Probe Heat and the PHCs

Wing ice costs you lift. Probe ice costs you the truth: a blocked pitot can make three air-data computers agree on the same wrong airspeed — and the industry learned the hard way that corrupted data is more dangerous than absent data. Probe heat is therefore the highest-stakes subsystem in ATA-30, and every design choice in it serves one word: independence. Four probe families, three channels, three computers. This article covers the machine; the twelve-alert failure family — including the "coherent but incorrect" trap — is article 11.

1. Four probe families, three channels, and the philosophy of three

Per FCOM DSC-30-50-10:

Electrical heating protects : ‐ Pitot probes ‐ Static ports ‐ Angle-Of-Attack (AOA) probes ‐ Total Air Temperature (TAT) probes.

The census comes from the navigation chapter — the ADIRS carries Pitot probes (3), Static pressure probes (STAT) (6), Angle of attack sensors (AOA) (3) and Total air temperature probes (TAT) (2), and per FCOM DSC-34-10-10-10, These sensors are electrically heated to prevent from icing up. Fourteen heated elements in all. Who runs them:

Three independent Probe Heat Computers (PHC) automatically control and monitor : ‐ Captain probes ‐ F/O probes ‐ STBY probes.

And the design intent in one AMM sentence — per AMM 30-31-00:

Ice protection of the probes does not affect the independence of the three detection channels (1, 2 and 3). Each channel is controlled and monitored by one Probe Heat Computer (PHC).

[!warning]- Why not one "probe heat master box"? One computer for fourteen probes would be cheaper — and a single box failure would strip heating from all three channels at once, exposing all three ADRs to icing together. The whole air-data safety case rests on the voting principle, which itself rests on failures being independent — the FCTM's phrasing is that it is highly unlikely that the aircraft probes will be obstructed at the same time, to the same degree and in the same way (article 11). One PHC per channel turns "unlikely to fail together" from a probability statement into a hardware structure. The independence runs deeper than the boxes: pitot, AOA and TAT elements heat on 115 V AC, the static ports on 28 V DC, and the PHCs themselves on 28 V DC — even the power paths refuse to share a single failure.

Each PHC's parish, per AMM 30-31-00:

The PHC controls and monitors heating of the probes given below: - one pitot probe - two static probes - one AOA sensor - one TAT sensor.

There are three PHCs, one per probe channel: - PHC 1 - PHC 2 - PHC 3 (Channel 3 does not include the TAT sensor).

[!warning]- There are only two TAT probes on the aircraft The standby channel has no TAT of its own — which is why the ECAM TAT-heat alert exists only in CAPT and F/O flavours, and why the MEL lists an installed quantity of two (article 13). The standby system's promise is the survival triad — attitude, airspeed, altitude; total air temperature serves performance and thrust management, not survival, so the third channel simply doesn't carry one.

2. Who switches it on

Per FCOM DSC-30-50-10:

The probes are heated : ‐ automatically when at least one engine is running, or in flight ‐ manually, when the flight crew switches ON the PROBE/WINDOW HEAT pushbutton switch.

The automation's senses (AMM): each PHC reads ten discrete inputs — four from the LGCIUs (two air/ground signals plus their validity bits), two from the EIUs reporting engines running, plus the ON command, a CMC test line, a reset, and a discrete tied to airspeed above 50 kt. Air/ground picks the power level (§3); engines-running arms the automatic start. The logic — and the pushbutton — are shared with window heat (article 05): one button, two customers. The manual position has an official purpose beyond habit — per AMM 30-31-00:

In case of severe icing condition on ground, automatic operation can be overriden by means of the PROBE & WINDOW HEAT pushbutton switch (6DG).

Anticipated heating: in freezing weather you warm the probes before engine start rather than after. Pushbutton semantics (FCOM DSC-30-50-20): AUTO — heated in flight, or on the ground (except the TAT probes) once an engine runs; ON — blue light, everything heated now, TAT still excluded on the ground. And the note that belongs in every cold-morning flow:

Note: Ensure that the probe covers are removed before setting the PROBE/WINDOW HEAT pb-sw to ON.

The cold-weather procedures give the blunt reason — the covers must come off to prevent them from melting onto the probes (article 09).

3. Power levels: the half-wave trick

Per FCOM DSC-30-50-10:

On the ground, the TAT probes are not heated and pitot heating operates at low level (the changeover to normal power in flight is automatic).

How "low level" is made, per AMM 30-31-00:

There are two heating levels for the pitot probe: * on the ground: half-wave heating, * in flight: full-wave heating.

Half-wave rectification simply throws away every second half-cycle of the AC supply — power halves with no transformer and no regulator, just one switching element. Why derate on the ground at all? In flight the probe sits in ram airflow and sheds heat as fast as it makes it; parked, still air barely cools it, so full power would cook the element, shorten its life — and brand the ramp agent who touches it. The TAT goes further: no ground heating at all. A total-air-temperature probe measures ram rise; on the ground there is no ram, so heating it would only cook the measurement — which is exactly why an abnormally heated TAT on the ground rates its own alert, TAT HEATED ON GND: if it's warm when it shouldn't be, the reading is already lying (article 11).

4. Monitoring: current is the thermometer

A PHC never measures probe temperature directly — it watches current. Heater elements have a known resistance; too little current means an open circuit or missing heat, too much means a short. Per AMM 30-31-00:

(a) For pitot probe - in flight when the current I is lower than 0.9A or greater than 6A - on ground when the current I is lower than 0.4A or greater than 4A. (b) For TAT sensor When the current I is lower than 0.8A or greater than 4A. (c) For AOA sensor When the current I is lower than 0.12A or greater than 5A. (d) For L and R static probes When the current is lower than 1.3A or greater than 4A.

Don't memorise the numbers — read the structure. The pitot carries two threshold sets because half-wave and full-wave draw currents a factor of two apart; a single window would false-alarm in one regime or miss faults in the other. And TAT monitoring is suppressed where TAT heating doesn't exist — per the same section, Monitoring of the TAT sensors is inhibited on the ground. The PHC identifies and memorises the faulty element for maintenance, and one robustness detail deserves its own sentence: the temperature-control function is achieved by a hardware logic device independent of the software — a life-critical heater does not die with a software crash (the window-heat computer makes the identical choice, article 05).

5. Why the fault report travels through the ADIRU

Per AMM 30-31-00:

The PHC generates 5 discrete outputs to the Flight Warning Computer (FWC) via the Air Data/Inertial Reference Unit (ADIRU)

[!warning]- A broken heater wire reports to the air-data computer first Odd routing — until you ask who needs the news most. The first customer of "this probe's heating has failed" is not the crew: it is the ADIRU itself, which must know its own data source may be icing before anyone else does. The FWC is the second customer. This wiring diagram explains the entire procedural philosophy of article 11: probe-heat alerts are answered with ADR pushbuttons — source switching and shedding — because in the architecture, heating and data were bound together from the start. The five discretes carry: TAT heating state, pitot fault, left static fault, right static fault, AOA fault; a heating-availability discrete goes directly to the SDAC, and an ARINC 429 bus reports to the CMC.

Which channel feeds which instruments is settled by one QRH sentence that becomes the cornerstone of every source-switching decision:

ADR3 and STBY speeds use the data of the same probe.

The navigation abnormals extend it: ADR3 and STBY speeds use the data of the same probes. Therefore, standby instrument must be used with care. When the standby pitot's heating dies, "just fly the standby instruments" is not an escape — the standby indications and ADR 3 are poisoned as a pair. Teach it as three words: the standby isn't.

6. Operating it — and where the failures go

In normal service this is the chapter's only set-and-forget system: check the pushbutton at AUTO during cockpit preparation, use ON for cold-weather anticipated heating (covers off first), and never touch it again — until the ECAM calls. When it calls, the family rule from article 11 applies: first line is always PROBE/WINDOW HEAT ON, because per the procedures, In some failure conditions, probe heating may be recovered — sometimes the fault is in the command chain, not the heater. Then source-switch or shed ADRs as the ECAM names them — it can read the static/AOA/TAT heating states; you can't. Dispatch with a dead heater is article 13's business (visible-moisture fences, the chapter's only 3-day repair interval, and an "ADR considered inoperative" downgrade path). In the electrical emergency configuration, the survivors are the captain and standby pitot/AOA heaters (article 01) — and the smoke/electrical procedures then flip the usual seniority: whichever side keeps its heat keeps your trust, standby included-or-excluded accordingly (article 11).

Self-test

[!note]- Q1. How many heated probes are there, and how are they divided among the PHCs?

Fourteen: 3 pitot, 6 static, 3 AOA, 2 TAT. Each PHC runs one channel: one pitot, two statics, one AOA and one TAT — except PHC 3, whose standby channel has no TAT.

[!note]- Q2. Why three PHCs instead of one?

The air-data voting principle assumes failures don't happen together. One master box would be a common failure stripping all three channels at once; one PHC per channel — with even the heater supply types split between 115 V AC and 28 V DC — turns that assumption into hardware.

[!note]- Q3. What is half-wave heating, and why derate the pitot on the ground at all?

Passing only every second AC half-cycle — half power with one switching element. On the ground there is no ram airflow to carry heat away, so full power would overheat the element (and burn hands); the changeover to full-wave in flight is automatic.

[!note]- Q4. Why is the TAT not heated on the ground, and what does the TAT HEATED ON GND alert really tell you?

TAT measures ram rise, which doesn't exist statically — heating would falsify the reading. The alert means a TAT is being warmed when it must not be: its indication is already unreliable on the ground.

[!note]- Q5. How does a PHC detect a heating failure, and why does the pitot need two threshold sets?

By monitoring heater current against windows (too low = open circuit, too high = short). Half-wave and full-wave regimes draw currents a factor of two apart, so ground and flight need separate windows to avoid false alarms.

[!note]- Q6. Why do probe-heat procedures act on ADR pushbuttons rather than heating switches — and why must the standby instruments be used "with care"?

Because PHC fault reports route through the ADIRU: heating status and data validity are architecturally bound, so the response is a data response. And ADR3 and the standby speeds use the same probes — lose the standby pitot's heat and both are suspect together. The standby isn't.

Key takeaways

References

Protected probe families, three independent PHCs, automatic/manual activation and ground heating restrictions per FCOM DSC-30-50-10; pushbutton semantics and probe-cover note per FCOM DSC-30-50-20; probe census per FCOM DSC-34-10-10-10. Channel independence, PHC parish (including channel 3's missing TAT), severe-icing override, half-wave/full-wave levels, current-monitoring thresholds with ground TAT inhibition, ten discrete inputs, ADIRU-routed fault outputs and the hardware temperature-control device per AMM 30-31-00 (Description and Operation). Shared-probe sentence per the QRH air-data check procedure and the FCOM navigation abnormal procedures; heating-recovery line per the FCOM anti-ice abnormal procedures. The "philosophy of three", the thermometer reading of current monitoring and the "standby isn't" formulation 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.