Chesty Morgan, quotes from FMC Supplemantary Data Document explain this as well as the 240/FL100 in descent:
The FMC defaults to the economy speed modes for VNAV operation. In addition, by default, a transition climb speed limit is observed below the speed transition altitude during climb (250 knots below 10,000 feet in the USA), and a similar limit, but 10 knots less, during descent (240 knots below 10,000 feet in the USA). The 10 knot margin was added to reduce the probability of exceeding the transition speed limit in turbulence. The FMC increases the transition climb speed at heavier gross weights to equal the flaps-up full maneuvering speed of VREF40 + 70 knots (minimum transition climb speed).
FMC generates CAS/Mach speed schedule based on a fixed speed throughout the climb, as a function of cost index and initial climb weight. The speeds are calculated using a reference point 180nm downpath of origin, at which point the TOC wind component and predicted OAT are used to make corrections to the fixed speed. CAS is increased up to 40kts with 150kts headwind, decreased up to -20kts with 150kts tailwind, increased up to 4kts with ISA -40C and decreased up to -30kts with ISA +40C [no effect between ISA 0C to +10C]. Econ CLB Mach is set equal to ECON CRZ at the estimated initial cruise conditions. Thus to derive the Econ CLB Mach, estimate TOC Weight, Altitude, Wind and Cost Index are required. PERF INIT provides input area for all calculations.
Avenger, please provide references to your quotes regarding "VNAV makes 3 basic calculations based on the initial inputs to the FMC. up to 5000 ( terminal area) 5000-10000 (speed limited) above 15000", its areas of applicability, etc.
We all know a speed restriction in VNAV CLM is limiting acceleration, thus my post stating VNAV CLM acts as LVL CHG did not refer to this, merely to full climb scenario. In fact such speed limits are
not adhered to as minimum speeds based on Flap position are observed by VNAV, protecting you from flying too slow. On your next departure try entering 180kts at the third or so waypoint, engage VNAV and treract flaps, it will fly Flap Up speeds to your surprise.
Furthermore, I cannot find any references to VNAV modes or calcuations being different below 5000'.
The thrust reduction below 15,000' as well as 250/FL100 equally have nothing to do with VNAV as it is an autothrottle/FMC function, as such it is avaialble in ALL thrust modes with a closed IAS window, not just VNAV CLM.