Monitoring and Indication

Every number and every colour you read on the ELEC page arrives there from some control box's data line. Nothing on that page is generated by the display itself: the screen is the terminal of a network of sensing, and each field is the printed-out opinion of a specific box about a specific parameter. This article does two jobs that the component articles (00 through 17) left scattered. First, it pulls together the monitoring chains — who measures, through whom, and where it lands — into one panorama. Second, it turns the field-level wording of AMM 24-27 (AC page) and AMM 24-37 (DC page) into a reading dictionary: the exact green/amber thresholds, and one symbol that runs through every field — the amber XX.

The division of labour with ECAM ELEC Page is worth fixing at the outset. This article is the field dictionary — what each digit and each colour means, threshold by threshold. Article 20 is the page-shape reading — how the AC and DC pages look as a whole in the flight / ground / emergency configurations, the two MEMOs, and how the page is read against the overhead panel. Read this one to know what an amber 124 or an amber XX is telling you; read article 20 to know what the whole page looks like in EMER CONFIG.

1. Scope and boundaries

By the end of this article you should be able to answer five questions:

1. The signal-chain panorama of the ELEC page: which boxes report over ARINC 429, and which over discrete/analogue lines — and why the split?
2. What is the essential difference between an amber XX and an amber digit?
3. The green/amber thresholds of each row of the GEN field — and where does the overload amber criterion actually come from?
4. The thresholds of the DC-page battery and TR fields — and which two of them carry an "APU-start exemption"?
5. Why is the panel BAT voltmeter the only electrical instrument on the aeroplane that does not pass through ECAM — and when does it save you?

This article owns: the monitoring-chain panorama, the ELEC AC/DC page field semantics at AMM precision, and the BAT voltmeter. Left to article 20: the overall page form in the three configurations, the two MEMOs, and the panel-plus-page combined reading.

2. The monitoring-chain panorama

Reading the panorama is reading one design rule. Restated from the overview: the smart boxes report over ARINC 429 (full parameter set, with built-in BITE); the emergency and end-of-line items report over discrete or analogue lines (only "good/bad", or a single parameter). Almost every electrical parameter converges on the SDAC, which then drives the ECAM displays — so the SDAC is the single bottleneck through which nearly the whole ELEC page is built (the effect of an SDAC failure itself, with its dual-SDAC redundancy, belongs to ATA 31, not here).

The same picture as a signal flow (note that one line bypasses SDAC entirely, and one bypasses every computer):

 SMART BOXES ── ARINC 429 ──┐
  GCU 1/2   GEN load/V/freq, IDG oil temp
  GAPCU     APU GEN, EXT A/B                  ┌── SDAC ──┐
  ECMU 1/2  bus/corridor/shed (label 055)     │          │   ECAM
  BCL x3    battery V/I, arrow ───────────────┤          ├──► ELEC AC page
                                              │          │    ELEC DC page
 END ITEMS ── discrete / analogue ───────────┤          │
  CSM/G GCU   EMER GEN                        │          │
  static inverter   STAT INV                  └──────────┘
  TR cards x4   TR V/I, fault

  CBMU ── ARINC 429 ──► DMC ──► ECAM C/B page
  BAT 1/2/APU ── hardwire ──► BAT VOLTMETER (panel LCD, no computer)

Two end-of-chain consequences fall straight out of the diagram. The EMER GEN and STAT INV report only over discrete/analogue lines straight into SDAC — so on those fields you get a fault flag or a single parameter, never the rich parameter set a GCU gives. And the BAT voltmeter is wired straight off the batteries, touching no computer at all — the structural reason it survives when everything else is dark (§3).

3. The BAT voltmeter — the last instrument

The small LCD (FIN 3PV) on the overhead electrical panel, with its three-position selector (FIN 1PV: BAT 1 / BAT 2 / APU), is the only electrical instrument on the aeroplane that does not pass through ECAM:

"The voltage of one battery is permanently displayed whatever the aircraft configuration."

Per AMM 24-37-00. The weight is in "whatever the configuration": when the whole aircraft is dark — when even ECAM is gone — this display is still lit, because it is fed from the battery's own power through the hot-bus chain, not from any bus the network could lose. In the batteries-only case it is the primary instrument for watching "how much is left" (the star instrument of Battery-only Flight); and on a cold-and-dark first glance it is how you judge how much the batteries drained overnight.

A teaching way to hold the whole article: the ELEC page is the group consolidated accounts; the BAT voltmeter is the cash in your pocket. Each subsidiary (GCU / BCL / ECMU) files on a quarterly schedule (ARINC into SDAC, the consolidated report); a figure printed in red (an amber digit) is a performance problem, while a missing figure (an amber XX) is a subsidiary that has gone silent — the audit response to the two is completely different. And no matter how badly the group's reporting system crashes, the cash in your pocket — the hardwired battery voltmeter — you can count by reaching for it. On the worst day, it is the one number you can still trust.

4. The universal XX — "unknown" is not "broken"

Every field's NOTE in the AMM repeats one construction: a parameter is "replaced by amber XX when information … not available". Distil it into one reading law:

• An amber digit = the link is up, and the parameter is out of limits (a voltage of 125 V displayed as an amber 125).
• An amber XX = the link is down, and the parameter is unknown (the GCU / SDAC / BCL information is not available).

The two send you in opposite directions. An amber digit makes you investigate the quality of the electricity — the generator or the TR itself. An amber XX makes you investigate the path of the message — the source box or the SDAC chain. They are not two grades of the same fault; they are two different faults.

The XX information source is not even the same from bus to bus, and the AMM hides a useful detail here: the status information for the DC BAT BUS (3PP) comes from ECMU 1 or the BCL, whereas 1PP / 2PP / 4PP come from the normal SDAC chain. So a screen showing only the DC BAT cell as XX, everything else normal, points at the ECMU 1 / BCL side — it is not a global SDAC problem (if SDAC were the problem, the XX would not appear on one bus alone). This is the seed of a general reading method: a single XX cell tells you which source box went quiet, not that the whole monitoring system failed.

5. The AC ELEC page field dictionary

5.1 GEN 1/2

The AMM gives the green-zone thresholds verbatim:

"Digits for the generator load (with 1 % accuracy) are displayed: in green normally, in amber when the generator is overloaded (GCU discrete information). Digits for the generator voltage (with 1 V accuracy) are displayed: in green between 110 V and 120 V, in amber otherwise. Digits for the generator frequency (with 1 Hz accuracy) are displayed: in green between 390 Hz and 410 Hz, in amber otherwise."

Per AMM 24-27-00. The number following GEN (the 1 or 2) is coloured on its own logic: white when that engine is running (or when the information is unavailable), amber when the engine is stopped. So "GEN 1" can carry three colours at once across its three characters — the label speaking the generator's health, the number speaking the engine's state, and the digits speaking the parameter quality.

The load amber has a precise threshold, and FCOM quantifies it: the load digit goes amber only above 108 %, sustained for more than 10 seconds:

"The GEN load is normally green. It becomes amber, if the load is greater than 108 %, for more than 10 s."

Per FCOM DSC-24-20.

[!warning]- Two event layers — do not merge "display amber" with "protection trip"

The "load digit > 108 % for > 10 s → amber" rule is a display threshold — it changes a colour on the ECAM, nothing more. The GCU's differential / overcurrent protection that actually trips the line contactor (the "+ 15 A" logic in GCU and AC Generation Control) is a protection threshold — a different source, a different layer. Reading the load amber as "the protection just tripped" is a layer error: the colour change is watched at 108 %/10 s, the trip is watched by the GCU discrete. One can happen without the other.

5.2 IDG

The IDG oil-outlet-temperature field has its own colour ladder:

"Outlet oil temperature: Normally green. Green, pulsing in case of advisory (above 151 °C). Amber in case of overheat (above 185 °C)."

Per FCOM DSC-24-20. The field also carries two independent legends: DISC (amber, appears when the GCU detects the IDG has been disconnected, otherwise not shown) and LO PR (amber, low oil pressure detected while the IDG is turning — **inhibited when N2 < 14 %**, to avoid a false report during start/shutdown, the same source as the OIL LO PR trigger in Generator and IDG Failures). The IDG **symbol** itself is white when normal and amber for oil temp > 185 °C, low oil pressure, or disconnected; the IDG number is white with the engine running and amber when stopped. (The IDG field is treated component-side in Integrated Drive Generator.)

5.3 APU GEN, EMER GEN, STAT INV, EXT and the source arrows

• APU GEN has three states: MASTER SW OFF (standby) → white label, no digits; MASTER ON with the GEN pb OFF → APU GEN amber + white OFF; running → the same principle as GEN 1, with the fault criterion coming from the GAPCU rather than a GCU (see APU Generator and GAPCU).
• EMER GEN: with the EMER GLC open → white label (amber if faulty), no digits; with the GLC closed → voltage and frequency displayed (see Emergency Generator (CSM/G)).
• STAT INV: the same principle as EMER GEN, with the contactor judged via 3XB (see Static Inverter).
• EXT A/B: shown as a ground frame only; available / supplying / information-unavailable all give a white label plus parameters (GAPCU data).
• AC source arrows: green when that source's contactor is in line / supplying — the GEN arrow green when the GLC is in line; the APU GEN / EXT arrow green when it supplies at least one AC bus, otherwise white. The arrow is the unified "who is supplying" indication.

[!warning]- The EMER GEN amber-voltage condition is verbatim, not a typo

The AMM gives the EMER GEN voltage-digit amber condition as "… in amber when the generator voltage is between 110 V and 120 V and with ENG1 and ENG2 OFF." This is the opposite of the generator green-zone rule (where 110–120 V is the green band) — but it has been checked back against the source and it is verbatim, not a transcription error. It is a conditional special case: only with both engines stopped (ENG 1 + ENG 2 OFF), the EMER GEN voltage is flagged amber even when it sits in the otherwise-normal 110–120 V band (presumably a RAT-configuration cue — but the source does not state the why, so it is recorded as written and not over-interpreted).

6. The DC ELEC page field dictionary

6.1 Batteries (BAT 1/2/APU)

XX = the BCL information is unavailable. The APU battery exemption: the abnormal voltage / current values reported by the ECB during an APU start are not displayed amber — the display side of the start-surge exemption in article 12.

6.2 TR (1/2/ESS/APU)

The AMM gives the battery and TR digit thresholds, including both APU-family exemptions, verbatim:

"[BAT] Digits for the voltage (1 V accuracy): in green when the voltage is > 25 V, in amber otherwise. Digits for the absolute value of the current (1 A accuracy): in green normally, in amber when the discharged current is over 5 A. [APU battery] Same principle as for battery 1 except that when there are abnormal voltage and current values during the APU start from ECB, they are not displayed in amber. [TR] Digits for the voltage (1 V accuracy): in green when the voltage is between 25 V and 32 V, in amber otherwise. Digits for the current (1 A accuracy): in green when the current is over 2 A, in amber otherwise. [APU TR] Same principle as for TR1, except that the abnormal voltage range for a green display is 25 to 33 V and the value is not displayed in amber during the APU start. The current value is displayed in amber only when the APU contactor is open."

Per AMM 24-37-00. So the APU TR carries two privileges: its green band is widened to 25–33 V, its values are not flagged amber during an APU start, and its current row goes amber only when the APU contactor is open (a no-load 0 A is the normal idle state — see article 10).

[!warning]- The TR green-zone upper limit splits three ways — and the real split is inside the AMM

The TR voltage green-zone upper limit is given as 25–31 V by FCOM DSC-24-20 ("… in amber, when the voltage is less than 25 V, or greater than 31 V"), 25–32 V by AMM 24-37-00 (the DC page, the quote above), and 25–31 V by AMM 24-27-00 (the AC-page TR symbol). The genuine discrepancy is therefore inside the AMM itself — its AC-page section says 31 V, its DC-page section says 32 V — and FCOM stands with the AC-page figure. At pilot level take the 31 V of FCOM / AMM-AC-page as the working answer, with the AMM-DC-page 32 V noted alongside; do not force the two into one.

6.3 DC bus rows, arrows and the ESS-TR source

• DC bus rows: green = powered, amber = unpowered, XX = status information unavailable. The XX source differs by bus — 3PP from ECMU 1 / BCL, the rest from the SDAC chain (§4) — and a TR-digit XX = the SDAC chain is unavailable, distinct from a battery XX = the BCL. The single-cell-XX-points-to-a-different-box method extends straight to the DC page.
• Battery charge/discharge arrow: with the battery line contactor (6PB1 / 6PB2 / 5PB) open → no arrow; with it closed → (1) current information unavailable shows green with no arrow; (2) charging = an arrow into the battery frame, green (↓ green); (3) discharge = an arrow toward the DC BAT BUS, amber (↑ amber). This is how a pilot reads charge/discharge state at a glance — an amber ↑ discharge is the normal state in the emergency / batteries-only configuration, where the battery is sustaining the ESS network. Per AMM 24-37-00.
• ESS-TR supply-source indication: a four-state field marking which source is currently feeding the ESS TR — AC1 / AC2 / EMER GEN / XX (AC1 white when 3XC-A is closed and AC1 is powered, amber when not; AC2 likewise via 3XC-B; EMER GEN white when the EMER GLC is closed and the CSM/G is healthy, amber when faulty; otherwise XX). In a failure configuration this is the key indication of who is feeding the ESS TR. Per AMM 24-37-00.

7. How the page is called up

There are two paths (per AMM 24-27 and AMM 24-37). Automatically — a Class 1 failure on the AC or DC network, together with a request from the FWS (Flight Warning System), calls up the corresponding page (the common "ELEC page auto-display" mechanism running through every component article). Manually — the ELEC (AC) or ELEC DC key on the ECAM control panel. This is why every caution in the failure articles (21–30) comes paired with "the page is called up": that is the standard FWS action, and it happens without a key press.

8. Flight-deck scenarios

1. GEN 1 voltage shows an amber 124. The link is up and the parameter is out of limits (> 120 V) — check the generator's voltage regulation (article 02; the OV protection may be about to act). If instead it shows an amber XX, the GCU 1 data link is down — the generator may still be producing perfectly good power (read the GLC state from the arrow separately).
2. Only the DC BAT cell shows XX, everything else normal. The source specificity (the ECMU 1 / BCL chain) — think ECMU 1 or BCL first, not a global SDAC failure.
3. During an APU start the DC page shows "APU BAT 22 V / 80 A", still green. The start exemption (the ECB knows this is a surge condition). The same figures appearing outside a start are a real abnormality.
4. Deep in EMER CONFIG the whole screen has crashed. The panel BAT voltmeter is still there — select BAT 1 / BAT 2 in turn; > 25 V is the number you care most about (article 30's primary instrument).

[!warning]- Common misconceptions — predict, then check

Read each statement, decide true or false, then check the truth in brackets.

1. "Anything amber on the ELEC page is a power-quality problem — go check the generator / TR." — Half true. An amber digit is link-up / out-of-limits (check the quality of the electricity); an amber XX is link-down / parameter-unknown (check the path of the message — the source box or the SDAC chain). The two send you in opposite directions (§4).
2. "Only the DC BAT cell shows XX, so the SDAC bottleneck must have failed." — False. The DC BAT BUS (3PP) status comes from ECMU 1 or the BCL, not the normal SDAC chain — a single XX points at the ECMU 1 / BCL side, not a global SDAC problem (§4).
3. "The GEN load cell went amber, so the load is over 100 %." — False. The display threshold is > 108 % sustained > 10 s (FCOM), not 100 %; and the actual trip is a different layer — the GCU's differential / overcurrent discrete (article 02). Display colour (108 %/10 s) and protection trip (GCU discrete) are two event layers (§5.1).
4. "DC page shows APU BAT 22 V / 80 A discharge — low voltage and heavy discharge, that's a fault." — It depends. During an APU start these surge values are not flagged amber (the ECB knows it is a surge, still green); the same figures outside a start are the real abnormality (§6.1).
5. "The whole screen has crashed — even ECAM is gone — so all the electrical instruments are blind." — False. The panel BAT voltmeter is the only electrical instrument that does not pass through ECAM, fed from the battery's own power (hot bus), "whatever the aircraft configuration" — still lit when the aircraft is dark, the primary power-monitoring instrument of batteries-only flight (§3 / article 30).

A compact memory aid for the whole dictionary: smart boxes report over ARINC, end items over discrete, the voltmeter over hardwire; an amber digit is out-of-limits, an amber XX is a broken link; 110–120 V and 390–410 Hz on the AC side; battery > 25 V and discharge > 5 A on the DC side; the APU family is exempt during start.

Self-test

[!note]- Q1. What are the two tiers of the monitoring chain, and which item bypasses every computer?

Smart boxes report over ARINC 429 → SDAC (GCU / GAPCU / ECMU on label 055 / BCL — full parameter set, with BITE); the emergency and end-of-line items report over discrete / analogue lines straight into SDAC (CSM/G GCU, static inverter, TR monitoring cards). The CBMU is the exception that runs ARINC → DMC to the C/B page. And the BAT voltmeter is read straight off the batteries by hardwire, touching no computer at all — the reason it survives a total display loss.

[!note]- Q2. Amber XX versus an amber digit — what is each telling you, and where do they differ by bus?

An amber digit = link up, parameter out of limits → investigate the quality of the electricity (the generator / TR itself). An amber XX = the information link is down, parameter unknown → investigate the path of the message (the source box or the SDAC chain). The DC BAT BUS (3PP) status source is special — ECMU 1 / BCL, not the SDAC chain — so a lone XX there points at that side rather than at SDAC.

[!note]- Q3. Give the GEN-field thresholds on the AC page.

Load at 1 % accuracy, with the amber criterion being the GCU overload discrete (not a simple percentage) — FCOM quantifies the display amber as > 108 % for > 10 s; voltage green 110–120 V; frequency green 390–410 Hz; GEN label amber = GCU judges a fault; the number amber = that engine is stopped. With the pb OFF the GEN goes amber and a white OFF replaces the digits.

[!note]- Q4. Give the DC-page thresholds and the two start exemptions.

Battery: voltage green > 25 V, discharge amber > 5 A (XX = BCL link down); FCOM adds an upper > 31 V amber. TR: voltage green 25–31 V (FCOM baseline; AMM 24-37 gives 25–32 V, noted alongside), current green > 2 A. The two exemptions: the APU battery and the APU TR are not flagged amber during an APU start; and the APU TR's green band is widened to 25–33 V, with its current going amber only when the APU contactor is open.

[!note]- Q5. What is the special status of the BAT voltmeter?

It is the only electrical instrument that does not pass through ECAM (the LCD 3PV plus the selector 1PV). It displays the selected battery's voltage permanently, in any configuration, fed from the battery's own power (hot bus). In a total-display-loss or batteries-only case it is the primary power-monitoring instrument; on a cold-and-dark first glance it tells you the overnight battery state.

Key takeaways

References

Per AMM 24-27-00 D/O (the AC-page readable-parameter list, the two call-up paths, the full GEN-field semantics — overload = GCU discrete, 110–120 V, 390–410 Hz, the XX rule — the APU GEN three states, and the EMER GEN / STAT INV / EXT frames, including the verbatim EMER GEN amber-voltage condition with ENG 1 + ENG 2 OFF); AMM 24-37-00 D/O (the BAT voltmeter 1PV / 3PV and its "permanently displayed whatever the aircraft configuration", the DC-page battery thresholds > 25 V / discharge > 5 A with the BCL XX, the TR thresholds 25–32 V / > 2 A, the APU-family start exemptions and the 25–33 V wide band, the charge/discharge arrow logic, the ESS-TR four-state source, and the bus status-source difference 3PP = ECMU 1 / BCL); FCOM DSC-24-20 (the GEN load > 108 % for > 10 s display amber, the IDG outlet-temperature 151 / 185 °C ladder, the TR voltage baseline "less than 25 V or greater than 31 V", and the FCOM-side cross-checks of each field). The ARINC-vs-discrete tiering, the "unknown ≠ broken" disposition split, and the consolidated-accounts-vs-cash analogy are integrative syntheses of the above and contain no facts from outside the library.

Independent study material, not an Airbus publication. Refer to current operator FCOM, FCTM, and QRH for operational use.