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Fire Detection - Fire & Smoke - Airframes & Aircraft Systems #58 — Transcript

by Aero & Air · 1,839 words · 332 segments · language en · Watch on YouTube

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0:00in this lesson we will discuss the fire
0:03detection systems fitted to all aircraft
0:08a fire cannot exist without three things
0:11being present these are
0:16eight or a source of ignition
0:20Buhl material that will burn
0:25and oxygen this is known as the triangle
0:29of fire
0:32take any one of the three away and the
0:35fire will go out
0:40a fire in an aircraft is extremely
0:43dangerous so to protect the aircraft the
0:46crew and the passengers aircraft must
0:49have fire detection and extinguishing
0:51systems fitted in all areas where a
0:53potential fire risk may exist following
0:56failure of or leakage from any component
0:59or associated equipment
1:03the specific areas that require
1:05protection are the engines
1:10the auxiliary power unit or APU
1:15the main wheel wells
1:19and the cargo compartments
1:23these areas are known as designated fire
1:26zones and they must have detection
1:29systems fitted to warn the crew of a
1:31fire
1:34in the case of the engines and the
1:36auxiliary power unit they are isolated
1:38by fireproof bulkheads known as
1:41firewalls normally manufactured from
1:43titanium or stainless steel which will
1:46contain any fire to the immediate area
1:48and prevent it from spreading to the
1:50rest of the aircraft
1:56each designated fire zone must have a
1:59fire detection system which is capable
2:02of providing rapid detection and warning
2:04of a localized fire or overheat
2:06condition
2:08there are a number of detection systems
2:10in use and we will now take a look at
2:12them
2:18differential expansion detectors use the
2:21fact that when heat is applied to
2:23different materials they will expand at
2:26different rates
2:30consists of a pair of contacts mounted
2:33on a spring bow assembly
2:37fitted with in an expansion tube which
2:39is mounted on a base
2:42the spring bow is fixed to the tube at
2:44the top and the bottom
2:48when heat is applied the tube expands at
2:51a greater rate than the bow drawing the
2:53contacts together
2:57when the contacts come together they
2:59provide power to the warning circuit
3:04drop in temperature will cause the tube
3:07to shorten the contactor open and the
3:10warning to stop
3:13this type of detector usually
3:15incorporates a short time today before
3:17the warning is activated to prevent
3:20false warnings due to vibration
3:24differential expansion detectors are
3:27often used to monitor engine cooling air
3:29outlets
3:33to provide internal engine overheat
3:35warning
3:37in the event of this warning being given
3:39the pilots will normally be required to
3:41shut down the engine
3:49engine and auxilary power unit fire
3:52zones are often fitted with continuous
3:54loop fire detectors commonly known as
3:58fire wire systems
4:02a detector unit consists of a stainless
4:04steel tube
4:08central electrode insulated from the
4:10tube
4:14of temperature sensitive material
4:22FireWire detectors operate on the
4:24principle of their core material having
4:27either a negative coefficient of
4:29resistance with temperature or a
4:32positive coefficient of capacitance with
4:35temperature in one system both methods
4:39are used together
4:45in the resistive type of firewire
4:49as the temperature increases
4:54the resistance of the insulating
4:56material decreases
5:00this causes the current flow between the
5:02central electrode and the outer tube to
5:04increase
5:07until at a predetermined temperature
5:09sufficient current will flow and the
5:12warning system will operate
5:16if the temperature drops back below a
5:18preset value the system will
5:20automatically reset and the warning will
5:23stop
5:25this type of detector will react and
5:28give a fire warning if there is an
5:30isolated temperature increase for
5:33instance from a direct flame
5:37or if the temperature of the entire
5:39detector increases possibly because of a
5:41hot gas leak
5:45because the loop is connected to the
5:47control unit at both ends
5:51the system will continue functioning
5:53with a single break in the firewire
5:58one particular problem with this type of
6:00system is that a short circuit between
6:02the case and the inner electrode will
6:05give a false fire warning
6:11in the capacitive type of system the
6:14detector acts as a long capacitor with
6:17the core material acting as the
6:18dielectric
6:22and the case and the inner electrode as
6:24the polls
6:26the case and the inner electrode are
6:28connected across an AC supply
6:33an increase in temperature causes a
6:35change in the properties of the inner
6:37core material
6:39this change increases the capacitance of
6:42the detector
6:46the firewire is polarized by the
6:48application of half wave rectified
6:50alternating current from a control unit
6:52which it stores and then discharges as a
6:55feedback current
6:58as the capacitance increases with
7:00temperature increase so will feedback
7:03current
7:06once the feedback current reaches a
7:07predetermined level it activates the
7:10fire warning
7:14as with the resistive type this system
7:16will reset itself once the temperature
7:18drops below a preset level
7:22this type of detector will also react
7:24and give a fire warning if there is an
7:27isolated temperature increase for
7:29instance from a direct flame
7:33or if the temperature of the entire
7:35detector increases
7:39again as with the resistive system the
7:42loop is connected to the control unit at
7:44both ends
7:48so the system will continue to operate
7:50normally in the event of a single break
7:52in the firewire
7:56this system has the advantage over the
7:58resistive type in that a short-circuit
8:00grounding of the element or system will
8:03cause the capacitance to fall to zero so
8:06will not result in a false warning
8:16fire wires are positioned around engine
8:19fire zones in a continuous double loop
8:21[Music]
8:24the two loops are normally known as loop
8:26a and loop B
8:30the two loops operate independently both
8:34loops need to detect a fire to initiate
8:36the warning
8:40this way spurious warnings caused by one
8:43loop giving a false fire indication
8:45possibly due to a short-circuit are
8:47prevented
8:50warning of any loop failure may be
8:52displayed on the fire detection panel or
8:55electronic system display unit
9:01the last system we are going to look at
9:03is the gas-filled detectors system
9:07this system consists of a number of
9:09sensing elements which look very similar
9:11to fire wire and they are used in a very
9:14similar way
9:17the tube has a central core made up of a
9:21material which will give off hydrogen
9:23gas when heated to a high temperature
9:28during the manufacturing process helium
9:31gas is forced into the tube under
9:32pressure the tube is then sealed
9:38to aid explanation of the gas-filled
9:40detector type of system we have included
9:43a pressure gauge this gauge is not
9:46fitted on aircraft detection systems
9:52the tube is monitored by two pressure
9:54switches
9:56one is closed the tubes normal pressure
9:59this is known as the integrity switch
10:05the other which is open at normal
10:06pressure is known as the alarm switch
10:12as with fire wire the detectors are
10:15positioned around the fire zones in a
10:16double loop with both loops again being
10:19required to detect a fire before a fire
10:22warning is given
10:28when the Cassville detect a tube is
10:30heated locally by a flame the helium
10:33pressure may not increase sufficiently
10:34to trigger the warning
10:37however hydrogen gas is released from
10:39the core material and the pressure in
10:42the tube will build up
10:43[Music]
10:46this increase of pressure is sensed by
10:49the alarm pressure switch which will
10:50close
10:54this will send a signal to a control box
10:56which will initiate a fire warning
11:00you
11:03in the event of a hot gas leak the
11:05temperature increase will be less than
11:07that caused by a flame
11:10but it'll be felt over a larger area of
11:12the detector
11:15this increase in temperature may be
11:17insufficient to cause the release of
11:19hydrogen gas but it will cause
11:22sufficient increase in the pressure of
11:23the helium gas to close the alarm
11:26pressure switch and initiate a fire
11:28warning
11:36should the to be damaged and the helium
11:38gas be released from the core
11:43pressure will fall
11:46and the normally-closed integrity
11:48pressure switch will open
11:52this will cause a fault indication on
11:54the control panel or electronic systems
11:57display
12:04whichever detection system is used fly
12:08tech indications of an engine fire will
12:10normally be both aural and visual
12:16warning will normally be a bell or a
12:18klaxon or possibly a continuous
12:20repetitive chime
12:24visually there may be a master fire
12:26warning caption
12:30and a steady red fire warning light for
12:32each engine
12:32[Music]
12:38before flight amines must be available
12:41to test the fire warning circuit
12:44a fire test selector is therefore
12:46provided for the flight crew
12:49with differential expansion detectors a
12:51simple continuity test is carried out on
12:54the wiring between the detector or
12:56detectors and the warning light
13:00a satisfactory test is indicated by
13:02illumination of the warning light
13:11when a fire test is initiated on a fire
13:14wire system a continuity test is carried
13:17out
13:20on both the outer tube
13:24and the inner electrode
13:29satisfactory test is indicated by a full
13:31fire warning being displayed for all
13:33engines being tested
13:37should there be a break in either the
13:39tube or the electrode or the firewire
13:41for a particular engine
13:45then the test will fail and a fire
13:48warning will not be given for that
13:49engine
13:55when a fire test is carried out on the
13:57gas filled system again the integrity of
14:00the loops is tested
14:04but also if the gas has leaked out and
14:06the integrity switch is open
14:11and the test will fail
14:18in some fire wire and gas field detector
14:20systems a warning is given to notify
14:23crews if a single fire loop should fail
14:26in flight
14:29the system may now automatically switch
14:31to single loop operation with the fire
14:34warning being given if the serviceable
14:36loop alone senses a fire depending on
14:39aircraft type a limited number of
14:42sectors may be permitted to be flown in
14:44the single loop mode
14:54that is the end of the lesson
14:57here is a summary of the main points
15:01click to go onto the next one once you
15:03are sure you understand the point being
15:05displayed
15:08the areas of an aircraft fitted with
15:10fire protection systems are the engines
15:13the auxiliary power unit the main wheel
15:17base and the cargo compartments
15:25differential expansion fire detectors
15:28use the fact that when heat is applied
15:30to different materials they will expand
15:32at different rates
15:35remember that these detectors usually
15:37have a time delay built into their
15:39warning circuits to prevent false alarms
15:42caused by vibration
15:53resistance and capacitive firewire
15:55systems have the detector connected at
15:58both ends
16:00so with a single break the detector will
16:03continue to function the system test
16:07checks the continuity of the loop as
16:08well as the warning system so if there
16:11is a break the test will fail
16:21you
16:26if a resistive firewire is
16:28short-circuited it will give a fire
16:30warning
16:36the gas-filled system uses the fact that
16:39the pressure of the gas in the detector
16:41increases with increasing temperature to
16:44trigger a fire warning
16:48when a test is carried out on the gas
16:50filled system the integrity of the loops
16:52is tested if the gas has leaked out the
16:56integrity switch will open and the test
16:58will fail
17:04finally flight-deck indications of an
17:07engine fire will normally be both aural
17:10and visual
17:19you

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