Window Heat, Wipers and Rain Repellent

This article is about seeing out. Instruments can fly the aeroplane, but somebody still has to look up on short final — so ATA-30 fields three weapons for the glass: electric heat against ice and fog (the WHC's thermostat act), wipers against ordinary rain, and a chemical repellent against the heavy stuff. They share the same two panes and, when they fail, the same verdict criterion: can the pilot flying still see? — which is exactly how the MEL grades them (article 13).

1. Window heat: mission, structure, automation

Per FCOM DSC-30-40-10:

The aircraft uses electrical heating to anti-ice each windshield and defog the cockpit side windows.

[!warning]- The side windows are not anti-iced Read the sentence's precision: windshields get anti-ice, side windows only defogging. The windshield faces the oncoming flow; the side panes sit in the boundary layer at a shallow impingement angle, where the real enemy is cabin-humidity fog, not impact ice. Hence one power level only for the windows — per the same section, Only one heating level exists for the windows — and the most relaxed dispatch item in the chapter (all four side-window heaters may be inoperative unconditionally, article 13).

The organisation:

Two independent WHC, one on each side, automatically regulates the system, protects it against overheating, and indicates faults.

Each side's sub-system is one windshield, two side windows (one sliding, one fixed) and one Window Heat Computer — and within a side, per AMM 30-42-00, the windshield temperature regulation and the window temperature regulation are independent. Left/right separation is the same philosophy as the three PHCs (article 04): one box's death blinds one side, never both — the hardware base under the MEL's "PF side" logic. Automation, shared button and all, mirrors probe heat:

Window heating comes ON: ‐ Automatically when at least one engine is running, or when the aircraft is in flight ‐ Manually before engine start, when the flight crew switches ON the PROBE/WINDOW HEAT pb-sw.

The AMM adds the symmetric exit — heating is automatically stopped after the last engine shutdown — and a maintenance quirk worth knowing: the WHC's fault memory is not cancelled after supply cutoff. The memory is deleted by ground/flight transition. Cycling power on the ground clears nothing; only a flight cycle wipes the slate.

2. The thermostat act: 35–42 °C, cut off at ±60 °C

Per AMM 30-42-00:

The WHC controls the temperature of the windshield and the windows between 35°C and 42°C.

Body temperature, roughly — the goal is not to melt thick ice but to keep the surface just above the dew point and freezing point, without pumping thermal stress into a pane that is also a pressure-bearing structural part (the window build-up in the ATA-56 documentation shows the heating film as one layer among structural plies, each of which is able to sustain individually the pressurization loads). The tolerance-included envelope is 31–48 °C.

Sensing has a borrowed-thermometer arrangement:

Since the sliding window is not provided with a temperature sensor, the temperature sensor of the fixed window, serves to control the temperature of the sliding window.

That trick is legal only because of how the panes are wired — The side windows (sliding and fixed) are supplied in series — one current through both, identical heating, so measuring one measures the pair. The price of series wiring: one broken element cools both panes together, which is why the ECAM window alert comes per side, not per pane (article 12).

The protection lines are the section's hardest exam material:

- +60°C: corresponds to an overheat or to the sensor in open circuit. - -60°C: corresponds to the sensor in short circuit.

[!warning]- −60 °C doesn't mean "too cold" — it means the thermometer is lying The WHC reads a resistive sensor. Break the wire and resistance goes infinite — the computer reads an impossibly high temperature and cuts off as overheat. Short the sensor and resistance collapses — the computer reads an impossibly low one. The two lines together say one thing: a reading outside ±60 °C means either real overheat or a dead sensor, and blind heating is unacceptable either way. In the baseline logic an overtemperature or sensor loss cuts the whole side (windshield and windows together); the later-standard WHC (FSIP) keeps two sensors permanently wired with automatic changeover and segregates the cut off of the windshield heating and the window heating. And one guarantee sits below all software — per the AMM, a hardware device independent of the microprocessor performs the +60 °C cutoff. Same creed as the PHC: life-critical protection doesn't crash with the code.

Power levels complete the family resemblance to the pitot's half-wave trick:

Two heating power levels are available for the windshield: - 37.25 W/dm2 on the ground - 74.5 W/dm2 in flight only (not allowed on the ground).

A clean 1:2 ratio (the AMM states the low/high ratio as one half). "Not allowed on the ground" is not bureaucracy — with no ram cooling, full power drives the pane toward heat damage, and that phrase is the physics behind the QRH's hot-ramp reset scenario in §5. Side windows: a single 17 W/dm² level, ground and flight alike.

3. Wipers: 95 and 140 sweeps, three speed lines

Per FCOM DSC-30-60-10, Each front windshield has a two-speed electric wiper, each crew member owning a rotary selector. The AMM quantifies the speeds — slow operation (SLOW position) (95 cycles/min), fast operation (FAST position) (140 cycles/min) — and OFF parks the arm. The park position is more thoughtful than it looks:

The wipers have a park position clear of the area of visibility with the wiper blades just lifted from the windshield surface to avoid sand accumulation which could cause scratching of the windshield.

Parked blades hover just off the glass so grit cannot embed under the rubber and scratch on the next sweep. Speed limits come in three layers, and the layering matters more than the single number in the limitations chapter (maximum operating speed 230 kt). Per AMM 30-45-00:

The windshield wiper system operates satisfactorily until the aircraft speed of 200 kts. The operation of the windshield wiper system is possible between 200 and 230 kts but its performance can be unsatisfactory. After the operation of the wipers at a speed higher than 230 kts, a check of the system performance is necessary.

Read it as 200 satisfied / 230 limit / above-230 inspection — not "anything below 230 is fine". And for the day an arm jams away from its park position, the AMM's reassurance is unambiguous: the flight can continue in all safety without any speed restriction. (Distinguish that in-flight verdict from the MEL's ground decision about a continuously running wiper, which must be deactivated before dispatch — article 13.)

4. Rain repellent: chemistry instead of muscle

The principle first, because it explains every usage rule. Per AMM 30-45-00, the fluid is designed to increase the surface tension of water:

causes water to remain in the form of droplets (similarity with mercury), facilitates evacuation of these droplets under dynamic effect of airstream ; the droplets do not wet the windshield surface.

Wipers sweep water; repellent makes water unable to stand — it beads like mercury and the airstream blows the beads away. Which means it needs two things to work: rain to bead, and airflow to clear. Hold that thought for the prohibitions. Usage window and coverage, per FCOM DSC-30-60-10: In moderate to heavy rain, the flight crew can spray a rain repellent liquid on the windshield to improve visibility, and The window is covered by spray after about 30 s.

The hardware chain: a nitrogen-pressurised can on the cockpit rear wall — The can contains 475 cm3 of usable rain repellent fluid of type 3 — feeding one time-controlled solenoid valve and one three-orifice nozzle per side. Per the AMM, The timing device is set at 0.4 second — one push, one metered shot; per the FCOM, To repeat the cycle the flight crew must push the button again. Quantity is read three ways: a pressure gauge (When the needle is in the yellow sector the bottle should be replaced — and the AMM adds that in the yellow REPLACE band roughly ten applications remain, worth a spare can if the forecast is grim), a transparent chamber with a float, and the note that even a fully empty can leaves the gauge assembly holding enough for several more flights. The anti-clogging design is the elegant bit: purge air from the air-conditioning hot manifold, at 4 ± 1 psi above cabin, breathes through the nozzles whenever the system is idle, so residue never dries in the orifices.

Now the rules, which all follow from "needs rain, needs airflow":

(10) The system is inhibited on the ground (connection with the Engine Interface Vibration Monitoring Unit (EIVMU)).

(FCOM crew-level version: inhibited on the ground with engines stopped.) And the discipline paragraph, per AMM 30-45-00:

(c) In very light rain, the application of the rain-repellent fluid on the windshield does not noticeably improve visibility ; therefore the use of the wipers should be recommended for its lower cost price. (d) Never spray fluid on a dry windshield. In the event of inadvertent application do not operate the wipers before the rain wets the windshield.

The SOP nails it to the cockpit-preparation flow:

CAUTION Never use rain repellent to wash the windshield and never use it on a dry windshield.

[!warning]- Why a dry spray is worse than no spray On dry glass the undiluted fluid dries into a tacky film; run the wipers through it and you smear a uniform haze across the pane — the system meant to improve visibility permanently degrades it, and only a ground wash recovers the pane. Hence the paired rule: after an inadvertent dry application, hands off the wipers until rain has wetted the glass. Application count, by contrast, is deliberately open — per the AMM, one or two shots normally cover a takeoff or an approach-and-landing, but the number of applications is unrestricted so as to improve visibility. Visibility outranks fluid economy.

Configuration note: some aircraft carry a hydrophobic windshield coating instead — and per the AMM, where a coating is applied, the rain repellent fluid must not be applied on the windshield. Know which fit your aircraft has.

5. Two resets and one smell

Hot-ramp phantom alert. A parked aircraft, packs off, strong sun — the glass warms toward the cutoff line by itself. Per the QRH system-reset table:

On ground: If the air conditioning packs are OFF with the OAT above 40 °C, and/or the windshield is under direct sunlight, a spurious A.ICE L(R) WSHLD(WINDOW) HEAT may trigger. In that case, select both air conditioning packs to ON and wait at least 5 min for the cockpit temperature to decrease. After, use the appropriate WHC reset pb on the OVHD RESET panel.

Sequence is everything: cool first, reset second — reset a still-hot pane and the alert simply re-arms. And note the table's own caveat: Associated heating is interrupted, when the reset pb is pulled.

Arcing. A localised breakdown in the heating film calls for the opposite reset philosophy. Per the FCOM miscellaneous abnormals: In case of electrical arcing, pull the Window Heat Computer reset button that is located on the affected side. Here pulling the button is the remedy — it de-powers that side's heating so the arc dies. Cool-and-restore versus pull-and-leave: two reset stories, opposite directions (article 12 pairs them with the cracked-window drill).

The smell. The repellent can lives in the cockpit, and its fluid has a signature. Per the FCOM smoke procedures:

• If the smell is similar to that of orange peels, suspect a toxic leak of rain repellent fluid • If the smell is similar to that of pine needles, suspect a non-toxic leak.

Orange peel in the cockpit air: think of that can first.

6. Operating notes and the CAT 2/3 gate

Daily rhythm: check RAIN RPLNT pressure and quantity during cockpit preparation (with the never-dry-spray caution), PROBE/WINDOW HEAT at AUTO (ON for cold-weather anticipated heating, article 09); wipers by rain, repellent by heavy rain; window heat itself needs no hands all day — first engine start brings it on, last shutdown takes it off.

The low-visibility gate is where these systems earn their MEL grading. Per the QRH's required-equipment table for CAT 2 and CAT 3:

WINDSHIELD HEAT (L or R windshield) 1 for PF WINDSHIELD WIPERS OR RAIN REPELLENT (if activated) 1 for PF

Only the pilot flying's pane must heat, and only the PF needs either a wiper or the repellent — the two rain weapons cover for each other. That single table row explains the MEL asymmetry in article 13: PF-side windshield heat or wiper failures cap the aircraft at CAT 1, while the same failure on the PM side leaves CAT 3 DUAL untouched — a fault you can sometimes solve with crew seating.

Self-test

[!note]- Q1. What does window heat do for the windshield versus the side windows, and how many power levels does each get?

Windshield: anti-ice and defog, two levels (37.25 W/dm² ground / 74.5 W/dm² flight — full power not allowed on the ground). Side windows: defog only, one level (17 W/dm²), ground and flight.

[!note]- Q2. State the regulation band and both cutoff lines. What does −60 °C actually detect?

Regulation 35–42 °C. Cutoff at +60 °C (real overheat or sensor open circuit) and −60 °C (sensor short circuit). −60 °C is not cold protection — it is the electrical signature of a lying thermometer, and blind heating is cut either way.

[!note]- Q3. The sliding window has no temperature sensor. How is it controlled, and what makes that legal?

The fixed window's sensor governs both panes — legal because the two are supplied in series: one current, matched heating, measure one and you've measured both. The cost: one broken element cools the pair.

[!note]- Q4. Give the wiper speed layers and the jammed-arm verdict.

Satisfactory to 200 kt; possible but possibly unsatisfactory 200–230 kt; above 230 kt requires a post-use system check. An arm that stops away from park and won't return: flight may continue in complete safety, no speed restriction.

[!note]- Q5. Why must rain repellent never touch a dry windshield, and what if it does?

The fluid works by beading water for the airstream to remove — dry, it just dries tacky; wipers then smear a uniform haze that only a ground wash removes. After an inadvertent dry shot: do not run the wipers until rain has wetted the glass.

[!note]- Q6. Parked, OAT 42 °C, packs off — A.ICE R WSHLD HEAT triggers. Order of actions?

Both packs ON, wait at least five minutes for cockpit temperature to fall, then pull-and-reset the affected WHC on the overhead RESET panel. Reset first and it re-triggers; and while the reset button is out, that heating is interrupted.

[!note]- Q7. For CAT 3, whose windshield must heat and whose wiper must work?

The pilot flying's, in both cases — and for rain removal, a wiper or activated rain repellent satisfies the requirement. PM-side failures don't touch the landing capability.

Key takeaways

References

Window-heat mission, dual WHCs, automatic activation and power-level statements per FCOM DSC-30-40-10; wiper and rain-repellent crew descriptions, 30-second coverage, metered repeat and gauge indications per FCOM DSC-30-60-10/-20; repellent moderate-to-heavy-rain limitation and wiper maximum speed per FCOM LIM. Regulation band, ±60 °C cutoff meanings, series-wired windows with the borrowed sensor, power figures, FSIP segregation, hardware cutoff, auto-stop at last shutdown and fault-memory behaviour per AMM 30-42-00 (Description and Operation); wiper speeds, park design, speed layers, jammed-arm clearance, repellent principle, can and metering data, purge-air arrangement, ground inhibition and usage discipline per AMM 30-45-00 (Description and Operation); window structural plies per the FCOM ATA-56 window description. Hot-ramp reset per the QRH system-reset table; arcing reset per the FCOM miscellaneous abnormal procedures; smell signatures per the FCOM smoke procedures; CAT 2/3 equipment rows per the QRH operational-data tables. The "thermometer lying" framing, the beading physics narrative and the PF-seat observation are integrative syntheses of the referenced material. Hydrophobic-coating fit varies by aircraft configuration.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.