TRANS-POLAR Flight - OPERATIONAL CONSIDERATIONS
|COLD FUEL MANAGEMENT
Because of the extended flight duration and the prevalence of very cold air masses on the polar routes, the potential exists for fuel temperatures to approach the freezing point. However, current airplane systems and operating procedures provide confidence that fuel will continue to flow unobstructed to the engines in all plausible cold-weather conditions likely to be experienced on polar routes.Properties of fuel at very low temperatures.
The fuel freezing point is the temperature at which wax crystals, which form in the fuel as it cools, completely disappear when the fuel is rewarmed. (This should not be confused with the fuel becoming cloudy upon cooling, which results when water dissolved in the fuel freezes, forming a suspension of very fine ice crystals. Airplane fuel and engine systems are designed to handle water ice crystals safely.)
The Jet A fuel specification limits the freezing point to a maximum of –40°C; the Jet A-1 limit is –47°C maximum. In Russia, the fuels are TS-1 and RT, which have a maximum freezing point of –50°C. (Note: Because specifications may vary by country, operators should ensure that they are using the appropriate fuel procurement specification for the fuel being dispensed.)
The maximum freezing point for some jet fuels can vary by the geographical region in which the fuel is refined or uplifted. Test methods for determining the fuel freezing point also introduce variability; reproducibility is approximately 2.5°C.
Some operators in the United States measure the actual freezing point of delivered Jet A fuel at the time of dispatch. Data show that the freezing point of delivered Jet A fuel is approximately 3°C lower than the specification maximum of –40°C. Table 1 shows the results of a study completed at several airports in the United States to verify the actual freezing point of Jet A fuel as delivered to the airplane. (An airline must verify the freezing point of the loaded fuel at dispatch if the airline uses a value other than the maximum specification.)However, the fuel freezing point is not what dictates fuel flow to the boost pumps. The critical condition of cold fuel in an airplane fuel tank, in terms of flight safety, is its propensity to flow toward and into the boost pump inlets. Pumpability, or flowability, depends on the pour point of the fuel, defined as the lowest temperature at which the fuel still flows before setting up into a semirigid state. Generally, the pour point is approximately 6°C lower than the fuel freezing point. However, the exact relationship between freezing point and pour point depends on the source of the crude oil and the refining processes.
Because jet fuel is a mixture of many different hydrocarbon molecules, each with its own freezing point, jet fuel does not become solid at one temperature as water does. As fuel is cooled, the hydrocarbon components with the highest freezing points solidify first, forming wax crystals. Further cooling causes hydrocarbons with lower freezing points to solidify. Thus, as the fuel cools, it changes from a homogenous liquid to a liquid containing a few hydrocarbon (wax) crystals, to a slush of fuel and hydrocarbon crystals, and finally to a near-solid block of hydrocarbon wax. Because the freezing point is defined as the temperature at which the last wax crystal melts, the freezing point of jet fuel is well above the temperature at which it completely solidifies (fig. 4).
Refueling airplanes at different stations creates a blend of fuels in the tanks, each with a unique freezing point. The resulting fuel freezing point in each tank can vary widely. The flight crew must operate with caution and not automatically assume that the freezing point of the uplifted fuel is the actual freezing point of the fuel on board. Boeing published a procedure for estimating the freezing points of blends of Jet A and Jet A-1 fuel in service letter 747-SL-28-68 (Nov. 4, 1991).
If the freezing point of the fuel on board cannot be determined using the published procedure, Boeing suggests using the highest freezing point of the fuel used in the last three fuel uplifts. For example, if Jet A-1 fuel was used for two uplifts and Jet A fuel was used for one uplift, then a –40°C freezing point would be used for the current refueling. If Jet A-1 fuel was used in three consecutive refuelings, then a –47°C freezing point may be used for the current refueling. In the 747- 400 and 777, if the fuel freezing point is projected to be critical for the next flight segment, Boeing advises the transfer of wing tank fuel to the center wing tank before refueling. This makes it possible to use the freezing point of the fuel being uplifted for that flight segment.
systems and temperature measurement.
When the fuel temperature on the 747-400 reaches –37°C, a FUEL TEMP LOW message is activated, and the fuel temperature displayed on the EICAS changes color from white to amber. The 747-400 system automatically defaults to the limit associated with the highest freezing point of fuel approved for use on the 747, which is –37°C for Jet A fuel. When the fuel-temperature-sensing system is inoperative, the FUEL TEMP SYS message is displayed. The flight crew then is instructed to use total air temperature (TAT) as an indication of fuel temperature. (Instructions for this procedure are contained in the master minimum equipment list.)
The 777 has a fuel temperature probe located between ribs 9 and 10 of the left main tank. The probe is approximately 12.6 in from the lower wing skin and is located one rib over, approximately 40 in outboard, from the aft boost pump inlet. Because the left wing tank contains a single heat exchanger, its fuel can be slightly colder than that in the right wing tank, which contains two hydraulic heat exchangers.
Fuel temperature on the 777 is displayed in white on the primary EICAS in the lower right corner. If the fuel temperature reaches the established minimum, the indication turns amber in color and the FUEL TEMP LOW advisory message is displayed (fig. 5). The 777 system automatically defaults to the limit associated with the highest freezing point of fuel approved for use on the 777, which is –37°C for Jet A fuel. However, the EICAS message can be set to other values. For example, if Jet A-1 fuel is used, the message can be set to –44°C (fig. 6).
On the 777, the fuel temperature can be entered in two ways: as the minimum fuel
temperature or fuel freezing point. Both options provide an indication at 3°C above the fuel freezing point. Fuel temperature is not displayed during fuel jettison.
On the MD-11, a fuel temperature probe is located in the outboard compartment of tank no. 3 and another is in the horizontal stabilizer tank. At 3°C above the fuel freezing point, the probe in the no. 3 tank signals a FUEL TEMP LO message display in the flight deck. To establish when the message should be displayed, the flight crew can enter the freezing point of the fuel being carried or select the type of fuel being carried. When the crew does not enter a value or specify the type of fuel, the system defaults to Jet A fuel, which has a freezing point of –40°C; a message displays at –37°C.
The temperature probes in the 747-400, 777, and MD-11 are located where the bulk of the fuel is coldest. However, some fuel may be colder than the fuel measured by the probes, such as the fuel that is in contact with the lower wing skin. This creates a temperature gradient in the fuel tank from the wing skin to the location of the probe.
As fuel travels to the boost pump inlets, the bottom, cold layer flows through small flapper valves located on solid tank ribs next to the bottom wing skin. These valves are used to control fuel slosh. Thus, the cold fuel tends to flow toward the boost pump inlets. Because the probes are located near the bottom of the tank, the temperature reading is representative of the critical fuel temperature in the tank.
affecting fuel temperature.
The size and shape of the tanks significantly affect how quickly the fuel temperature is affected by wing skin temperatures. A tank with a high surface-to-volume ratio transfers heat through the wing surfaces at a higher rate than a tank with a low surface-to-volume ratio. Thus, fuel temperature is affected at different rates depending on the airplane model and tank design. For example, because the 747-400 outboard main tanks are long and narrow and have about half the total fuel volume of the 777 main tanks, the surface-to-volume ratio on the 747-400 main tanks is much higher. This means that heat transfer through the wing surfaces is greater on the 747-400, and the fuel temperature changes faster than it does on the 777. On the MD-11, the outboard compartments of tank nos. 1 and 3 have the highest surface-to-volume ratio. The next highest ratio is that of the horizontal stabilizer tank. These tanks are the most critical for fuel flowability at low temperatures on the MD-11.
Fuel is managed differently on the 747-400, 777, and MD-11, but in all cases, the wing main fuel tanks are the last to deplete. On some models, fuel in tanks with high surface-to-volume ratios is held until near the end of a flight. Whether a tank is full or partially depleted of fuel alters the rate at which the fuel temperature changes.
During long-range operations at high altitudes, fuel tank temperatures can approach the freezing point of fuel. On long flights, the fuel temperature tends to adjust to the temperature of the aerodynamic boundary layer over the wing skin. This boundary layer temperature is slightly lower than the TAT because theoretical TAT is not achieved. Initially, TAT is much lower than the fuel probe temperature because of the thermal lag of the fuel. Thermal analysis of the 747-400, 777, and MD-11 airplanes shows that the fuel tank temperature is driven more by TAT than airplane configuration.
and procedures with low fuel temperatures.
When fuel temperature decreases to 3°C above the freezing point, a message of FUEL TEMP LOW displays in the 747-400 and 777 flight decks; the message FUEL TEMP LO is displayed in the MD-11 flight deck. If this condition is reached, the flight crew must take action, as described below, to increase the TAT to avoid further fuel cooling.
In consultation with airline dispatch and air traffic control, the flight crew decides on a plan of action. If possible, the action should include changing the flight plan to where warmer air can be expected. Another action is to descend to a lower altitude. The required descent would be within 3,000 to 5,000 ft of optimum altitude. In more severe cases, a descent to 25,000 ft might be required. Recent experience on polar routes has shown that the temperature may be higher at higher altitudes, in which case a climb may be warranted. The flight crew also may increase airplane speed; an increase of 0.01 Mach results in a TAT increase of 0.5° to 0.7°C. (It should be noted that any of these techniques increases fuel consumption, possibly to the point at which refueling becomes necessary.)
It takes approximately 15 min to 1 hr for a change in TAT to affect the fuel temperature. The rate of cooling of the fuel is approximately 3°C/h. A maximum of 12°C/h is possible under the most extreme cold conditions.
A minimum in-flight fuel temperature advisory message provides a margin of safety under all atmospheric and operational conditions to ensure that the fuel will continue to flow to the boost pump inlets. Besides the 3°C margin between the advisory message temperature and fuel freezing point, there typically is a 6°C difference between the freezing point and pour point of fuels, which provides an additional margin. A review of the service history of transport airplane operations worldwide for the past 40 years does not show a single reported incident of restricted fuel flow because of low fuel tank temperatures. This service history affirms that the criteria used to establish the advisory message are adequate and conservative.
However, flight crews on polar routes must be knowledgeable about fuel freezing points. Flight crews also must be cognizant of the en route fuel temperature and the possible need for corrective action to ensure continued safe, routine polar operations.
aids for flight planning.
Measuring the actual freezing point of the fuel being uplifted can be a valuable step in the flight planning process for flights during which fuel freezing point is a concern. In general, actual fuel freezing points tend to be about 3°C below the specification maximum requirement. Details on measuring the freezing point when fuel is being uplifted are available to airlines through Boeing Field Service representatives.
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