Indicating — Speed in Percent, EGT Rake Averaging, the Self-Erasing APU Page
What the crew actually sees of the APU is two numbers — speed (N) in percent and EGT in °C — on the SD APU page, plus the ON/AVAIL lights. This article covers how those are sensed and why the page appears and disappears on its own.
The indicating system monitors the APU speed and the Exhaust Gas Temperature (EGT)... The power indicating system monitors the APU speed in revolutions per minute (rpm)... The EIS shows the APU speed in percent on the APU system page... The temperature indicating system monitors the APU EGT. Two rakes, each with two thermocouples, transmit a signal to the ECB 59KD. — AMM 49-70-00
1. Speed — sensed in rpm, shown in percent
Per AMM 49-71-00, two speed sensors (between the load compressor and the air-intake housing) feed the ECB; the ECB passes a speed signal to the EIS, which shows N in percent on the SD APU page.
[!note]- Percent, not rpm — because the APU is a constant-speed machine (integrative synthesis) The sensors measure rpm but the display is percent (AMM 49-71-00), because 100 % N = the constant 41730 rpm the ECB governs (00). A pilot reads N% as "how far through the start / is it at governing speed," never as a power setting — there is no APU power lever.
2. EGT — two rakes, averaged, with single-rake fallback
Four EGT thermocouples are installed in the exhaust cone at the turbine discharge... Each thermocouple rake is connected to the ECB 59KD. The ECB 59KD reads the average temperature of the two thermocouple rakes... If a failure of one thermocouple rake occurs, the ECB 59KD takes the value of the remaining thermocouple rake and transmits this EGT signal to the EIS.
Per AMM 49-72-00, EGT is measured by four thermocouples in the exhaust cone at the turbine discharge, grouped as two rakes (two thermocouples each). The ECB normally shows the average of the two rakes, and on a single-rake failure it falls back to the remaining rake — the EGT indication survives one rake failure.
[!note]- EGT is the start's health signal (integrative synthesis) EGT is read at the turbine discharge, so it is the direct measure the ECB's timed acceleration logic and hot-start protection work from. The rake redundancy keeps that signal alive — important because a lost EGT signal would blind the very protection that guards the turbine.
3. The APU page that erases itself
Per FCOM DSC-49-20, the SD APU page appears automatically during the start and walks through a visible sequence:
- flap fully open → APU starter energised;
- N = 7 % → ignition ON;
- N = 50 % → starter de-energised, ignition off;
- N = 95 % → the START pb ON light goes off, AVAIL comes on (the APU can now supply bleed and electrical power);
- 10 s later the APU page disappears from the ECAM display.
On a manual shutdown (MASTER SW off): the ON light goes off; after 15 s (for No-Break Power Transfer) the APU shuts down and AVAIL goes off; if APU BLEED was on, an 85 s cooling period at 82 % runs first; at N 7 % the air inlet flap closes.
[!warning]- The page is a transient tool — AVAIL is the steady-state cue The APU page is shown only during start/shutdown transients and self-erases 10 s after AVAIL (FCOM DSC-49-20). In steady operation the crew reads AVAIL (N ≥ 95 %), not the page. The numeric N%/EGT matter mainly while the start is in progress or a fault recalls the page. The 7 %/50 %/95 % marks here are the same ones in ignition and starting — the page is just their cockpit face.
4. Analyzers — health and life data (maintenance layer)
Per AMM 49-73-00, the ECB BITE keeps fault data and the memory module keeps APU life data, sent over ARINC 429 to the CMS/DMU → MCDU, ACMS reports (APU MES/IDLE <13>, APU SHUTDOWN <14>) and ACARS. The life logic covers usage statistics, health monitoring (T5 and bleed pressure during each main-engine start) and life usage (low-cycle-fatigue / stress-rupture life of the first two turbine stages from calculated T4). For the crew this surfaces only as SERVICE DATA on the MCDU; the rest is the maintenance layer.
[!note]- The APU watches every engine start it assists (integrative synthesis) Health monitoring records T5 and bleed pressure during each main-engine start (AMM 49-73-00) — i.e. the APU logs its own performance in its primary ground role of supplying engine-start bleed. Together with the ECB life data (hot starts, turbine life), this is how a degrading APU is caught before it fails to start an engine.
5. Counterintuitive points
[!warning]- N is shown in percent because the APU runs at one speed 100 % = 41730 rpm constant; N% is a start/governing cue, not a power setting (AMM 49-71-00).
[!warning]- The APU page disappears by itself 10 s after AVAIL It is a transient display; steady-state monitoring is via AVAIL, not the page (FCOM DSC-49-20).
Self-test
[!note]- Q1. How is APU speed sensed and displayed? Two speed sensors → ECB → EIS, shown as N in percent (100 % = 41730 rpm constant).
[!note]- Q2. How is EGT measured, and what happens if a rake fails? Four thermocouples = two rakes, averaged by the ECB; on a single-rake failure it uses the remaining rake.
[!note]- Q3. When does the SD APU page appear and disappear? Appears automatically at start; disappears 10 s after AVAIL (N ≥ 95 %).
[!note]- Q4. What does the APU health monitoring record about engine starts? T5 and bleed pressure during each main-engine start — performance trending of its ground bleed role.
Key takeaways
| Point | Detail |
|---|---|
| Speed | 2 sensors → ECB → EIS; shown N in % (100 % = 41730 rpm) |
| EGT | 4 thermocouples = 2 rakes, ECB averages; single-rake fallback |
| APU page | auto-appears at start; sequence 7 %/50 %/95 %; self-erases 10 s after AVAIL |
| Shutdown cues | MASTER off → 15 s NBPT → AVAIL off; APU BLEED → 85 s cooling at 82 %; flap closes at 7 % |
| Analyzers | BITE + life data (usage / health monitoring of each engine start / turbine LCF life); crew sees SERVICE DATA on MCDU |
References
- AMM 49-70-00 / 49-71-00 / 49-72-00 (Indicating; Power; Temperature) — two speed sensors → ECB → EIS shows N in percent; four EGT thermocouples = two rakes, ECB averages the two rakes and falls back to the remaining rake on a single-rake failure, shown in °C on the SD APU page.
- AMM 49-73-00 (Analyzers) — ECB BITE fault data + memory-module life data over ARINC 429 to CMS/DMU/MCDU, ACMS reports (MES/IDLE <13>, SHUTDOWN <14>), ACARS; life logic = usage statistics, health monitoring (T5/bleed pressure each main-engine start), life usage (LCF/stress-rupture of first two turbine stages from T4).
- FCOM DSC-49-20 — APU page auto-displays during start (flap open → starter → 7 % ignition → 50 % starter off → 95 % AVAIL), self-erases 10 s after AVAIL; manual shutdown 15 s NBPT, 85 s cooling at 82 % if APU BLEED was on, flap closes at N 7 %.
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.