Page 166 - The Jet Engine

Fire protection

the current that operates the warning indicators. A 17. At high Mach numbers, the considerably higher
thermistor may be used as a single point detector or temperature levels may be such as to render the
as a continuous element sensor. thermistor or thermocouple fire detection system
unsatisfactory. Thermal detectors that sense either a
15. Another form of continuous element sensor temperature rise, or a rate of temperature rise, may
takes the form of a capacitor consisting of a tube therefore prove most suitable.
containing a dielectric material with a conductor
running through the centre. A voltage difference is 18. Alternatives to the above types are surveillance
applied between the tube and the centre conductor. detectors that respond to light radiation from a fire.
As the temperature increases then the properties of These may be made so sensitive that they respond
the dielectric change with a corresponding change in only to the ultra-violet and infra-red rays emitted from
the value of capacitance. This change of capacitance a kerosine fire.
is displayed as a fire warning.
FIRE CONTAINMENT
16. The gas filled detector consists of stainless 19. An engine fire must be contained within the
steel tubing filled with gas absorbent material and in power plant and not be allowed to spread to other
the event of a fire or overheat condition the parts of the aircraft. The cowlings that surround the
temperature rise will cause the core of the sensing engine are usually made of aluminium alloys, which
loop to expel the absorbed active gas into the sealed would be unable to contain a fire when the aircraft is
tube causing a rapid increase in pressure. This build static. During flight, however, the airflow around the
up of pressure is sensed by the detector alarm cowlings provides sufficient cooling to render them
switch. Should the sensing loop become damaged fireproof. Fireproof bulkheads and any cowlings that
causing a loss of the pressurized gas, an integrity are not affected by a cooling airflow, and sections of
switch will indicate a detection loop fault on the cowlings around certain outlets that may act as
appropriate engine. Fire indication is given by a 'flame-holders', are usually manufactured from steel
warning light and bell. or titanium.

Fig. 14-3 A continuous element fire detecting system.

156