ECB — Full-Authority Digital Control, the Two OBRMs, and Life-Time Data

Everything in the previous articles — fuel metering, IGV position, surge-valve modulation, ignition windows, the start sequence — is scheduled by one box: the ECB (Electronic Control Box) 59KD. It is the APU's full-authority digital controller, the direct equivalent of an engine FADEC. This article ties the system together at the controller level.

The Electronic Control Box (ECB) 59KD is a full authority digital electronic control... It has full authority on the APU from the start initiation, through the acceleration and the governor speed, to the shutdown. The ECB continuously monitors the APU and controls the APU operation. — AMM 49-61-00

1. Full authority — the whole life of every run

Per AMM 49-61-00, the ECB has full authority over the APU from start initiation → acceleration → governor (constant) speed → shutdown. There is no separate hydromechanical governor underneath it (contrast a main engine): the ECB is the governor.

To hold the constant 41730 rpm = 100 % N at any load, the ECB continuously monitors:

• APU speed (from the two speed sensors)
• the temperatures (EGT, etc.)
• the load-compressor airflow (the bleed demand)

and continuously controls three actuators by separate torque-motor currents:

• fuel flow (the fuel control assembly metering valve, 04)
• IGV position (the bleed throttle, 02/03)
• surge-control-valve position (03)

[!warning]- One controller balances fuel, IGV and surge valve simultaneously to hold constant speed This is the integrating insight of the whole chapter: at a fixed shaft speed, the only way to meet a changing bleed demand without surging or overspeeding is to co-ordinate fuel and airflow. The ECB does exactly that — three separate torque-motor currents (fuel / IGV / surge valve) driven from one speed-and-temperature picture (AMM 49-61-00). Open the bleed valve and the ECB simultaneously opens the IGVs, trims fuel for the new load, and rides the surge valve — all to keep 41730 rpm steady. That is why you never set APU "power"; you set a bleed/electrical demand and the controller does the rest.

Beyond those three, the ECB also commands the starter motor, the igniters, the fuel control shutoff valve, and the air intake flap (08) — so the start sequence, ignition windows and inlet are all one coordinated schedule.

2. What the ECB does — four jobs

Per AMM 49-61-00, the ECB provides:

• automatic operation control — the crew commands ON/OFF; the ECB runs the sequence
• constant-speed control — holds 100 % N across all loads
• protection — on failure or overload it acts (up to automatic shutdown, 13)
• supply of pneumatic and shaft power management — it governs how much bleed/load the APU delivers

It also runs LRU monitoring and tests, but those belong to the maintenance layer rather than the flight-crew picture.

3. The two OBRMs and life-time data

The ECB contains two On-Board Replaceable Modules (OBRM): ‐ the control and monitoring module, ‐ the communications module... The ECB keeps the APU life-time data (the serial number, the operating hours, the number of starts, the unsuccessful starts, the hot starts and the turbine life consumed) in parallel with the memory module.

Per AMM 49-61-00, the ECB is built from two On-Board Replaceable Modules (OBRM):

• the control and monitoring module — the controller proper
• the communications module — talks to the aircraft over ARINC 429

The ECB also keeps the APU life-time data — serial number, operating hours, number of starts, unsuccessful starts, hot starts, and turbine life consumed — held in parallel with a memory module, so the two cross-load each other when one is replaced.

[!note]- The APU remembers its own abuse (integrative synthesis) The ECB logs hot starts and unsuccessful starts and tracks turbine life consumed (AMM 49-61-00). This is why the start protections matter for the airframe, not just the moment: a string of hot starts is recorded against the turbine's life. It also explains the 3-attempt / 60-min discipline of 06 — the controller is counting.

The ECB sits adjacent to the bulk cargo compartment (right side, FR71–72) in an ARINC 600 rack — i.e. it is not in the tailcone fire zone with the APU itself, so a tailcone fire does not directly take out the controller.

4. Counterintuitive points

[!warning]- There is no governor under the ECB — the ECB is the governor Unlike a main engine with a hydromechanical fuel-control backup, the APU's speed is held purely by the digital ECB trimming fuel against speed/temperature (AMM 49-61-00). A critical ECB failure is therefore one of the few things that forces an automatic shutdown (13).

[!warning]- One box, four actuators, one constant speed Fuel, IGV, surge valve, plus starter/igniters/shutoff/inlet flap — all one coordinated schedule from a single speed-and-temperature picture (AMM 49-61-00). The crew sets a demand, never a power.

Self-test

[!note]- Q1. What is the span of the ECB's authority? Full authority from start initiation → acceleration → governor speed → shutdown (AMM 49-61-00).

[!note]- Q2. Which three actuators does the ECB drive by separate torque-motor currents, and why? Fuel flow, IGV position, surge-control-valve position — co-ordinated to hold constant 41730 rpm = 100 % N against changing bleed load.

[!note]- Q3. What are the two OBRMs? The control-and-monitoring module and the communications module (ARINC 429).

[!note]- Q4. What life-time data does the ECB keep, and where? Serial number, operating hours, starts, unsuccessful starts, hot starts, turbine life consumed — in parallel with a memory module (cross-loaded on replacement).

Key takeaways

References

• AMM 49-61-00 (ECB — Description and Operation) — full-authority digital control from start initiation through governor speed to shutdown; constant-speed control; monitors speed/temperatures/load-compressor airflow; continuously controls fuel flow + IGV position + surge-control-valve position by separate torque-motor currents; also controls starter motor, igniters, fuel control shutoff valve, air intake flap; two OBRMs (control-and-monitoring + communications, ARINC 429); APU life-time data (serial number, hours, starts, unsuccessful/hot starts, turbine life consumed) held in parallel with memory module; located adjacent to bulk cargo compartment FR71–72.
• FCOM DSC-49-10 — ECB controls and monitors the APU (overview).

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.