Altitude Climb Performance

The altitude climb performance required of an airframe boost pump is specified in terms of altitude achieved versus time in minutes, flow required with respect to altitude, the fuel tank temperature with respect to altitude, and the minimum pump pressure rise required versus altitude. The temperature versus altitude represents the cooling of the fuel through heat transfer to the cold ambient atmosphere and boiling of the fuel that is experienced in a climb event.

Immediately upon take-off, as the aircraft gains altitude, the dissolved air in the fuel will begin to evolve. The rate of this air evolution will depend upon the climb rate of the aircraft. Presently, there is no accurate way to determine this rate of air evolution. This volume of evolved air is handled in the design of the pump by judicious oversizing of the inlet based upon experience and empirical design parameters. Further assistance in handling the volume of vapor is provided in tank bottom mount pumps through pump element inlet tip vapor vents and discharge hub vapor vents. Snorkel inlet pumps can receive vapor handling assistance through the liquid ring reprime element. Any remaining air and fuel vapor mixture is compressed as it passes through the increasing pressure within the inducer and is redissolved into the fuel. Therefore, the pressurized fuel delivered to the engine is free of vapor.

As the altitude increases, a tank pressure will be reached that will equal the vapor pressure of the fuel. At this point, especially with wide cut fuels, the vapor release will become quite violent (boiling). Here, the rate of vapor evolution will be determined by the capacity of the tank vent system. As the light ends leave the fuel, the vapor pressure of the fuel will decrease. This is referred to as "weathering" of the fuel. After the maximum cruise altitude is reached, the vapor pressure of the fuel will reach equilibrium with the altitude pressure.

The altitude climb test is the most difficult test to perform on airframe boost pumps because it must be a single fuel pass test without recirculation to accurately reproduce an actual climb condition. This requires approximately 1500 gallons (5,680 liters) of fuel for one test on an average commercial airliner fuel boost pump element. Figure 5 presents the results of an altitude climb test on the fuel boost pump depicted in Figures 3 and 4.

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