CRUISE
When the flight segment is too short to
permit the optimum altitude, the most fuel-efficient profile is a climb
until intercept of the descent profile.
It is an unfortunate - although often
unavoidable - fact that the efforts made to maximize fuel efficiency in
cruise can very quickly be negated by the inability of ATC to approve a
request.
It is important that ATC maintain a
constant awareness of the impact of assigned altitudes on fuel efficiency.