Fire on an aircraft is fought in three acts (see it, confirm it, smother it), and each act has its own hardware. This lesson works through the smoke detectors of the cabin and holds, the fire detectors wrapped around the engines, the extinguishing bottles behind them, and then turns to a humbler protection: keeping the windshield readable in rain.
Three questions from this lesson — one straightforward, one that applies it, one that pushes a little further. Nothing is recorded, and you can draw a new set as often as you like.
An example of the warnings, indications and function test of a fire-protection system is:
The three windshield rain-protection systems for an aeroplane work as follows:
In cruise, loop A of an engine's double-loop fire-detection system shorts where it has chafed against a clamp. Loop B senses normal temperatures. The system:
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Smoke detectors guard the spaces nobody watches: cargo compartments, lavatories, avionics bays. Two sensing principles do the work:
Optical (photoelectric) sensor — a dark chamber with a light source and a photocell deliberately placed out of the beam: clean air scatters almost nothing and the cell sits in darkness, but smoke particles scatter the beam sideways, light lands on the cell, and the alarm follows.
Ionising sensor — inverts the logic: a tiny radioactive source ionises the chamber's air so a minute current flows between charged plates; smoke particles catch and carry off the ions, the current falls, and the falling current is the alarm.
One triggers on light appearing, the other on current disappearing.
Warnings, indications and tests: a detected smoke event drives an aural warning and a red or amber annunciation naming the compartment — the lavatory smoke chime, the cargo SMOKE light on the overhead panel or electronic display. A function test from the flight deck or maintenance panel stimulates the sensing circuit and must reproduce the full warning chain, lights and sound together: a test that stays silent has found a fault.
Engine and APU fire zones are too hot, windy and oily for smoke sensing; their detectors feel heat instead, through two sensing principles: in one, heat changes a sensing element's electrical properties — its resistance falls and its capacitance rises as temperature climbs, and the monitoring unit trips on either; in the other, heat raises the pressure of a gas sealed in a tube, and a pressure switch trips.
Bimetallic spot detector — two bonded metals that expand differently; heat bends the pair until contacts close. Cheap and simple, but it watches only its own spot.
Continuous loop — metres of thin sensing element (the resistance-and-capacitance kind) snaked around the whole fire zone, so a fire anywhere along its length trips the same alarm: the standard engine installation.
Gaseous (gas-filled) loop — the sealed-gas principle drawn out into a tube around the zone; general heating raises its pressure gradually, and an intense local hot spot releases extra gas from the tube's core material, so it answers both an overall fire and a concentrated one.
Why two loops: a sensing element living clamped to a vibrating, oil-misted engine can chafe, kink or short, and a single loop's fault is indistinguishable from its fire. Fitting two independent loops through the same zone and demanding both agree means a damaged loop alone cannot raise a false FIRE warning — while a real fire, heating both loops together, sounds it at once. If the system finds one loop failed, it reverts to trusting the survivor alone, degraded but alive.
Warnings, indications and tests for fire follow one grammar across types: a red light in or beside the fire handle of the affected engine, a master warning and the fire bell. The function test (the fire test switch on the overhead panel) exercises the loops' continuity and the warning chain, and the crew expects the full chorus: every handle lit, bell ringing, master warning flashing. Silence from one engine's light during the test is a failed detector, not a healthy one.
The built-in fire-extinguishing system is a set of pressurised bottles and the plumbing to aim them. Its components:
Spherical bottles — filled with extinguishing agent and pressurised with nitrogen, each opened by a squib, a small explosive cartridge that bursts the discharge valve when fired electrically, fast and irreversible.
Distribution lines and spray nozzles — in the protected zone.
Fire handle — arms it all: pulling the handle isolates the engine (fuel, hydraulics, bleed air, generator) and arms the squibs; turning or pressing it fires a bottle.
Certification requires two discharges available for each engine (CS 25.1195(c)), which the crossed plumbing of the figure provides from two shared bottles: if the first shot fails to kill the fire, a second is always there. Cargo compartments extend the idea with a dump-then-meter schedule, one bottle flooding the hold at once and another bleeding in slowly to hold the concentration for the long diversion.
Wipers — the mechanical answer, with selectable speeds, used for taxi, take-off and approach; not used on a dry screen, where they scratch.
Rain repellent — a liquid sprayed onto the wet screen on command, making the glass so water-shy that drops bead and the airstream carries them away; applied only to a wet screen, where the rain is heavy enough to need it.
Hydrophobic coating — the same water-shedding idea baked permanently onto the glass at manufacture, working passively every flight and slowly wearing over the years.
The two smoke principles and three rain answers are straight recall; the double-loop AND logic and the two-discharge rule are the reasoning marks, and the squib is the exam's favourite component name. The overview, following the learning objectives:
Explain the optical and ionising smoke sensors, with example warnings and function tests
Explain the fire-detection principles (resistance/capacitance, gas pressure) and applications (bimetallic, continuous loop, gaseous loop)
Explain why double-loop systems are used, and give example fire warnings and tests
Explain the built-in extinguishing system, its components, and the two-discharge requirement
Explain wipers, rain repellent and coatings