Ice Detection and the Visual Ice Indicator

The protection systems answer what to do about ice; the detection system answers when. The A330's answer is a three-piece sensing kit: two singing probes on the nose (when the song goes flat, there's ice), a titanium "ice bolt" between the windshields, and a pair of lights that throw beams on the leading edges at night. Their shared job description is adviser — they prompt, and they never touch an anti-ice switch. This article covers the physics, the two-level signalling, the inhibitions and the system's one famous blind spot. The alerts it drives are handled in article 12; the blind spot gets its full treatment in article 10.

1. Position: adviser — and part-time fuel economist

Per FCOM DSC-30-70-10:

The ice detection system does not control the ENG or WING anti-ice systems.

First principle of the whole article: advisory only. Outputs are ECAM messages and MEMO behaviour; the switches stay with the crew. Why fit it at all? The AMM gives two reasons, and the second is routinely forgotten. Per AMM 30-81-00:

The purpose of the advisory ice detection system is to enable: - better detection of icing conditions - cutting off of the anti ice system when the latter is no longer necessary (fuel saving).

[!warning]- Half the system's job is telling you to switch OFF Anti-ice isn't free — engine and wing anti-ice both burn bleed air, and bleed air is fuel (a stuck-open engine valve costs measurable percent, article 13; the emergency-electrics fail-open case adds roughly 1.5 %, article 03). "The icing has ended" is information worth exactly as much as "icing has begun" — which is why the MEMO-pulsing behaviour in §5 exists.

The kit, located: two ice detectors on the forward lower fuselage — per the AMM, at the six-o'clock position — plus the lighted visual indicator between the windshields (§6), plus the wing-scan lights from the exterior-lighting chapter.

2. The physics: a tuning fork that counts ice

Per AMM 30-81-00:

The sensing probe is driven magnetostrictively to vibrate at its resonant frequency of 40 KHz. Then, the weight of the accreted ice causes the frequency to decrease.

The hardware:

The probe consists of a magnetostrictive nickel alloy tube of approximately 0.25 in. (6.35 mm) diameter with an exposed length of 1 in. (25.4 mm).

And the calibration that turns physics into a signal:

A 133 Hz drop is correlated to 0.020 plus or minus 0.005 in. (0.508 plus or minus 0.127 mm) of ice and is considered as the detection trip point. When the trip point is reached, the probe is deiced by means of internal heaters to prepare for subsequent detections.

Causal chain in one line: ice has mass → mass slows the vibration → 133 Hz down = one "elementary detection" → self-heat clean → wait for the next. This is a system that counts ice in half-millimetre coins rather than measuring thickness continuously — a mud-flecked guitar string that gets wiped after every flat note.

[!warning]- The probe de-ices itself — not for protection, but to count again The sensing element must be allowed to ice (or there is nothing to measure) and must be cleared promptly (or one accretion would alarm forever). The ice → count → self-clean → wait cycle means the output is really an accretion-rate signal: how often it trips is how fast ice is growing. That rate-counting is the entire foundation of the SEVERITY logic below.

3. Two signal levels: count one, count seven

Per AMM 30-81-00:

The ice detector sends the ICE signal when a thickness of 0.5 mm of ice is accreted on its sensing element. This signal is maintained for 60 seconds. If new ice detections occur within 60 seconds, the ICE signal is maintained for 60 seconds after the last detection.

The ice detector generates the SEVERITY signal when a number of 7 elementary ice detections is reached. The threshold for the SEVERITY signal corresponds to approximately 5 mm of ice accreted on the most critical protected surface of the wings (wing tips).

The two levels drive the two condition alerts (triggering text here; the procedures live in article 12). ICE DETECTED — per the FCOM anti-ice abnormals:

This alert triggers when ice is detected in flight above 1 500 ft (with TAT < 10 °C) and the engine anti-ice is off.

SEVERE ICE DETECTED — per the FCOM severe-ice procedure:

This alert triggers when heavy ice is detected in flight above 1 500 ft (with TAT < 10 °C) and the WING ANTI ICE pb-sw is set to OFF or ENG X ANTI ICE pb-sw is set to OFF.

Read the gradient: one count with engine anti-ice off → a prompt to switch the engines on; seven counts with anything still off → an upgrade telling you to arm everything. And notice the shared grammar — "…and the anti-ice is off". These alerts do not announce ice; they announce ice while your configuration disagrees with the recommendation. Switch on, and they have nothing left to say.

The 5 mm figure closes the loop planted in article 01: the limitations chapter defines severe ice accretion as approximately 5 mm; the detector translates that into seven elementary detections (wing-tip equivalent); the abnormal procedures translate it into the minimum-speed package. Definition → sensor → procedure, one number through three layers. SEVERITY also carries a reset clause that explains a triggering subtlety:

The SEVERITY signal is reset and its processing is inhibited as long as the ice detectors receive an input indicating that the wing anti ice is supplied from the aircraft.

SEVERITY's mission is to talk you into wing anti-ice — once WAI air is flowing, the counter zeroes and holds its tongue, and starts counting afresh only after you switch WAI off. (This is why the wing-side control relays feed a status signal to the detectors — the handshake noted in article 02.)

4. Redundancy and fault grading: one mute is silent, two mutes make a caution

Per AMM 30-81-00, The system is composed of two ice detectors, for redundancy — and the voting is an OR: The ICE and SEVERITY signals used for the warning messages can be provided by one ice detector or (and) the other. Either detector's count stands. (Contrast the bleed-leak loops' AND voting in the ATA-36 leak article: ice detection would rather cry wolf than miss a wolf; leak detection would rather miss a beat than shut down half the network — voting logic follows consequences.) Fault handling:

A FAULT signal is sent when a fault is detected. In this case, the ICE and SEVERITY signals are inhibited.

A detector that doubts itself goes mute rather than babble. The grading:

the crew with a caution message (class 1 fault) when both ice detectors are failed - the maintenance crew with a class 2 fault message indicating that one ice detector has failed

[!warning]- One dead detector is invisible from the cockpit Single failure: no ECAM, no memo change — just a maintenance-class message in the central maintenance system. Defensible (the second unit covers), but the teaching point cuts the other way: when A.ICE DETECT FAULT does appear, you are already fully blind, not half blind. The procedure's answer is to hand the job back to you — ANTI ICE AS RQRD, by the article 01 definitions — and the dispatch-with-it story ends the same way: periodic visual checks of the windshield and wiper areas (article 13).

5. Three inhibitions, one number dispute, and the MEMO's three voices

Per AMM 30-81-00:

The advisory ice detection system is inhibited on the ground in weight-on-wheels configuration (in relation with LGCIU) because the ice detectors need a minimum relative airspeed, and below 1500 ft, which corresponds to FWC level 2 alerts inhibition phases. The ICE and SEVERITY signals are inhibited when the TAT is above 8°C to be in accordance with the Flight Manual which states that the icing conditions can only exist below 8°C and if moisture is present.

Three gates, three reasons: no airflow, no ice collection (ground); no distractions in the busiest 1 500 ft (standard alert-inhibition phasing); no nagging in air too warm to ice. But note the number dispute worth teaching honestly: the AMM's inhibition line says 8 °C, while the FCOM limitations definition and both alert triggers say 10 °C (TAT < 10 °C). This series follows its standing convention — teach the FCOM's 10 °C as the operational figure and flag the AMM's 8 °C as the documented inhibition value. Practical formula: answer 10 in the exam; remember 8 when the system stays silent at 9.

The MEMO speaks three ways, two of them on timers. Steady green WING A.ICE / ENG A.ICE — you selected it (the ENG line also appears if a valve fails open on electrical loss, article 03). ICE NOT DET — per the FCOM controls text, it appears in green if ice is not detected for 130 seconds after the pushbutton is set on: "you armed it, and I haven't seen ice." And the pulsing exit cue, per AMM 30-81-00:

When icing conditions are no longer detected, the two MEMO messages WING A. ICE and ENG A. ICE begin to pulse, advising to select the ANTI ICE/ENG 1 and 2 pushbutton switches and the ANTI ICE/WING pushbutton switch off.

The pulse's own timer sits in the AMM system description: the messages pulse when the pushbuttons are set and ice has not occurred for 190 seconds. Two numbers, two behaviours, no conflict: 130 s governs "it never showed up" (ICE NOT DET appears); 190 s governs "it went away" (the memos start pulsing). Both ride on the 60-second signal-hold skeleton of §3. Cockpit shorthand: 130 — nothing came; 190 — show's over.

6. The titanium bolt and the night lights

Per FCOM DSC-30-70-10, An external visual ice indicator, which is visible to the crew, is installed between the two windshields. Its true identity, per AMM 30-81-00:

It is installed in lieu of one windshield center-post retainer-bolt. It is visible by both pilots. Its illumination is controlled by the ICE IND &/STBY COMPASS switch located on the INT LT section of the panel 215VU. The indicator body is made of titanium.

A bolt-shaped ice magnet in the crew's eyeline — the most primitive instrument on the flight deck, and legally the most senior. The adverse-weather procedures name it as evidence: per the FCOM supplementary procedures, Evidence of ice accretion can be ice on the visual indicator (between the two cockpit windshields) or on the windshield wipers. Those two sightlines — plus the SEVERE ICE DETECTED alert — are the three tokens that convert wing anti-ice from may to must in article 10. For the dark, the exterior WING switch throws two beams per side — per FCOM DSC-33-20-20, These lights provide lighting on the wing leading edge and on the engine air intake to detect ice accretion.

7. The blind spot: ice crystals

The FCTM's weather chapter draws the boundary of everything above:

In addition, in areas of ice crystals, the flight crew should not expect significant icing of the airframe. This is because ice crystals bounce off cold aircraft surfaces.

This is why even the ice detection system does not detect ice crystals, because ice crystals do not build up on ice detectors and visual ice indicators.

[!warning]- The most dangerous ice is the kind neither the probes nor your eyes will show Ice crystals don't stick to cold skin — they ricochet. Where they do stick is on warm surfaces deep inside the engine, melting and refreezing — with engine vibration, power loss, damage or air-data icing as the harvest. The detector's silence certifies only the supercooled-droplet threat; it testifies to nothing about crystals. The human-sensor course — the windshield "rain" with its hiss, ice grains on the wipers, and above all the TAT indication pinned near 0 °C — is article 10's six-clue list.

Self-test

[!note]- Q1. Chain 40 kHz, 133 Hz and 0.5 mm into one causal sentence — and what happens at the trip point?

The probe resonates magnetostrictively at 40 kHz; accreted ice's mass drags the frequency down; a 133 Hz drop corresponds to about 0.5 mm of ice and scores one elementary detection — then internal heaters clean the probe to count again.

[!note]- Q2. What exactly is the SEVERITY signal, and why can SEVERE ICE DETECTED only appear with wing anti-ice off?

Seven elementary detections, calibrated to roughly 5 mm on the wing tips — the LIM definition of severe ice made electronic. While the detectors sense wing anti-ice air being supplied, SEVERITY is reset and inhibited; only with WAI off does the counter run.

[!note]- Q3. One ice detector fails in cruise. What does the crew see?

Nothing — a single failure is a maintenance-class message only. The cockpit caution (A.ICE DETECT FAULT) means both are gone: fully blind, back to judging icing conditions by definition and by eye.

[!note]- Q4. Name the three inhibitions and the two temperatures in tension.

Weight-on-wheels (the probes need airflow), below 1 500 ft (alert-inhibition phase), and TAT above 8 °C per the AMM — against the FCOM's 10 °C definition and trigger figure. Teach 10 as the operating number; expect silence between 8 and 10.

[!note]- Q5. Decode the MEMO's three voices and their two timers.

Steady WING/ENG A.ICE: selected (ENG also shows for a fail-open valve). ICE NOT DET at 130 s: armed but nothing seen since switch-on. Pulsing at 190 s without ice: conditions have ended — consider switching off. "130 — nothing came; 190 — show's over."

[!note]- Q6. Why do ice crystals defeat both the detectors and the visual indicator, and where is the real danger?

Crystals bounce off cold surfaces instead of accreting — nothing builds on the probe or the bolt. They melt and refreeze on warm engine internals: vibration, power loss, damage, probe icing. The detector's all-clear covers supercooled droplets only.

Key takeaways

References

Advisory character, non-control statement, twin probes and visual indicator per FCOM DSC-30-70-10; alert triggering conditions per the FCOM anti-ice and severe-ice abnormal procedures; evidence sentence per the FCOM adverse-weather supplementary procedures; wing-scan lighting per FCOM DSC-33-20-20; ICE NOT DET memo timing per the FCOM anti-ice controls text. System purpose including fuel saving, detector location, magnetostrictive principle and probe construction, trip-point calibration, ICE/SEVERITY definitions and hold times, WAI-driven SEVERITY reset, fault grading and signal inhibition, OR voting, inhibition conditions with the 8 °C rationale, pulsing advisory and the 190-second figure, and the lighted-indicator installation per AMM 30-81-00 and AMM 30-00-00 (Description and Operation). Ice-crystal behaviour per the FCTM weather-radar chapter. The counting-coins framing, the voting-logic contrast with leak detection and the "teach 10, remember 8" formula are integrative syntheses of the referenced material.

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.