Interesting discussion, since I think about this from time to time. As I am by no means an expert flight planner, but rather a simple pilot who must plan his own flights, I really try to pay attention to this area. Below are a few observations I have made. Please feel free to comment on them or point out any glaring errors in my logic or understanding of the concepts involved.
I get the distinct impression that that not all airframers take the same approach to the problem of determining a maximum range flight profile for planning purposes, and may not define optimum cruise altitude according to the same parameters. This may present a problem when attempting to universally define these terms. Consequently, the advice to consult the AFM performance planning section will guarantee that one is using the performance terminology as defined for that particular airframers product.
On one of the two aircraft I fly, the maximum range cruise profile stipulates a reducing mach number increment for each reducing increment of flight weight as fuel is burned off. An optimum altitude is assumed as determined by flight weight and atmosperic SAT. The resultant percentage of decrease in TAS between hour 1 and hour 6 appears to be aproximately equal to half the percentage of flight weight decrease during the cruise. AOA appears to increase throughout the flight. Of course, fuel flow decreases with each step climb and thrust reduction until by hour 6, cruise fuel flow has reduced by 23%, TAS has reduced by 13% while flight weight has reduced by 27% from initial hour 1 cruise. Following this profile appears to add perhaps 5% to the still air range of this aircraft compared to a constant long range mach or TAS profile. It's a draggy airframe and using this profile makes the fight seem interminable when we must use it to achieve maximum range!
The other airplane I have flown maximum range profiles in calls for a constant TAS throughout the cruise phase of flight with the obligatory step climbs. Much simpler planning! In this integrated EFIS aircraft, FMS calculated optimum cruise altitude is predicated purely upon fuel burn per NM equivalent still air distance (ESAD) and as limited by weight, altitude and pre-programmed or assumed temperatures at each altitude. Interestingly, the fuel flow reduces by a similar percentage per reduction in flight weight as in the other aircraft while this profile maintains a constant TAS as opposed to decreasing with weight. AOA is ever decreasing at each altitude increment as fuel is burned off until the next climb as dictated by the FMS optimum altitude computation. Once MOA is reached, (about 1/2 way through a max range flight) thrust is continually reduced to maintain scheduled TAS. The aircraft will easily exceed it's optimum altitude at any time, but will require a greater fuel flow to do so. (sometimes done to top weather or winds.)
In both cases, the max range profiles for these aircraft are predicated upon still air distances. They are based (for these aircraft) only on aircraft performance and no consideration is given to the effect of wind on either optimum altitude or cruise speed schedule. Consequently, optimal altitude and speed schedules provided by planning providers like Universal or Jeppesen for the actual conditions of the day may vary somewhat from the "book" figures. It's no small wonder that so many sophisticated flight planning software packages and service providers abound industry wide. Complicate the matter further by adding company specific cost indexing and the matter now completely eludes the ability of the majority of ordinary line pilots I know to maintain a grasp on with so many factors to be considered. It has become a specialist area of expertise where computers spit out the end product.
I am simply interested in being capable of determining whether or not the numbers make sense, and if not, why not? It is merely my intention never to be led down a primrose path by numbers spit out of a machine which were based on spurious input or invalid assumptions.
The bottom line in all this will come down to mission profile. Actual optimum altitudes and speed schedules will vary with what cost and mission performance factors are to be optimized, and under the conditions of the day.
As the US auto manufacturers are fond of saying, "Your actual mileage may vary!"
Best regards,
Westhawk