that part of the pneumatic circuit not belonging to the probe 10 a difference between the observed signal and the reference signal indicates the presence of particles in the internal volume should particles be detected a maintenance operation has to be carried out on the probe 10 in order to remove these particles otherwise if the difference is not significant the probe 10 is considered to be operational such a check is very quick to carry out this check may be carried out directly on the aircraft without removing the probe 10 this check may also be carried out during a maintenance operation on the probe 10 so as to verify that the particles notably those able to obstruct the drain hole 23 have indeed been removed advantageously the checking device 25 comprises information means if a difference between the observed signal and the reference signal exceeds a predefined threshold the predefined threshold may be stored in a memory of the checking device 25 the threshold may be defined by means of trials in which different sizes of particles inserted into the internal volume at different locations therein are tested the information means may be formed by a warning lamp 29 that an operator can observe during a checking operation the information means may also be relayed by means of a connector 30 of the device 25 during a checking operation by means of the device 25 it is possible for example to look for a resonant frequency of the internal volume this frequency depends on any obstruction of the drain hole 23 or the presence of particles in the internal volume advantageously the acoustic signal transmitted by the transmitter 26 may be the acoustic signal obtained by scanning a.