pattern_is_full:
An A320 will ideally always sip fuel more efficiently at 36,000 feet than at 24,000 feet - yet BA flies Paris-London at ~24,000. Why? Because it is a short trip, and the fuel costs to fight gravity and climb the extra 10,000 feet (and immediately start down again) are higher than the additional fuel burn from staying low.
I came up against this 35 years ago when introducing a new a/c type to a short-route operator. The aircraft was big, but operated at light fuel loads for this mission.
Their engineering dept. had reviewed the OEM tables and it appeared the min. fuel burn SOP would be to climb toward the optimum altitude, but stop climbing only when they intersected the descent profile.
We agreed that on paper, this appeared to burn least fuel; but it introduced shock cooling into the engine-life equation. So we discouraged their proposal, encouraging them to plan a few (5-10) minutes cruise just for engine cooldown.
(A similar situation to a parachute jump ship; climb all-out, then chop throttle for the drop and descent)