Signal Processing
A processor located in each sensor unit evaluates the physical signals recorded by the
probes. Thus, the detectors are independent and provide redundancy in case of a failure.
Each unit is equipped with a .Power On Self Test., an .Initiated Test. and a .Built In
Test Equipment. (BITE) which continuously monitor all components. The .Initiated Test.
is triggered by a test input for at least 500 ms.
The detectors are directly connected to the redundant Flight Warning Computers (FWC),
which also receive the information, like the Total Air Temperature (TAT), the Weight On
Wheel signal, the altitude and the positions of the Nacelle/Wing Anti-Ice pushbuttons. Any
fault indication is directly recorded on the Centralized Maintenance Computers (CMC) for
A330/A340 (Centralized Fault Data System for A320).

Procedure
If 0,5 +-0,13 mm of ice is attached to the probe of the detector, which equals a drop in frequency of 133 Hz, the amber ICE DETECTED warning appears on the Engine/Warning Display (E/WD) associated with a MASTER CAUTION and a single chime. The caution is active for 60 seconds. At the same time, the probe is heated until it is free of ice. This usually takes 1 second. The heater continues heating for an additional 6 seconds. In severe icing conditions, this de-icing cycle may be longer. If the heating time reaches 25 seconds, a fault warning is displayed. Both cases inhibit any other indication and cause a Class 2 failure message in the CMC or CFDS, associated to the failed ice detector. Further ice detection sets a new ice signal flag for 60 seconds. If there are 7 detections accumulated, a .SEVERE ICE DETECTED. warning (MASTER CAUTION, single chime, amber) is given. This corresponds to an ice accretion of approximately 5 mm on the protected wing surface.
This correlation has been validated in extensive flight tests. The .severity. counter is set to zero when the WAI is selected. The ice detection sequence is schematically shown in the figure below.

The above principle of icing condition detection depends on the ice accretion on the probe. This accretion is only achieved with a certain airspeed. During taxiing, the aircraft is too slow to accrete ice on the probes. An ice accretion due to frost, for example, would lead to a wrong indication. Thus, the system is inhibited by the FWC, if the aircraft is on the ground and below 1500 ft of altitude. All mentioned indications are not announced if the TAT is above +8 °C.
A3.3.3 Primary Ice Detection System
The major difference between a Primary Ice Detection System and the above Dual Advisory Ice Detection System is that the Primary Ice Detection System replaces the AFM/FCOM procedure either by an ice detector indication (manual system) or by an automatic system activation (automatic system). This can be achieved through an increased system redundancy and higher equipment integrity. A possible procedure for a primary automatic ice detection system is shown in the table below. Compared to the
　
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