Air Data Reference: Probes to Parameters

The previous article built the skeleton. This one follows the blood: from a column of pressure at a probe orifice to the speed tape and the altitude window on the PFD — what corrections, conversions, and thresholds sit in between. Every link in the chain hides an exam point. Why does the ND drop TAS at low speed? Why does extending flaps change the static-pressure correction? Why is the maximum-allowable-speed line a curve that bends downward with altitude?

More importantly, only by mastering this chain does the unreliable-airspeed handling later in the chapter become reasoning rather than recall: once you know how a number is computed, you know how it can lie.

1. Five computations and three aircraft-specific corrections

Per AMM 34-13-00:

The ADR software performs five basic computational elements which are under the air data calculations as follows: - pressure altitude functions (ALT/ALT rate) - Mach calculation (M) - airspeed calculation (CAS/TAS) - temperature calculation (SAT/TAT) - output signal processing.

Beyond those five sit three aircraft-specific corrections: static-source error correction (SSEC), angle-of-attack correction, and maximum operating speed (VMO/MMO). "Aircraft-specific" means they are drawn from data table 1 by an aircraft/configuration recognition code — the same ADIRU part number installed on a different type or configuration selects different correction laws. The ADR also monitors itself continuously. Per AMM 34-13-00:

The continuous monitoring detects and annunciates faults in the ADR during normal operation. Faults are stored in Non Volatile Memory (NVM) BIT and sent to the Central Maintenance System (CMS) via digital words.

2. The static path — average, correct, become altitude

The two static ports are first digitised individually at the ADM layer (on the channel-3 side the averaging happens inside the ADM, since that side has only two ADM buses). The software then averages the left and right statics into Psm and applies the SSEC. Per AMM 34-13-00:

From the average static pressure (Psm), the software calculates the corrected static pressure based on the SSEC factor:

The relationship is Psc = Psm × (1 + G1/1000 + G2/1000). The two gains come from tables. Per AMM 34-13-00:

- G1 depends on flaps and Mach is interpolated in a table (2 flaps position and 19 Mach values are considered) - G2 depends on Mach and AOA corrected and is interpolated in a table (6 AOA values and 18 Mach values are considered).

Why correct static pressure at all? A static port measures the local pressure at the orifice; the fuselage bends the flow so that local pressure ≠ free-stream static, and the error varies with configuration (flaps), speed (Mach), and attitude (AOA) — which is exactly the three-variable table above. The fact that extending flaps changes the static correction also explains why the SFCC appears in the ADIRU's discrete-input list: flap position changes the AOA correction law and the SSEC gain G1. The corrected static then becomes altitude. Per AMM 34-13-00:

The altitude is derived from the corrected static pressure. The conversion from pressure to altitude is based on the geopotential altitude tables of the US Standard Atmosphere, 1962. The accepted range is (-2000 ft, +50,000 ft).

Altitude rate is not a simple differentiation of altitude but a chain rule. Per AMM 34-13-00:

The altitude rate is calculated by multiplying the rate of change of the corrected static pressure by the derivative of altitude with respect to corrected static pressure:

Note, though, that the vertical-speed tape you normally read on the PFD is the IR's baro-inertial VS (next article); the pure barometric altitude rate is only its stand-in after the IR is lost.

3. The pitot path — total-pressure correction and the CAS/Mach thresholds

Total pressure gets its own correction (Pt = Ptm + (Ptm − Psm)/Psm × 0.38) — the pitot orifice is likewise inside the fuselage flow field, and the correction scales with the dynamic-pressure ratio. Then, per AMM 34-13-00:

The CAS and Mach computation is based on the conversion of the impact pressure: impact pressure = Pt - Ps.

CAS converts through the standard sea-level speed of sound (661.4746 kt) and standard pressure (1013.25 hPa); Mach uses Qc/Ps. Two thresholds are hard-wired into the output logic. Per AMM 34-13-00:

For accuracy reason, the CAS is output with a valid (N.O) SSM only after 30 kts. If the CAS is below 30 kts, the label 206 indicates 0 kt with SSM=NCD.

If Mach is below 0.1, the label 205 indicates 0 with SSM=NCD.

That is the mechanism behind the empty speed tape for the first few seconds of the take-off roll: not a fault — below 30 kt the dynamic pressure is too small to measure accurately, so the ADR simply does not output. The speed pointer comes alive only from 30 kt, and the take-off callout chain ("BELOW 80 kt", "ONE HUNDRED KNOTS") begins downstream of that threshold.

4. The temperature chain — TAT → SAT → TAS, subtracting its own body heat

The TAT platinum-resistance element, after lead-resistance compensation (subtracting 1.04 Ω), converts to temperature via the Callender–Van Dusen equation. But the probe is heated, and heating drives the reading high, so, per AMM 34-13-00:

The computation also compensates the TAT value for heating effect. The ADR knows when the sensor is heated by the state of the TAT Heat input discrete from the PHC.

The compensation law, supplied by the sensor manufacturer, is a function of Mach and air density — fast, high-altitude flow carries away more heat, so the compensation shrinks. That also explains the ice-and-rain chapter's rule that TAT is not heated on the ground (with no airflow, the compensation law cannot cover it). The same page carries a cross-chapter detail: once a probe-heat fault discrete reaches the ADR, per AMM 34-13-00:

In this case the ADR operates as a relay which sends these information signals to the FWC for warning purposes.

So the A.ICE probe-heat cautions of the ice-and-rain chapter actually route their warning through the ATA-34 ADR.

SAT and TAS are pure formulas: SAT = TAT/(1 + 0.2 M²) − 273.15; TAS = 38.96695 × M × √SAT (kelvin). The threshold, per AMM 34-13-00:

For accuracy reason, the TAS is output with a valid (N.O) SSM only when CAS is above 60kts.

An invalid TAS cascades to the IR's wind computation (wind = GPIRS ground-speed vector − TAS vector; below 100 kt TAS the IR does not compute wind at all). That is the full reason chain for the blank wind readout on taxi: CAS < 60 → TAS = NCD → wind = NCD.

5. The AOA chain — one resolver working, one on standby, dead below 60 kt

Per AMM 34-13-00:

The A/C is wired in the AOA unique selection, so the ADR computes the AOA with the reading of one resolver. The second resolver is used as a back-up.

The correction is AOAc = AOAind/K + I, where K and I depend on system number and slat/flap configuration — the SFCC's four discretes select among three correction laws (all open = normal law, 1A+2A grounded = alternate law 1, all four grounded = alternate law 2). The low-speed threshold mirrors CAS. Per AMM 34-13-00:

For a CAS less than 60 kts: AOAc = 0 and status matrix is coded NCD.

At low speed the vane hangs down and measures the direction of gravity, not the airflow, so the ADR would rather output nothing. The angle-of-attack protection of the flight-control chapter and the BUSS "speed" of the abnormal chapter both stand on this ±0.3° vane.

6. Baro correction — the FCU's two buses and two altitudes

Per AMM 34-13-00:

The ADR receives the baro correction introduced on the FCU by each crew member. The ADR uses the values in millibars (labels 234 and 236 from the FCU).

The corrected altitude is calculated by a shift of the pressure altitude to a value corresponding to the entered baro correction.

Note the architecture: each ADR receives both the CAPT and the F/O baro settings, computing label 204 (captain-side corrected altitude) and label 220 (first-officer-side corrected altitude) separately, and converting to inHg for return. So "the two sides set different baro" does not confuse any single ADR, but it does trip the FWC comparison monitors — the source of the NAV BARO REF DISCREPANCY (STD/QNH mismatch) and NAV BARO VALUE DISAGREE (value mismatch) cautions later. The FCU's label 275 also broadcasts each side's hPa/STD/QNH/QFE selection state — the bus trace of the QFE option, which is fitted in the configuration covered here.

7. VMO/MMO — a red line that bends, and a gear-down switch

Label 207, "maximum allowable airspeed," is the data source for the PFD barber pole. Per AMM 34-13-00:

The software determines the normal or alternate VMO/MMO law selected by the VM0/MM0 input 2 discrete state; then computes the label 207 Maximum Allowable Airspeed. This airspeed is based on the Maximum Allowable Airspeed under a certain altitude and on the Maximum Allowable Mach above this altitude. It is always indicated in knots and decreases when the altitude increases.

"Decreases when the altitude increases" is the mechanism behind the barber pole descending during a constant-Mach climb: the same M0.86 corresponds to a lower CAS the higher you go. The basic law is 330/.86; the alternate law (ALT 2) is 255/0.60 — activated by whom? Per AMM 34-13-00:

The normal position of this switch is open and it is grounded after crew action for particular flights (ferry flight for example).

That switch is the landing-gear-down VMO/MMO selection — for a gear-down ferry flight it pushes the whole barber pole down to 255/.60 (consistent with the QRH OVERSPEED note "255/.60 in case of dispatch with landing gear down").

8. The speed/Mach maximum-difference table — check before you write it up

The FCOM ADIRS controls-and-indicators section gives a "TSM value" table. Per FCOM DSC-34-10-10-20:

The following table details the TSM values used to determine if a maintenance task is due when a difference between Speed/Mach indications is observed.

They are provided in order for flight crews to determine if indeed maintenance actions should be requested.

(ISIS Mach is not displayed below M0.50 in climb / M0.45 in descent — a table footnote.) Reading it: at FL300, ADR 1 and ADR 2 differing by 5 kt exceeds the 3-kt criterion — write it in the log; ISIS and an ADR differing by 6 kt is within the 8-kt tolerance — normal scatter. This is the "sensory health chart," the twin of the RVSM altitude-difference table (the 55–145 ft family) later — one governs speed, one governs altitude.

The ADR also runs a denser background health net (maintenance words): ADR-vs-EEC Ps/Pt/TAT consistency, ADR-vs-ISIS CAS/altitude consistency, ADR-vs-GPS altitude-rate comparison, L-R static failure, even a "bird-strike monitor" bit and "gear/ground-effect/reverser correction applied" bits. A single ADR's view of the world is far richer than the PFD shows; these bits normally reach only the maintenance system, but they are the raw material behind the NAV IAS DISCREPANCY / ALT DISCREPANCY comparison cautions.

9. BUSS — when all three ADRs are untrustworthy, fly a different physical quantity

BUSS (Back-Up Speed Scale) is the fitted, non-reversible "last speed indication." Per FCOM DSC-34-10-10-30-10:

The BackUp Speed Scale (BUSS) enables to fly the aircraft when airspeed indications are unreliable.

The BUSS is displayed on both PFDs when the flight crew turn off all ADRs.

The activation of this BUSS is not reversible.

The BUSS information is based on the angle of attack (AOA), and depends on the slat/flap configuration.

The AMM makes the mechanism explicit:

The CAS usually displayed on the PFD is replaced by the median value of the three AOA coming from the ADRs via the IR bus.

Three points to chew on. First, the quantity it displays is not speed at all but an energy margin derived from angle of attack — so the scale is not numbers but five colour bands (red/amber FAST, green, amber/red SLOW, plus a green target arrow); "too low an AOA = too fast":

Too low an AOA means too fast an airspeed, and vice versa.

Second, the data travels the IR bus — the ADRs are off, but the raw AOA values keep flowing through the inertial section's bus (which is why data survives all-ADR-OFF). Third, it takes the median, not the average — pick the middle of three, and a single mad vane cannot drag the result. Altitude re-sources in step. Per FCOM DSC-34-10-10-30-10:

Since the GPS altitude is less accurate than the barometric altitude, the last two insignificant digits are dashed.

The vertical-speed indication also disappears. The associated ECAM caution (NAV BKUP SPD/ALT ON …) belongs to the reversible BUSS variant with a pushbutton; in the non-reversible configuration covered here the trigger is simply "push all three ADR pushbuttons off." The procedure and operating discipline (no speed brakes, change configuration wings-level, the green-band edge technique, "if it does not respond to stick input, disregard it") are developed in the unreliable-airspeed article. The mechanism to fix here: BUSS lives on AOA, so anything that breaks the AOA-to-speed relationship — extending the speed brakes, for one — lies to it.

Key numbers

The three NCD thresholds in one line: 30 kt makes speed live, 60 kt makes AOA and true airspeed live, Mach 0.1 makes Mach live.

Self-test

[!note]- Q1. On what variables do the SSEC gains G1 and G2 interpolate, and why does extending flaps change the static correction? G1 on flaps + Mach; G2 on Mach + corrected AOA. Flaps change the fuselage flow field around the static ports, so the static-source error — and its correction gain G1 — changes with flap position.

[!note]- Q2. Accelerating through 25 → 45 → 70 kt, when do the speed tape, the ND TAS, and the wind readout each come alive? Speed tape at 30 kt (CAS threshold). TAS and wind require CAS > 60 kt (TAS threshold); wind additionally needs TAS ≥ 100 kt. Below those, each is NCD by design.

[!note]- Q3. Who supplies the TAT heating-compensation law and on what does it depend? How does that relate to "no TAT heat on the ground"? The sensor manufacturer; it is a function of Mach and air density. On the ground there is no airflow, so the compensation law cannot cover the self-heating — hence TAT is not heated on the ground.

[!note]- Q4. On a gear-down ferry flight, where does the barber pole sit, and what pushes it there? 255/.60. The landing-gear-down VMO/MMO selection switch (normally open) is grounded by crew action, selecting the alternate VMO/MMO law.

[!note]- Q5. What physical quantity does BUSS actually display, why the median, and why the IR bus? An energy margin derived from AOA, not speed. Median of three AOA so a single failed vane cannot skew it. Data travels the IR bus because the ADRs are off but the inertial section still passes the raw AOA.

Key takeaways

References

• FCOM DSC-34-10-10-20 — speed/Mach maximum-difference (TSM) table.
• FCOM DSC-34-10-10-30-10 — BUSS definition, non-reversible activation, AOA basis, GPS altitude.
• AMM 34-13-00 — the five computations, SSEC, CAS/Mach/TAS/SAT, AOA correction, baro correction, VMO/MMO, output thresholds, monitoring, output accuracy table.

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.