If you look at where your baber pole is above 13,200, you'll find that is is always at exactly Mach 0.48 [MN=TAS/LSS, where LSS= local speed of sound, get the TAS off your GPS, and LSS=39*sqrt(OAT+273)].
Mach 0.48 is listed in the 1900D AFM as the Mmo.
Many textbooks (like Handling the Big Jets, Davies) state that above Mach .5, the prop is subject to huge innefficiencies due to Mach effects at the tip, hence the .48 restriction.
Besides, structural limitations are due to CAS (or EAS if you consider compressiblity effects, which would only give you about 5% error at the top of the 1900D envelope), not TAS.
Last edited by betaboy; 14th Mar 2005 at 02:45.