getting to grips with COLD WEATHER OPERATIONS 寒冷天气运行(22)

●  
Large aircraft are controlled by servo jacks, which remove all hinge moment problems, known in icing on aircraft with mechanical gearing. FBW (Fly-By-Wire) systems further enhance that effect.

●  
Large aircraft generally have thermal de-icing systems that are fully evaporative.

The execution of an icing certification test in natural icing is difficult. Finding 3 inches of ice, a number of times, in the real atmosphere, regardless of the season, is an impossible challenge. Furthermore, after accreting such an ice, by the time the pilot exits the cloud and executes the certification maneuvers, most of the ice will be gone by combined effects of buffet during stalls, erosion, evaporation and sublimation. This is why natural ice shapes, once identified, are reproduced in shapes of plastic foam and glued to aircraft leading edges. Handling and performance tests are performed in that configuration.
Flight tests in natural icing are still done to demonstrate the efficiency of the de-icing systems, including failure cases. This is necessary because no plastic foam would ever reproduce the intense thermodynamic process which develops between the ice and the airframe.
As the certification method is overprotective for large jets, some reasonable assumptions have been made. One is very important and often unknown by pilots: Aircraft are not certified for sustained icing in configurations with slats and flaps out. Aircraft behavior was thoroughly checked with foam ice shapes, as picked up in the clean configuration. Then, as stated, slats and flaps were deflected and all reference speeds for take off and landing verified. It is assumed that an aircraft wouldn’t stay long enough in icing conditions with slats/flaps extended to pick up such an amount of ice that performance would be modified.
The certification rule has proved to be more than adequate for large jets, as no accident or significant incident has ever occurred in the category due to in-flight icing. However, it should not be interpreted as a total protection against unlimited icing. Unlimited icing is totally unlikely to occur, but pilots must remain conscious that there is a limit somewhere, which could be given in terms of exposure time, coupled with ice accretion rate. Again, that limit has never proved to be encountered in 30 years of large jet transport.

A3 IN-FLIGHT ICE PROTECTION

A3.1 Ice Protection Means
There are three principle methods of protecting the airframe from ice accretion. Namely, mechanical, electrical heating or hot bleed air are used to de-ice and/or anti-ice the critical surfaces of the aircraft.
A3.1.1 Hot bleed air
Hot air is usually used on aircraft with jet engines. These systems are referred to as anti-ice systems, as they run continuously and are usually switched on before ice accretes. The heated surfaces thus prevent icing. Bleed air ice protection systems can also be used to remove light accumulations of ice. However, the amount of energy to evaporate accreted ice being very high, bleed air ice protection systems cannot be considered as fully effective de-icing systems. All Airbus wing and nacelle (engine intake) ice protection systems use hot bleed air type anti-icing.
　
中国航空网 www.aero.cn
航空翻译 www.aviation.cn
本文链接地址：getting to grips with COLD WEATHER OPERATIONS 寒冷天气运行(22)