up and down

[JanniCash]

Back to the topic,

also the optimal, most economic altitude for jets depends on the gross weight. Fully loaded, with all tanks full this is much lower than when the
tanks are down to 20%. Therefore the most economic way to fly a jet over a long distance is to perform a technique that is called "step climb". You
start out at the Flight Level (FL) that is above the optimum for your takeoff weight minus estimated climb burn. When your weight has dropped enough
so that your current optimum is half between your current FL and the next available for your direction, you climb to that level. Assuming the
remaining flight distance is long enough to gain back what you spend for the climb. That way your aircraft stays pretty close to the optimum altitude.

The documentation I have for the 737-200 ADV with JT8D-15 power plants for example shows an optimum altitude of FL330 around 110000 lbs, FL350 around
100000 lbs and FL370 around 90000 lbs gross weight. The fuel penalty for flying below your optimum can be quite significant. Again, for a 737-200
flying at .74 Mach 8000 ft below optimum, the fuel penalty is as high as 15% and still an impressive 6% for 4000 ft below, while it is only 1% for
flying 2000 ft above optimum.

If there would be no ATC forcing you to specific flight levels, one could stay on a constant, very slow climb and maintain optimum. If there would be
no ATC.


Jan