7.3 Ice and Rain Control System Inspection and Limits

Anti-Ice Versus Deice, and the Methods Tested

The first exam distinction is anti-ice versus deice. Anti-ice systems are turned on before entering icing and run continuously to prevent ice from forming — heated pitot tubes, thermal bleed-air wings, and TKS weeping wings are anti-ice. Deice systems are operated after ice accumulates to remove it — the pneumatic boot is the classic deicer. Choosing the wrong category leads to wrong operation: running a boot continuously, or cycling it before enough ice builds, allows ice bridging, where ice forms a shell over the inflated boot that later inflations cannot break.

The second distinction is what is protected: lift/control surfaces, engine inlets, air-data probes (pitot/static), stall-warning sensors, and crew visibility. Stay inside the affected boundary — a failed wiper does not prove a pitot-heat fault, and a dead pitot-heat circuit does not prove a boot problem.

Thermal bleed-air anti-ice extracts hot air from a turbine engine's compressor and routes it through perforated piccolo tubes inside wing, stabilizer, and inlet leading edges; ejectors mix bleed air with cooler ram air to control temperature. It requires extensive ducting, control valves, and overheat protection, and bleed leaks are an inspection item. Electric (electrothermal) heat powers resistance elements in pitot tubes, static ports, stall vanes, propeller boots (via slip rings/brushes), and heated windshields.

Pneumatic deice boots are rubber leading-edge coverings with internal bladders that inflate from an engine-driven air pump (or vacuum/regulated pressure) to crack ice, then deflate flush. TKS weeping wing is a fluid anti-ice/deice system: a pump forces glycol-based fluid through formed titanium panels laser-drilled with roughly 800 tiny holes per square inch; the fluid weeps out, coats the leading edge, propeller, and windshield, and lowers the freezing point of impinging water so it runs off without freezing.

Rain Control and Inspection Limits

Rain control protects visibility by three approved means. Windshield wipers clear water mechanically; blades must match the windshield, park correctly, and sweep within the designed arc — a wiper that parks wrong can block vision or scratch the transparency, and wipers are not used on dry glass. Chemical rain repellent (or hydrophobic surface coatings) causes water to bead and blow off; only the approved fluid/coating may be used, because incompatible chemicals attack transparencies.

Pneumatic rain removal blows a sheet of high-velocity (often heated) air across the windshield to push water off, common on jets. Windshield alcohol systems spray isopropyl alcohol for combined anti-ice and rain repellency on some aircraft. Heated windshields also keep the panel above the freezing/fogging point and warm it for bird-strike resilience.

** Pneumatic boots are easily ruined by poor practice: petroleum products and unapproved solvents attack the rubber, and scrapers cut the surface, so only the cleaner specified in the maintenance manual is used. When a boot inflates weakly, separate the air source, timer/distributor (control) valve, plumbing, clamps, check valves, leaks, and boot condition before replacing the visible boot — the fault is often upstream.

Electric ice protection adds burn hazards: a heated pitot tube can reach skin-burning temperature within seconds of power-on, and ground operation of pitot heat is time-limited. Troubleshoot electric heat by verifying breaker, power, switch, control, wiring, ground, and element resistance against approved data before condemning a "cold" probe.

Ice Detection and System Components

Many aircraft carry ice detectors that alert the crew or automatically trigger protection: a vibrating-probe detector senses ice mass by a change in its resonant frequency, while optical detectors sense ice optically. The mechanic checks detector security, heating, and test response by procedure. Engine inlet anti-ice (bleed-air or electric) protects the inlet lip and probes so ingested ice cannot damage compressor blades — a powerplant-related but airframe-routed system.

The pneumatic-boot system has identifiable parts worth knowing: an engine-driven air pump (often the same pump that supplies vacuum instruments, with pressure tapped off), a regulator and relief valve, a distributor/timer (control) valve that sequences inflation, check valves to hold boot deflation by vacuum, and the boots themselves bonded to the leading edge. Boots are normally held deflated against the airfoil by suction so they do not disturb airflow in clear air; they inflate only on command. When inflation is weak or uneven, the timer/distributor and air supply are checked before the boot is condemned.

The consistent exam theme: identify the installed system, classify it as anti-ice or deice, use its specific procedure, and avoid any action that damages the protected surface. A mechanic does not release an ice-protection system based on general weather knowledge — the task is to confirm the installed system meets the instructions for continued airworthiness through operational checks, resistance/current checks, pressure checks, plumbing and boot inspection, and approved cleaning.

A common exam trap is assuming a "cold" heated probe is failed: verify breaker, switch, control logic, wiring, ground, and element resistance first, because the element itself is frequently not the fault.