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A3.3 Ice Detection
A3.3.1 General
The definition of icing conditions, as .visible moisture and less than 10°C TAT. has proven to be rather conservative. As already stated, when icing conditions are present (as per the definition), it does not necessarily mean that ice accretes on the aircraft. On the other hand, there are situations where the icing conditions, as per the AFM, are difficult
for the flight crew to identify, e.g. during flight at night-time.
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Although at night it is possible to check visibility with the headlights on, or to estimate the visible length of the wing, visibility depends only on the size of the particles, not on the encountered liquid water content, which is important for ice accretion. For instance, big ice crystals do not harm the wing but they reduce visibility more than water droplets. Over the past years, a large number of ice detection technologies have been successfully developed to enable the identification of ice accretion on the airframe and/or the presence of icing conditions. Generally, these technologies enable:
.
 A decreased crew workload,

.
 Increased safety for ground or flight operations in icing conditions,

.
 Fuel saving.
The following paragraph provides a brief overview of the ice detection principles that are
most commonly used in service:

 

(a) Visual cue:
The pilot is provided with visual cues (specific or not specific) to decipher the icing
conditions that are encountered.
The following information can be extracted from these cues:

.
 Beginning of ice accretion;

.
Type of icing encountered (rime, glaze and mixed);

.
 Ice thickness;

.
 Accretion rate;

.
 End of ice accretion, if the cue is periodically de-iced.

Potential use:

.
To determine the icing conditions, in order to apply the AFM/FCOM procedures (for activating the various ice protection systems),

.
Last element to be free of ice, indicating the end of the specific icing procedures, if any,

.
To detect particular icing conditions (supercooled large droplets, or ground icing).

(b) Detection of icing conditions:
The detector is intended to detect icing conditions in flight and to provide crew indication or to automatically actuate the system whenever the aircraft is flying in icing conditions that accrete more than a specified thickness. These detectors are generally intrusive to the airflow.
The following are the most common types of intrusive detectors which have been or are currently being used:
.
Vibrating finger (piezoelectric, magnetostrictive or inductive transducers) with measurement of the resonant frequency variation. This technology, known as Rosemount (now BFGoodrich) detector, is today the most widely used for aircraft ice detection systems, including the Airbus Dual Advisory Ice Detection System.
　
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