.
Vibrating surface (through piezoelectric transducers) fitted on a finger end, with measurement of the stiffness variation of a membrane. Such a system developed by Vibrometer is being certified on Dash 8-400.

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It should be noted that the above technologies do not identify icing conditions in the meteorological sense, they only detect icing conditions indirectly. Whereas the detectors can provide a signal homogenous with an accretion rate, based upon their sensitivity, they do not reflect how much ice is collecting on the aircraft’s critical surfaces. The detectors are mounted on the airframe, at a location, which is the most sensitive to ice accretion,
i.e. the location where ice is first encountered. The correlation between ice accumulation at this sensitive location and ice accretion (and accretion rate) on critical aircraft surfaces
(e.g. the engines inlets and the slats), are tested and validated in extensive flight and wind tunnel testing.
(c) Detection of ice accretion:
The detectors are intended to detect any type of ice which forms on a specific surface,
such as leading edge of wings or the air intake and the wing upper surfaces. They are
operative for in-flight or on ground icing and provide crew indication or automatically
actuate the system whenever the aircraft is within icing conditions that accrete more than
a specified thickness. These detectors are generally non-intrusive (flush-mounted). They
are integrated and detect ice formation over their sensing surfaces. Some of them are
able to measure the ice layer thickness, or to distinguish ice from other contaminants
(water, slush, de/anti icing fluids, etc). Most of these detectors provide a limited sensing
surface, which does not necessarily reflect the status of the whole surface to be
monitored.

There are several types of ice detection systems, depending on their technologies, their
uses and their level of integrity /reliability.
Usually, the following definitions are employed:

● Ice detector
An ice detector is generally designed to provide a signal when the aircraft is operating in
icing conditions either in flight or on the ground at static.
For in-flight application, the ice detector signal is used for ice protection system activation
and the respect of AFM icing procedure.

● Advisory system
The detector sends an informational advisory signal to the pilot. The pilot still has the responsibility to detect the icing conditions, or the presence of ice (or other contaminants such as snow, slush...) and to take the appropriate action, as required by the AFM/FCOM. There are no safety objectives linked to the detection system.
● Primary system
The detector sends a signal, reliable enough to be used as a primary information to warn the pilot. The consequences of an undetected system failure must be established in order to design a robust system architecture.
　
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