Fire protection systems

Fire protection systems on aircraft usually consist of two separate operating systems with associated controls and indicators.  One system is for fire or overheat detection and the other is for fire suppression or extinguishing.  In some cases the systems can be interconnected so extinguishing takes place automatically when a fire is detected whereas the extinguishing was manual in early days.

Followings are some main facts that a Fire Detection and Extinguishing system must ideally be capable of,

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•  The fire warning system must accurately indicate that a fire has been extinguished and indicate if a fire re-ignites even after the first attempts of extinguishing.

• Since the system is exposed to all the critical areas of aircraft that must be durable and resistant to damage from any oil, water or other fluids that may be present in the area where it is installed.

• The system must include an accurate and effective method of testing so that the flight crew and engineers can check the integrity of the system.

• The system must be easily inspected, removed and installed.

• The system and its components must be designed so that the possibility of false indications is unlikely.

• The system must require a minimum of electrical power and must operate from the aircraft electrical system without the use of inverters or other special equipment to ensure safe operation even in some electrical system failures.

Detection systems must be capable of providing rapid detection of fire or overheat conditions and warn the crew by means of a red light and an audible signal on the flight deck indicating the area where some corrective action is required. However Fire detection systems will not automatically operate the main power unit extinguishers which may cause the main power plants(engines) to shutdown all of a sudden . But, in some installations the fire detection system may shut down APU and may operate on APU fire extinguisher automatically.

 

Hydrualic System-Functional Tests

After installation & reinstallation/Major maintenance of components it may be necessary to carry out adjustments to micro-switches, limiters, sensors...etc and to ensure the correct operation of warning devices and instrumentation, also operating systems will need to be tested to check for correct operation. This is generally referred to as functional testing. 

 All system functional tests will be specified in maintenance manuals and these must be always done in accordance with maintenance manuals(IAW MM) . Prior to any functional tests, it will be necessary to inspect the system(s) to be operated, this inspection should generally cover the followings,

• Quality and completeness of the maintenance task requiring a functional test.
• Checking of reservoir levels.
• Checking of accumulator pressures.
• Applying external gas pressure to reservoirs, if required.
• A thorough check of the area around services to be operated, to check for clearance,FOD, tools...etc.
• Correct connection of appropriate power sources (electrical and hydraulic).

It should be noted that the specific tests applied to particular aircraft and systems will be stated in the maintenance manual and all the steps must be done in accordance with the Manual only. 

     Functional tests are designed to test for correct sequence and speed of operation. Usually, if a major component replacement has taken place the system will be operated at low pressure/slow speed, to ensure correct operation before the full pressure/flow tests. During these tests all aspects of the system operation will be tested, this may include the timing of the operating sequence. The results of the functional test as Sluggish, slow, erratic operation or high temperature of fluid during operation gives indications of the faults in the system such as Aeration,Internal Leakages....etc. where it may cause the further inspection of the maintenance job.

Ram Air Turbine (RAT)

A ram air turbine (RAT) is a self contained turbine, or propeller, unit with a hydraulic pump attached, these are generally only used in emergency situations. The RAT is mounted on a shaft that provides swivel couplings for the pump supply line and pressure pipelines, when stowed it is held in position by a lock, aircraft skin contour is maintained by doors closing over the RAT stowage.RAT may be deployed into the airflow by selection of a switch on the flight deck, but is more usually deployed automatically when main system pressure drops below a predetermined value.

The RAT must be capable of operation over a broad speed range, maintaining normal hydraulic pressure, whether the aeroplane is at maximum speed and height or slowing for landing. To accomplish this the propeller is of a variable pitch design, controlled by a mechanical governor. An increase or decrease in RAT speed will change the centrifugal force on fly weights and blade pitch will be adjusted to maintain a constant RPM during operation .

A volumetric fuse or flow control valve is usually fitted to the RAT allowing the propeller to attain operating speed, before being loaded by the pump, this allows full system pressure to be achieved approximately 4-5 seconds after deployment into the air stream.

Image shows RAT deployed for maintenance.this RAT is stowed in the flap track fairing.the place of stowage can be different from aircraft to aircraft. generally it may be under the wings,under the belly, or at sideways of the aircraft.