Вот что было недавно на форуме simhq:
TAS isn't available from FB, but we figured out how to calculate it earlier on in this long thread:

TAS(kmph)=((IAS/3.6)*(1+ALT/15000))*3.6

IAS is parameter #30 from FB and comes out as kmph, thus the 3.6 conversion factor for pure SI (Standard International) units in the calculation.

ALT is parameter #40 and it's in meters.