The problem with modern aircraft is that they have largely been built to fly efficiently in a 1930's procedural concept. Indeed if you ask Airbus or Boeing flight engineers the most efficient low fuel/emissions way to fly their aircraft - including taking advantage of wind and temperature and assuming no air traffic control airspace structures or other aircraft - they will have stand up arguments. The aircraft systems will not 'cruise climb', they will not follow an altitude profile once in cruise climbing when planned, without an input from the crew whereas the FMS will follow a track. Wind fields are simplified to winds at a few waypoints on the track, so the aircraft will not optimize by changing track only by changing level (as in FLT LVL Recommend) . Many of these limitations are to make the aircraft fly efficiently in a fixed route ATC system. In 2025 the ocean track structures in the Atlantic will be withdrawn, upper level route structures are already being simplified in Europe, and many ocean areas are getting more 'thin routes'. So what aircraft systems need to be changed to fly with fuel efficiency and schedule efficiency in the future airspace systems where there will be no restrictions on the '4 dimensional trajectory' of the aircraft? It will not need to fly 'in trail' at 0.84M but will be able to fly with almost complete freedom following a speed and altitude profile. But it will need to be able to fly it's 4D trajectory accurately within a lateral, vertical and longitudinal containment - and current FMC's cannot do that.