FIRE DETECTION – PRINCIPLE OF OPERATION
The firewire element takes into account two types of high temperature
conditions:
• the first type is a general overheat condition. In this case if the
temperature around the firewire element increases over a preset limit.
As a consequence the sensor acts in accordance with the law of
gases: if the volume of the firewire element is held constant, its
pressure will increase as temperature increases. The helium gas in the
tube exerts a pressure which closes the high pressure switch that
generates the ENG FIRE message.
• the second type is a fire condition at a short section of the firewire
sensor element. In this case the core material releases a large volume
of hydrogen gas so that the internal pressure of the firewire element
increases very quickly and the gas in the tube exerts a pressure which
closes the high pressure switch that generates the ENG FIRE
message. After the situation is corrected, the material reabsorbs the
hydrogen and the system returns to a stand-by mode.
In case of leakage of the helium gas from the firewire element, the pressure
will decrease and operates the low pressure switch which generates the
FIRE DET message.
FIRE EXTINGUISHING – PRINCIPLE OF OPERATION
Refer to AW139–Rotor Flight Manual–4D for emergency procedures to be
applied in case of fire (on the ground or in flight).
NOTE
The command to extinguish fire is done via the FIRE switch on the
FIRE EXTG panel that, after been pressed, illuminates the amber
segment ARM.
The extinction of the fire does not implies the automatic re-opening of the
fuel shut-off valve automatically closed when the switch FIRE EXTING has
been moved from the central position.
In case the first shot to extinguish fire fails, a second shot is available
moving the switch to the second bottle indication.