Some observations ..
Vb is described as a factor on the relevant stall speed and considers a range of things - including
(a) wing loading, geometry and lift curve
(b) density (ie altitude)
(c) design cruise speed
(d) design gust velocities (which vary with height)
(e) weight (to which stall speed relates)
At higher cruise levels where Mach becomes limiting, the balance changes to consider high and low speed buffet.
As in all these certification matters, nothing is a guarantee, only a set of reference information. Caution is always an admirable attribute as the bumps get nastier.
Epsom Hold 2
Consider that extreme fluctuations in IAS generally indicate fluctuations in the static pressure sensed rather than actual speed variations. The ASI is only a pressure diff gauge, not a speedo, and is very sensitive to static errors. In general, if the IAS is varying at any significant rate, then the reading should be viewed with a grain of salt until quasi steady conditions return.
[ 03 November 2001: Message edited by: john_tullamarine ]