From:rdd@netcom.com (Robert Dorsett)Organization:Netcom Online Communications Services (408-241-9760 login: guest)Date:14 Oct 94 02:23:34References:1

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In article <airliners.1994.1618@ohare.Chicago.COM> lmassaro@manta.nosc.mil (Larry Massaro) writes: >On a couple of transcon flights, I have noticed that that pilot has >sometimes spoken about leveling off at an intermediate FL to burn >off fuel. Is it really more efficient to burn fuel first at lower >cruise altitudes then climb to a final? Does anyone have any >data/equations etc. which indicate this is true. My first impression >would be that winds aloft would be the more probable reason >for flying at the lower "less eficient" altutude initially. > There are two considerations. The heavier you are, the higher the stall speed gets. Therefore, the heavier you get, the faster you have to go. The faster you go, the closer you'll get to the airspeed limitations of the airplane. At a certain point, the weight vs. speed curves define the maximum altitude of the airplane. The second primary consideration is efficiency. Saturn V didn't haul all its stages to the Moon, and neither does a 747 haul all its fuel to cruise altitude. The less fuel to haul, the more efficient the climb will be, and the more fuel you will save. All this translates into operating econ- omies. Most airliners use a climb to some "initial" altitude. Depending on ATC considerations and the length of the flight (which defines the optimal altitude), the airplane could have several more "steps," which should be pretty much imperceptible to the passengers. Crews attempt to fly to as optimal a flight altitude as possible. It is a common practice. What is bad is when ATC constraints keep an airplane to a low altitude for an unusual period of time. Jockeying for altitude has ramifications on range, ground speed, and overall operating economies and passenger satisfaction. To give an example, here are some values for the 737-200, with JT8D-9 engines. FL = flight level, = altitude / 1000. Schedule is the speed schedule, in Mach. This is a high-speed schedule for the 737, and less efficient. Optimal weight is the computed economical weight for a specific configuration. Limit weight is the cruise thrust limit weight at that al- titude, for ISA+0. FL Schedule Optimal weight Limit weight 370 0.78 75,000 lbs 90,600 lbs 350 0.78 82,000 lbs 99,700 lbs 330 0.78 90,000 lbs 108,500 lbs 310 0.78 99,000 lbs 117,700 lbs 290 0.78 108,000 lbs 120,000 lbs There's a pretty dramatic change there, amounting to two or three hours of flying time, which is probably why one rarely sees these airplanes at FL370. "Winds aloft" enters into the equation to some degree, but the above are the *limits*, regardless of how beneficial the winds turn out to be on flight planning (i.e., fuel load and initial selected altitude for the desired altitude). -- Robert Dorsett Moderator, sci.aeronautics.simulation rdd@netcom.com aero-simulation@rascal.ics.utexas.edu