Ice, rain and fire protection

Ice spoils a wing's shape, adds weight and blocks pitot tubes and intakes. Anti-ice systems stop ice forming; de-ice systems remove it after it forms.

• Pneumatic boots (de-ice): rubber on the leading edges that inflates in sequence to crack ice off, then is held flat by vacuum.
• Thermal anti-ice: hot engine bleed air piped inside wing leading edges and engine inlets.
• Electric heat: pitot tubes, static ports, windshields, propeller blades and sensors.
• Fluid (TKS): glycol fluid weeps through tiny holes in the leading edges and propellers.

Rain is cleared with windshield wipers, rain repellent coatings, or a blast of bleed air across the windshield.

Fire detection

• Thermal switch (spot detector): closes at a set temperature.
• Thermocouple: makes its own voltage and responds to the rate of temperature rise, so it catches a fast fire.
• Continuous loop: a long sensing element routed through the nacelle; its resistance or capacitance changes when any part of it is heated.
• Pressure (gas) type: a sealed tube of gas that expands with heat and closes a switch.
• Smoke detectors in cargo bays and lavatories, and carbon monoxide detectors in cabins.

Extinguishing

Engine and APU fires are fought with agent (often Halon 1301, being replaced by newer agents) from pressurized bottles, released by an electrically fired squib (discharge cartridge). On many transport aircraft, indicator discs outside the fuselage show what happened to a bottle: a missing red disc means a thermal discharge (the bottle overheated and released itself), and a missing yellow disc means a normal discharge (the crew fired it). Bottles are checked by their pressure against a temperature chart, or by weight.