Low Fuel Temperature
Fuel temperature changes relative to total air temperature. For example, extended operation at high cruise altitudes tends to reduce fuel temperature. In some cases the fuel temperature may approach the minimum fuel temperature limit.
Fuel freezing point should not be confused with fuel ice formation caused by frozen water particles. The fuel freezing point is the temperature at which the formation of wax crystals appears in the fuel. The Jet A fuel specification limits the freezing point to -40°C maximum, while the Jet A-1 limit is -47°C maximum. In the Former Soviet Union, the fuel is TS-1 or RT, which has a maximum freezing point of -50°C, which can be lower in some geographical regions. The actual uplifted freezing point for jet fuels varies by the geographical region in which the fuel is refined.
Unless the operator measures the actual freezing point of the loaded fuel at the dispatch station, the maximum specification freezing point must be used. At most airports, the measured fuel freezing point can yield a lower freezing point than the specification maximum freezing point. The actual delivered freezing temperature can be used if it is known. Pilots should keep in mind that some airports store fuel above ground and, in extremely low temperature conditions, the fuel may already be close to the minimum allowable temperature before being loaded.
For blends of fuels, use the most conservative freezing point of the fuel on board as the freezing point of the fuel mixture. This procedure should be used until 3 consecutive refuelings with a lower freezing point fuel have been completed. Then the lower freezing point may be used. If fuel freezing point is projected to be critical for the next flight segment, wing tank fuel should be transferred to the center wing tank before refueling. The freezing point of the fuel being loaded can then be used for that flight segment.
Fuel temperature should be maintained within AFM limitations as specified in the Limitations chapter of the FCOM.
Maintaining a minimum fuel temperature should not be a concern unless the fuel temperature approaches the minimum temperature limit. The rate of cooling of the fuel is approximately 3° C per hour, with a maximum of 12° C per hour possible under the most extreme conditions.
Copyright . The Boeing Company. See title page for details.
4.10 FCT 737 (TM) October 31, 2006
737 Flight Crew Training Manual
Total air temperature can be raised in the following three ways, used individually or in combination:
.
climb or descend to a warmer air mass
.
deviate to a warmer air mass
.
increase Mach number.
Note: In most situations, warmer air can be reached by descending but there have
been reports of warmer air at higher flight levels. Air temperature forecasts
should be carefully evaluated when colder than normal temperatures are
anticipated.
It takes from 15 minutes to one hour to stabilize the fuel temperature. In most cases, the required descent would be 3,000 to 5,000 feet below optimum altitude. In more severe cases, descent to altitudes of 25,000 feet to 30,000 feet might be required. An increase of 0.01 Mach results in an increase of 0.5° to 0.7° C total air temperature.
Cruise Performance Economy
The flight plan fuel burn from departure to destination is based on certain assumed conditions. These include takeoff gross weight, cruise altitude, route of flight, temperature, enroute winds, and cruise speed.
Actual fuel burn should be compared to the flight plan fuel burn throughout the flight.
The planned fuel burn can increase due to:
.
temperature above planned
.
a lower cruise altitude than planned
.
cruise altitude more than 2,000 feet above optimum altitude
.
speed faster than planned or appreciably slower than long range cruise
speed when long range cruise was planned
.
stronger headwind component
.
fuel imbalance
.
improperly trimmed airplane
.
excessive thrust lever adjustments.
Cruise fuel penalties include:
.
ISA + 10° C: 1% increase in trip fuel
.
2,000 feet above/below optimum altitude: 1% to 2% increase in trip fuel
.
4,000 feet below optimum altitude: 3% to 5% increase in trip fuel
.
8,000 feet below optimum altitude: 8% to 14% increase in trip fuel
.
cruise speed 0.01M above LRC: 1% to 2% increase in trip fuel.
Copyright . The Boeing Company. See title page for details.
October 31, 2006 FCT 737 (TM) 4.11
737-300 - 737-500 For cruise within 2,000 feet of optimum, long range cruise speed can be approximated by using 0.74M. Long range cruise also provides the best buffet margin at all cruise altitudes. 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：737机组训练手册 Flight Crew Training Manual FCTM(49)
