mcdhu
Well my mate is up and running and his view is that the speeds chosen by the system were driven by the Cost Index (CI) input.
If you want more detail I quote from his to me:
You asked about the CI on an Airbus.
It is a complex subject and I would not claim to be an expert. Many pilots believe that they understand the process but then find that they cannot explain an observation, such as the one you asked about, because there is more to it than appears in the FCOM.
The primary and essential goal of the cost index is trip cost, or mission optimisation, and not speed control. It is a flexible tool to control fuel burn and trip time or a trade off between the two where the cost of time in the air against the fuel cost is a complex equation. The CI has aspects that assist airlines to optimise their operations to any number of factors including aircraft leasing and maintenance costs, or even flight crew costs. Therefore it is not just about speed or fuel flow during any phase of flight although short term changes to CI are often used when short term changes in the price of fuel are experienced.
For a given sector minimum trip cost is achieved by adopting operational speeds that properly proportion fuel and time related costs for a given CI. The IAS/IMN speed target will depend on a number of factors including the Take Off Mass, OAT and actual wind, and it will compensate for fluctuations in wind or differences between pre-flight inserted forecast winds and actual wind measured in flight. When the CI is low the initial flight profile will relate to best achievable rate of climb, and takes into account a calculated cross over altitude (IAS to IMN) usually between FL230 and FL270.
Cruise level optimisation is another CI factor but where a level is held down for operational reasons the lower the FL the lower the economic IMN, and the lower the aircraft gross mass the lower the economic IMN. These two factors probably come into play on a short sector where operationally it may not be best to plan to fly at a maximum achievable level from purely a performance point of view. ATC, SIDs, airways and STARSs (plus some crafty planning to avoid traffic) might all dictate a lower best altitude to fly. Such lower cruise levels may appear to confuse the CI process, but it still works effectively, even when the selected level is right from a performance point of view as well.
On the other hand it is not perfect and occasionally anomalies are evident, but when a profile is observed regularly it is doing it right even though the reasons may not be obvious.
Hope this helps.
Regards
JF