another way to express speed . The key thing is not to overspeed the A/C thus the barberpole. the higer altitude the lower IAS, even thoug the speed is Higher (TAS/ Mach)
On smaller A/C like a Cessna 172 its no barberpole installed because it will not fly at those great altitudes that will make this a problem. But there is no problem fitting any A/C with this system.
Why we actually use Mach instead of TAS I do not know.
