Engine Parameter Sensing

In the preceding articles the sensors played supporting roles; here they take the lead. The job of this article is to trace every engine number on the E/WD and SD upstream to where it is physically born. The payoff is interpretive: only when you know how a parameter is measured do you know when it can lie to you — single-element freezing, trim standardisation, differential correction. The threads planted across the earlier articles gather here into one master table.

1. The master sensing table

The indication block diagram (read directly) confirms three things at the equipment-number level: the once-per-rev sensor is an independent unit, a separate item from the EEC's dedicated alternator — the phase reference has its own hardware identity, not a signal skimmed off a speed probe; the IP turbine overheat detection uses two thermocouples, one forward and one aft of the disc — redundancy by position, bracketing the part it protects; and the convergence pattern matches the table — three N1 and three N2 probes into the OPU first, N3 via the PCU, eleven EGT thermocouples, the vibration RCC into the EIVMU.

2. Three spool speeds, three ways of measuring

"The two phonic wheels each have 60 teeth. There are three speed probes for each phonic wheel. Each speed probe has a magnet and a coil … When a tooth goes through the magnetic force of a probe, it causes an electrical pulse in that probe. The frequency of the pulses is in proportion to the speed of the shaft. This output is sent to the OPU."

The elegance of sixty teeth (synthesis): sixty pulses per revolution means the pulse frequency in hertz equals revolutions per second times sixty — numerically, revolutions per minute. No conversion arithmetic at all. The three measuring methods contrast neatly. N1 and N2 are measured "properly" — phonic wheel, three probes each, and the signals go to the OPU first: the bodyguard uses them, selects two healthy ones, and hands those to the EEC (article 04). N3 has no dedicated probe at all — it is embedded in the alternator's output frequency (the rotor is on the HP shaft; frequency is speed), which is why N3 and FADEC power live or die together. And the LP turbine carries its own additional phonic wheel at the turbine end — one wheel at the fan end (FBH), one at the turbine end, and the moment their readings split apart, the LP shaft has broken: the complete hardware picture behind the TOS comparison of article 05.

3. EGT: eleven dual thermocouples and two dissimilar-metal harnesses

"The EGT indicating system uses 11 dual thermocouple assemblies to measure EGT. … The EGT is trimmed through the Data Entry Plug (DEP)."

"Dual" means each of the eleven assemblies contains two independent thermocouple elements — one per EEC channel, so channels A and B each own a complete eleven-point average and borrow nothing. The harness names give the measuring principle away: one chromel harness and one alumel harness — the two dissimilar alloys of a K-type thermocouple. The hot junction is welded inside the NGV; the harness itself is the thermocouple's extended leg, which is why it must run the same alloy end to end — splice in ordinary copper and the junction would generate a spurious voltage of its own. The EEC digitises the average and applies the DEP's three-point calibration (850/900/920 °C, article 04) to produce the standardised cockpit value.

4. The vibration chain: one ear listening to three shafts

"Monitoring is performed by a vibration transducer on each engine. … Electrical signals are sent from the vibration transducer to the RCC (Remote Charge Converter) through the vibration junction box. The signal is then amplified by the RCC to be sent to the EIVMU (Engine Interface and Vibration Monitoring Unit)."

One accelerometer per engine, on the intermediate case. How does a single ear report three separate vibration values? The answer is in the EIVMU's signal processing: it holds all three spool speeds (from the EEC's bus), the once-per-rev phase reference (its dedicated FBH probe), and the raw accelerometer signal — and performs narrow-band tracking filtering. The vibration of an unbalanced shaft occurs at that shaft's own rotation frequency, so "listen only at the N1 frequency" yields N1 VIB, and likewise for N2 and N3 — a tuned receiver stepping through three stations (synthesis). The display scaling is defined precisely:

"The display between 0 and 10 units depicts a vibration between 0 and 3 ips (linear)."

The cockpit "unit" is a dimensionless engineering value: 1 unit = 0.3 inches per second of vibration velocity. The RCC exists because a piezoelectric accelerometer outputs a faint charge signal that must be amplified close to the source before travelling to the avionics bay (synthesis). And once-per-rev has a second career: it records the phase angle of the imbalance for N1 trim balancing, the data maintenance uses to place balance weights on the fan (article 29).

5. Turbine overheat detection: measuring the cooling air, feeding the IPTOS

"The system measures the cooling air temperature on the two sides of the IP turbine disk. If there is a failure of the cooling air system, the air temperature around the disk can become too high." — "The IPTOS function will limit IP shaft speed, such that the terminal speed will remain below the blade release threshold in the event of a subsequent IP shaft failure."

Two counter-intuitives in two sentences. First, the detectors do not measure the disc itself — they measure the cooling/sealing air on either side of the IP turbine disc (article 03's internal flows): a cooling-air failure is the precursor of disc distress, so the environment is watched rather than the patient. Second, the AMM places the IPTOS in the overheat-detection chapter: a deteriorating disc environment is exactly the context in which IPTOS clamps the shaft speed so that even if the IP shaft subsequently fails, the runaway turbine stays below the blade-release threshold — the hardware logic beneath the 30 %-or-shutdown protection of article 05 and the TURBINE OVHT alert of article 28. Two dual thermocouples bracket the disc, fore and aft — measure both sides, know the middle.

6. The B-channel single-element list — and an interpretation discipline

Gathering the discoveries of earlier articles: the EPR pressure transducers (article 07) and the fuel flow transmitter are wired to channel B only; channel A receives their data across the cross-channel link. The frozen-last-value behaviour of article 06 therefore applies to exactly these parameters: an EPR or FF that sits perfectly motionless deserves suspicion of the data link before trust in the engine. By contrast N1/N2 (three probes), EGT (eleven dual points) and T30 (three elements) are richly redundant — the list of "parameters that can lie" is short, and knowing it by heart is enough.

Two quieter monitoring points complete the table. T25 — IP compressor exit temperature, a single thermocouple hidden in a steel sleeve inside intermediate-case vane 4 — drives no control and triggers no alert; it feeds the engine condition-monitoring programme only, the slow medical file kept alongside borescope results and MCD findings. Nacelle temperature — a single long-probe thermocouple whose sensing tube spans Zone 3 from the combustor outer case front flange rearward — watches the bay, not the engine: a bleed duct leaking hot air into Zone 3 announces itself here first (the NAC advisory on the SD, article 15). Its signal chain has one unique quirk: the analog signal is trimmed by a factor of 0.788 inside the EEC before digitisation, then output over ARINC to the DMC for display and to the FWC for alerting — the NAC figure on the SD is a calibrated conversion, not a raw electromotive reading.

7. Where the sensing chains meet the failure chapters

Self-test

[!note]- Q1. What is N3's "sensor"? There isn't one. N3 is embedded in the dedicated alternator's output frequency — the rotor rides the HP shaft, so frequency is speed — entering the EEC via the PCU. The signal and the FADEC's power are the same physical phenomenon.

[!note]- Q2. Why is one EGT harness called chromel and the other alumel? They are the two dissimilar alloys of a K-type thermocouple — the harness is the thermocouple's extended leg and must remain the same alloy end to end, or every splice would become a spurious junction. Eleven dual assemblies give each channel its own independent 11-point average.

[!note]- Q3. How does one accelerometer report three vibration values? The EIVMU applies narrow-band tracking filters tuned to each spool's own speed — an unbalanced shaft vibrates at its rotation frequency, so listening per-frequency separates the shafts. The once-per-rev probe supplies the phase reference for trim balancing. Display: 0–10 units linear over 0–3 ips, so 1 unit = 0.3 ips.

[!note]- Q4. Does the TURBINE OVHT detector measure disc temperature? No — it measures the cooling air on the two sides of the IP turbine disc: cooling-system failure is the precursor the system watches for. The same chapter ties it to the IPTOS, which clamps shaft speed so that even a subsequent shaft failure stays below the blade-release threshold.

[!note]- Q5. Which parameters are B-channel single-element, and what is the interpretive rule? EPR (pressure transducers) and fuel flow (transmitter) — both wired to channel B only, with channel A served across the cross-channel link. If that link fails, the A-side value freezes at the last reading: a perfectly motionless EPR or FF means check the data chain before believing the engine.

Key takeaways

References

• AMM 77-11 (speed sensing, D/O) — phonic wheels and probes, OPU routing, alternator-frequency N3, the LP-turbine wheel, once-per-rev to the EIVMU.
• AMM 77-21 (EGT, D/O) — eleven dual assemblies, harness alloys, DEP trim, distribution to DMC/FWC.
• AMM 77-22 (turbine overheat, D/O) — cooling-air measurement on both disc faces, the IPTOS blade-release rationale.
• AMM 77-30 / 77-32 (vibration, D/O) — transducer, junction box, RCC amplification, the 0–10-unit/0–3-ips scaling.
• AMM 77-33 / 75-41 (T25, nacelle temperature, D/O) — condition-monitoring role; Zone-3 long probe and the 0.788 display trim.
• AMM 73-31 (fuel flow indicating, D/O) — transmitter position and channel-B connection.
• ASM (engine indication block diagram, read directly) — independent once-per-rev unit, fore/aft overheat thermocouples, signal convergence.
• Integrative synthesis (marked in text): the 60-teeth arithmetic; the tuned-receiver reading of tracking filters; the charge-amplifier rationale; the measure-both-sides observation; the trust-hierarchy framing.

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.