Originally Posted by
pattern_is_full
Trip distance is also important. 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. BA's dispatchers and bean-counters have studied the curves and determined that 24,000 or thereabouts (depending on the day's weather) is the most cost-effective altitude, all factors considered.
Great explanation PIF, found it quite helpful myself.
That last bit was what I was also thinking. In Colombia we get a lot of short trips, several of our routes are sectors of less than 35 minutes. We don't go higher than 22 or 24 thousand ft on those, and that usually even involves either taking off or landing at Bogota (elevation is 8360 ft). We even have one particularly flight with cruise at 18000 ft, flight time is 23 min.
The FMGC can give an optimum altitude higher than that, particularly if the aircraft is light, but the trip is so short we usually don't change more than +/-2000 ft (for a light/heavy aircraft, respectively) of our usual cruise altitude for that particular route, because it would be going up and straight back down again.
So there you go, another scenario in which is preferable to fly lower than usual.