Tyre Pressure Indication

The Tyre Pressure Indicating System (TPIS) monitors each wheel's pressure automatically and shows it continuously on the WHEEL page. Per AMM 32-49-00:

The TPIS automatically monitors the L/G tire pressures and continuously shows the values on the WHEEL page of the Systems Display (SD). The system includes an electronic sensor in each wheel and a Tire Pressure Indicating Computer (TPIC). The TPIC controls the operation of the system... It supplies data and warnings when the pressures are unsatisfactory.

(The AMM calls the box the TPIC; the FCOM calls it the TPIU — the same unit. This article uses TPIC.)

This article covers how it measures (strain gauge + temperature compensation), how it carries the signal off a turning wheel (a rotary transformer), how it judges "low" (against a datum, with thresholds that change with flight phase), and when the TYRE LO PR warning is suppressed.

 Measure (per wheel)        Transmit (per axle)        Compute            Display
 ──────────────────         ───────────────────        ───────            ───────
 sensor (strain gauge) ───► transmitter ────────────► TPIC 2GV ─────────► WHEEL page (SD)
 · wheatstone bridge        (rotary transformer:        (= FCOM TPIU)       pressure values:
 · temperature compensated   stator on axle /           · 520 ms full scan  · green = satisfactory
 · 0–100 mV (0–300 psi)      rotor on the wheel)        · 100 samples/wheel  · amber = unsatisfactory
 · replaceable on a live tyre                            · compares to datum  E/WD: TYRE LO PR
 datum sets: OBRM holds 16, ARINC 600 pin selects the A330 set                + MASTER CAUT + single chime
 thresholds change with flight phase (CMC ARINC label 126)                    (suppressed take-off 80 kt–1500 ft / landing touchdown–80 kt)

1. The sensor — strain gauge, temperature-compensated, replaceable live

Per AMM 32-49-00:

The pressure sensor head is a strain gage type and contains a wheatstone bridge and other resistors for temperature compensation and slope rectification... The air pressure in the pressure sensor head when it changes, changes the resistance across the bridge. This changes the output voltage. Resistors in the circuit compensate for temperature changes. The output signals are between 0 and 100 mV (0-300 psi).

Why temperature compensation: the strain gauge's own resistance drifts with temperature, so the compensating resistors subtract the sensor's drift so the output reflects only the true pressure. (Note it compensates the sensor's drift, not the real thermal expansion of the gas — a genuinely higher pressure from heating is genuinely measured.)

A practical design — the sensor can be replaced on an inflated tyre. Per AMM 32-49-00:

Because the leakage of pressurized air through the sensor holder is small, it is possible to replace the sensor on an inflated tire. The operator can install a blanking cap on the sensor holder to prevent loss of tire pressure, when the sensor is not installed.

2. The transmitter — a rotary transformer across the spinning interface

The wheel turns, the airframe does not — so how does the signal get out? A transmitter (a rotary transformer). Per AMM 32-49-00:

The transmitter has two main parts, a stator and a rotor. The stator (primary windings) attaches to the carrier in the axle... The rotor (secondary windings) turns in the stator and connects to the sensor flying-lead. The rotor includes an electronic module. The module rectifies the 3.125 kHz power supply signal and gives a stable 10V DC voltage to the pressure sensor.

Energy and signal cross the rotating interface by electromagnetic coupling — contactless, with no slip-ring wear. The sensor output is amplified to a 60–166 kHz sine wave back to the TPIC.

3. Measurement rhythm — 520 ms scan, 100 samples averaged

Per AMM 32-49-00:

The sequence to power all of the sensors is continuous and takes 520ms. During the time a sensor is energized, it measures the tire pressure approximately 100 times. The average tire pressure value from these values is then sent four times to the TPIC.

The wheels are polled one at a time (not simultaneously), each averaged over ~100 samples to suppress single-measurement noise, refreshing the whole aircraft's tyre pressures every half-second.

4. Judging "low" — against a datum, thresholds by flight phase

The TPIC does not judge low pressure against a fixed psi value; it compares against a datum as a percentage difference, and the threshold changes with flight phase. Per AMM 32-49-00:

the TPIC compares the measured values from the sensors with the datum pressure values in the OBRM. The OBRM program specifies a maximum percentage for the difference between the measured value and the datum value. If the difference is more than the specified percentage the TPIC sends data which causes the measured value to show in amber... These specified percentages change during different flight phases.

The thresholds (AMM Table 1):

The design:

• Ground phases (1–4) are tight (11% / 15%): catch any low tyre before take-off — low tolerance.
• Flight phases (5–10) are looser (26% / 21%): pressures change with temperature and retraction after take-off, and nothing can be done about it then, so the threshold is relaxed to avoid false warnings.

Flight-phase information comes from the CMC (ARINC label 126). The datum values themselves: the OBRM holds 16 sets, and a pin on the ARINC 600 connector selects the A330 set — so the same TPIC hardware serves different types/configurations, with the datum chosen by a pin.

5. Indication and warning suppression

Per AMM 32-49-00:

When one tire pressure value or the tire pressure difference on one axle is unsatisfactory: the MASTER CAUT lights comes on / the E/WD shows TYRE LO PR / an aural warning is given (single chime). The warning messages are not shown: during a takeoff (between a speed of 80 knots and an altitude of 1500ft) / during a landing (between touch down and a speed of 80 knots).

Why those two windows are suppressed: take-off from 80 kt to 1500 ft, and landing from touchdown to 80 kt, are high-workload phases in which the tyre pressure cannot be acted on — a TYRE LO PR pop-up would only distract. The warning is deferred to when it can be handled. Note the WHEEL-page amber is not suppressed (it shows continuously); only the E/WD warning and chime are suppressed.

[!warning]- Five misconceptions this article corrects (1) The TPIS does not judge low pressure against a fixed psi — it compares against a datum as a percentage, with the threshold changing by flight phase (tight on the ground, loose in flight). (2) Temperature compensation does not convert a thermally-expanded pressure back to a cold value — it compensates the sensor's own drift; a genuinely higher pressure is genuinely measured. (3) Replacing a pressure sensor does not require deflating the tyre — the holder leaks little, so it can be replaced on a live tyre (with a blanking cap when no sensor is fitted). (4) A low tyre during take-off/landing does not immediately give TYRE LO PR — the warning is suppressed (take-off 80 kt–1500 ft, landing touchdown–80 kt), though the WHEEL-page amber remains. (5) It does not only monitor a single wheel's absolute pressure — it also monitors the difference between same-axle tyres (a slowly-deflating wheel first shows up as a difference).

Self-test

[!note]- Q1. What type is the sensor, why temperature-compensate, and what exactly is compensated?

A strain-gauge type with a wheatstone bridge. Temperature compensation is needed because the gauge's own resistance drifts with temperature; the compensating resistors subtract the sensor's drift so the output reflects only the true pressure. It does not convert thermal expansion to a cold value.

[!note]- Q2. How does the signal get off the turning wheel?

Through a transmitter that is a rotary transformer: the stator (primary) attaches to the axle, the rotor (secondary) turns with the wheel and connects to the sensor. Energy and signal cross the rotating interface by electromagnetic coupling — contactless, with no slip-ring wear.

[!note]- Q3. The two low-pressure thresholds change with what, and how do ground and flight differ?

With flight phase. Single tyre below datum: 11% in phases 1–4 (ground), 26% in phases 5–10 (flight). Same-axle difference: 15% then 21%. Tight on the ground to catch any low tyre before take-off; loose in flight because pressures change and cannot be acted on, avoiding false warnings.

[!note]- Q4. When is TYRE LO PR suppressed, and why?

During take-off (80 kt to 1500 ft) and landing (touchdown to 80 kt) — high-workload phases in which the tyre pressure cannot be acted on, so the warning would only distract and is deferred. The WHEEL-page amber is not suppressed; only the E/WD warning and chime are.

[!note]- Q5. How is the datum set chosen among the 16?

The OBRM holds 16 datum sets, and a pin on the ARINC 600 connector selects the A330 set — so the same TPIC hardware can serve different types/configurations.

Key takeaways

References

A330 specifics per AMM 32-49-00 (TPIS — strain-gauge sensor with temperature compensation and live replacement, the rotary-transformer transmitter, the 520 ms / 100-sample averaging, the datum comparison, the flight-phase threshold table (11/26/15/21%), TYRE LO PR with its suppression windows, the OBRM 16-set datum selection) and FCOM DSC-32-40 (the three components — sensor / transmission unit / TPIU — and the green/amber ECAM caution). The system diagram and threshold table are integrative syntheses of the AMM text and Table 1. The absolute datum psi values belong to AMM servicing / FCOM limitations; the ARINC 429 bit tables and part-level circuits are maintenance-layer. The handling of TYRE LO PR and the thermal fuse plug are in Wheel and Tyre Damage.

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.