getting to grips with COLD WEATHER OPERATIONS 寒冷天气运行(6)

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called “speed optimization”. The process will always provide the optimum speeds. In a situation where the runway is contaminated, that means as low as possible.

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The FLEXIBLE THRUST principle, used to save engine life by reducing the thrust to the necessary amount, is not allowed when the runway is contaminated. Operators can take advantage of the DERATED THRUST. The main difference between Flex thrust and Derated thrust is that, in the case of flexible thrust, it is allowed to recover maximum thrust (TOGA), whereas it is not allowed to recover maximum thrust at low speeds in the case of derated thrust.

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Moreover, the reduction of thrust makes it easier to control the aircraft should an engine fail (lesser torque). In other words, any time an engine is derated, the associated VMC (Minimum Control Speed) is reduced. This VMC reduction allows even lower operating speeds (V1, VR and V2) and, consequently, shorter takeoff distances. In a situation where the performance is VMC limited, derating the engines can lead to a higher takeoff weight.

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Different methods are proposed by Airbus Industrie to determine the performance on a contaminated runway. The methods differ by their medium (paper or electronic) and the level of conservatism and details they provide.

D.
 FUEL FREEZING LIMITATIONS

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The minimum allowed fuel temperature may either be limited by:

.
 The fuel freezing point to prevent fuel lines and filters from becoming blocked
by waxy fuel (variable with the fuel being used) or

.
 The engine fuel heat management system to prevent ice crystals, contained
in the fuel, from blocking the fuel filter (fixed temperature). The latter is often
outside the flight envelope and, thus, transparent to the pilot.

●  Different fuel types having variable freezing points may be used as mentioned in the FCOM. When the actual freezing point of the fuel being used is unknown, the limitation is given by the minimum fuel specification values. In addition, a margin for the engine is sometimes required. The resulting limitation may be penalizing under certain temperature conditions especially when JET A is used (maximum freezing point -40°C). In such cases, knowledge of the actual freezing point of the fuel being used generally provides a large operational benefit as surveys have shown a significant give-away.
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Although the fuel freezing limitation should not be deliberately exceeded, it should be known that it ensures a significant safety margin.

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When mixing fuel types, operators should set their own rules with regard to the resulting freezing point, as it is not really possible to predict it. When a mixture of JET A/JETA1 contains less than 10% of JETA, considering the whole fuel as JETA1, with respect to the freezing point, is considered to be a
　
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