🔵✈️ 306. Aviation: Fire Detection System Maintenance 🚁🎞️🕹️

Source: FAA-H-8083-32A, Aviation Maintenance Technician Handbook Powerplant, Volume 2, Pag: 9-10

Simulator - Model 3D

Fire detector sensing elements are located in many highactivity areas around aircraft engines. Their location, together with their small size, increases the chance of damage to the sensing elements during maintenance. 

An inspection and maintenance program for all types of continuous-loop systems should include the following visual checks. These procedures are examples and should not be used to replace the applicable manufacturer’s instructions.

Sensing elements of a continuous-loop system should be inspected for the following: 

1. Cracked or broken sections caused by crushing or squeezing between inspection plates, cowl panels, or engine components. 

2. Abrasion caused by rubbing of the element on cowling, accessories, or structural members. 

3. Pieces of safety wire or other metal particles that may short the spot-detector terminals. 

4. Condition of rubber grommets in mounting clamps, which may be softened from exposure to oils or hardened from excessive heat. 

5. Dents and kinks in sensing element sections. Limits on the element diameter, acceptable dents and kinks, and degree of smoothness of tubing contour are specified by the manufacturer. No attempt should be made to straighten any acceptable dent or kink, since stresses may be set up that could cause tubing failure.

6. Nuts at the end of the sensing elements should be inspected for tightness and the presence of a safety wire. Loose nuts should be retorqued to the value specified by the manufacturer’s instructions. Some types of sensing element connection joints require the use of copper crush gaskets, which should be replaced any time a connection is separated.

7. If shielded flexible leads are used, they should be inspected for fraying of the outer braid. The braided sheath is composed of many fine metal strands woven into a protective covering surrounding the inner insulated wire. Continuous bending of the cable or rough treatment can break these fine wires, especially those near the connectors.

8. Sensing element routing and clamping should be inspected carefully. Long, unsupported sections may permit excessive vibration that can cause breakage. The distance between clamps on straight runs, usually about 8–10 inches, is specified by the manufacturer. 

At end connectors, the first support clamp is usually located about four to six inches from the end connector fittings. In most cases, a straight run of one inch is maintained from all connectors before a bend is started, and an optimum bend radius of three inches is normally applied.

9. Interference between a cowl brace and a sensing element can cause rubbing. This interference may cause wear and short the sensing element.

10. Grommets should be installed on the sensing element so that both ends are centered on its clamp. The split end of the grommet should face the outside of the nearest bend. Clamps and grommets should fit the element snugly.

Fire Detection System Troubleshooting 

The following troubleshooting procedures represent the most common difficulties encountered in engine fire detection systems: 

1. Intermittent alarms are most often caused by an intermittent short in the detector system wiring. Such shorts may be caused by a loose wire that occasionally touches a nearby terminal, a frayed wire brushing against a structure, or a sensing element rubbing against a structural member long enough to wear through the insulation. Intermittent faults often can be located by moving wires to re-create the short.

2. Fire alarms and warning lights can occur when no engine fire or overheat condition exists. Such false alarms can be most easily located by disconnecting the engine sensing loop connections from the control unit. 

If the false alarm ceases when the engine sensing loop is disconnected, the fault is in the disconnected sensing loop, which should be examined for areas that have been bent into contact with hot parts of the engine. If no bent element can be found, the shorted section can be located by isolating the connecting elements consecutively around the entire loop.

3. Kinks and sharp bends in the sensing element can cause an internal wire to short intermittently to the outer tubing. The fault can be located by checking the sensing element with an ohm meter while tapping the element in the suspected areas to produce the short.

4. Moisture in the detection system seldom causes a false fire alarm. If, however, moisture does cause an alarm, the warning persists until the contamination is removed or boils away, and the resistance of the loop returns to its normal value.

5. Failure to obtain an alarm signal when the test switch is actuated may be caused by a defective test switch or control unit, the lack of electrical power, inoperative indicator light, or an opening in the sensing element or connecting wiring. 

When the test switch fails to provide an alarm, the continuity of a two-wire sensing loop can be determined by opening the loop and measuring the resistance. In a single-wire, continuousloop system, the center conductor should be grounded. 

Fire Extinguisher System Maintenance Practices

Regular maintenance of fire extinguisher systems typically includes such items as the inspection and servicing of fire extinguisher bottles (containers), removal and reinstallation of cartridge and discharge valves, testing of discharge tubing for leakage, and electrical wiring continuity tests. The following paragraphs contain details of some of the most typical maintenance procedures.

Fire extinguisher containers are checked periodically to determine that the pressure is between the prescribed minimum and maximum limits. Changes of pressure with ambient temperatures must also fall within prescribed limits. 

The graph shown in Figure is typical of the pressure temperature curve graphs that provide maximum and minimum gauge readings. If the pressure does not fall within the graph limits, the extinguisher container is replaced. The service life of fire extinguisher discharge cartridges is calculated from the manufacturer’s date stamp, which is usually placed on the face of the cartridge. 

The cartridge service life recommended by the manufacturer is usually in terms of years. Cartridges are available with a service life of 5 years or more. To determine the unexpired service life of a discharge cartridge, it is usually necessary to remove the electrical leads and discharge line from the plug body, which can then be removed from the extinguisher container.  

Be careful when replacing cartridge and discharge valves. Most new extinguisher containers are supplied with their cartridge and discharge valve disassembled. Before installation on the aircraft, properly assemble the cartridge in the discharge valve and connect the valve to the container, usually by means of a swivel nut that tightens against a packing ring gasket.

If a cartridge is removed from a discharge valve for any reason, it should not be used in another discharge valve assembly, since the distance the contact point protrudes may vary with each unit. Thus, continuity might not exist if a used plug that had been indented with a long contact point were installed in a discharge valve with a shorter contact point.

The preceding material in this chapter is general in nature, addressing the principles involved and general procedures to be followed. When actually performing maintenance, always refer to the applicable maintenance manuals and other related publications pertaining to a particular aircraft.

Aviation Fire Detection and Extinguisher System Maintenance Procedures

Maintenance Category	Inspection or Fault Item	Probable Cause or Defect	Recommended Action or Procedure
Continuous-Loop Inspection	Sensing element sections	Cracked, broken, or abraded sections caused by crushing/squeezing between components or rubbing on cowling, accessories, or structural members	Perform visual checks during inspection and maintenance.
Continuous-Loop Inspection	Spot-detector terminals	Pieces of safety wire or other metal particles that may short the terminals	Perform visual checks during inspection and maintenance.
Continuous-Loop Inspection	Rubber grommets in mounting clamps	Softened from exposure to oils or hardened from excessive heat	Inspect condition of rubber grommets.
Continuous-Loop Inspection	Sensing element sections	Dents and kinks	Check against limits specified by manufacturer; make no attempt to straighten any acceptable dent or kink to prevent stresses that cause tubing failure.
Continuous-Loop Inspection	Nuts at the end of sensing elements	Loose nuts or missing safety wire	Inspect for tightness and presence of safety wire; retorque loose nuts to specified value; replace copper crush gaskets any time connection is separated.
Continuous-Loop Inspection	Shielded flexible leads	Fraying of outer braid caused by continuous bending of cable or rough treatment breaking fine wires near connectors	Inspect outer braid for fraying.
Continuous-Loop Inspection	Sensing element routing and clamping	Long, unsupported sections permitting excessive vibration that can cause breakage	Inspect routing and clamping carefully; maintain distance between clamps (usually 8–10 inches on straight runs; first support clamp 4–6 inches from end connector; 1-inch straight run before bend; 3-inch optimum bend radius).
Continuous-Loop Inspection	Sensing element and cowl brace	Interference causing rubbing, wear, and shorting of the sensing element	Inspect for interference.
Continuous-Loop Inspection	Grommets and clamps on sensing element	Improper installation or loose fit	Install grommets so both ends are centered on clamp; split end facing outside of nearest bend; ensure clamps and grommets fit element snugly.
Troubleshooting	Intermittent alarms	Intermittent short in detector system wiring (loose wire touching terminal, frayed wire brushing structure, or sensing element rubbing structural member wearing through insulation)	Move wires to re-create short and locate the intermittent fault.
Troubleshooting	False alarms / warning lights (no engine fire or overheat)	Sensing element bent into contact with hot parts of the engine or shorted section in loop	Disconnect engine sensing loop from control unit; examine for elements bent into contact with hot parts; if none found, isolate connecting elements consecutively around loop.
Troubleshooting	Intermittent short in sensing element	Kinks and sharp bends causing internal wire to short intermittently to outer tubing	Check sensing element with an ohmmeter while tapping element in suspected areas to produce the short.
Troubleshooting	False fire alarm (persistent)	Moisture contamination in detection system	Remove contamination (or allow to boil away) until loop resistance returns to normal value.
Troubleshooting	Failure to obtain alarm signal when test switch is actuated	Defective test switch or control unit, lack of electrical power, inoperative indicator light, or open in sensing element/connecting wiring	Measure resistance by opening the loop (for two-wire system) or ground center conductor (for single-wire system) to test continuity.
Fire Extinguisher Container Maintenance	Container pressure checks	Pressure not falling within prescribed minimum and maximum limits on pressure-temperature curve graph	Check pressure periodically against temperature curve graphs; replace extinguisher container if pressure falls outside graph limits.
Fire Extinguisher Container Maintenance	Discharge cartridge service life	Expired service life (calculated from manufacturer's date stamp)	Determine unexpired service life by removing electrical leads and discharge line from plug body to read date stamp; replace per manufacturer recommendations.
Fire Extinguisher Container Maintenance	Cartridge and discharge valve reinstallation	Cartridge removed from discharge valve and reused in another assembly, leading to lack of electrical continuity due to varying contact point protrusion distances	Do not reuse a removed cartridge in another discharge valve assembly; properly assemble new cartridge in valve and connect valve to container using swivel nut against packing ring gasket.