Lessons Learned About the Teach-in Function

When critical cabin emergency lights are not illuminating, it may lead to a NO-GO condition. If these faults remain undetected due to incorrect system monitoring set-up, some of cabin emergency lights may not illuminate when required. There is a risk that the aircraft could be dispatched with a NO-GO condition. If it is the case that faulty lights or components are hidden from the system test or monitoring, then only a complete visual check will identify all of the emergency lights which are not illuminating.

It is important for cabin crew to perform a thorough visual inspection of all emergency lights, as described by the Cabin Crew Operating Manual’s Ground Check, as well as visual checks by maintenance personnel during troubleshooting.

The Emergency Light System Test is triggered either centrally from Multifunction Control Display Unit (MCDU on A320/A330/A340 family aircraft), On-board Maintenance Terminal (OMT for A350XWB/A380) or locally on the EPSU itself (fig.1). The EPSU starts a test routine that includes a self-test and a test of all the electrical components connected to it, including its battery and the emergency lights.

This test is a measurement of the actual current on each output compared with a reference value that is stored in the memory of the EPSU.  If the tests detects differences in these values outside of pre-defined limits, this will generate a fault indication. In the case of a short circuit, an EPSU internal fuse will disconnect the output. The EPSU on A320/A330/A340 family aircraft is a thermal fuse, which requires removal of the unit to replace the fuse in a repair centre. An EPSU installed on A350XWB and A380 aircraft has a resettable solid state fuse.

When an EPSU is installed on the aircraft it must first learn the correct reference values. This is managed by the EPSU “Teach-In” procedure, which shall only be performed only after the assembly of the system is complete and a visual inspection to first verify the proper function of all emergency lights. 

During the Teach-In procedure, all outputs are operated for a short time to capture their specific current. This value is stored in the EPSU as the reference value. It is used for comparison with the actual electrical loads, which are measured at the outputs of the EPSU during the System Test. 

Teach-In starts automatically on A320/A330/A340 family aircraft, and for A380 aircraft, when an EPSU is connected to an aircraft which is powered, or when a battery pack is reconnected to the EPSU (fig.2). On A350XWB aircraft, the Teach In is manually triggered by pressing the test button on the EPSU for more than 5 seconds.

Teach-In function on electronic units like the EPSU allows for common part numbers to be used across a fleet of aircraft with different models, and with their various cabin configurations. Aircraft cabins are customized by each airline. The Emergency Lighting System must therefore be configurable to operate correctly in a customized cabin layout. It also allows for common EPSU part numbers to be used with both the older emergency light systems fitted with traditional light bulbs, and the newer systems that have LED lights requiring much less electrical current.

After any Emergency Light System configuration change, such as a cabin layout modification or replacing a system’s traditional “bulb” lights with modern LED lights, the Teach-In procedure is necessary to determine the correct reference value to store in the EPSU memory for the system test. Before starting the Teach-In, a visual inspection to verify that all lights are operative is required. The Teach-In procedure should only be done strictly in accordance with the applicable AMM/MP.

The Teach-In function is not to be used during troubleshooting of the Emergency Light System and the EPSU. Teach-In function is not a system reset function. There is NO Emergency Light fault RESET function on the EPSU. 

(fig.3) Emergency Light System Fault – DO NOT USE TEACH-IN FUNCTION TO CLEAR A FAULT

Initiating the EPSU Teach-In procedure in an Emergency Light System that has inoperative lights or defects present has three main consequences. To avoid these consequences, DO NOT clear Emergency Light System faults, with their associated ECAM message, by either disconnecting the affected EPSU’s battery, swapping EPSU positions or replacing an EPSU before first identifying and rectifying the root cause of the fault message.

• Hidden Defects

After the Teach-In procedure is completed on the EPSU, it will no longer detect the defect is present and a fault message will not be displayed. The defect is effectively hidden from the system test because the EPSU has stored a new reference value that was measured with the defective component in the system. (fig. 3)

• False Fault Message

If the defect is later detected during a visual inspection and it is then repaired, the EPSU test will display a fault message even though there is no longer a defect present in the system. This is because there will be a difference between the measured electrical loads and the false reference value captured during the Teach-In when the defect was still present. This could generate confusion for maintenance personnel and it may lead to lengthy troubleshooting with the risk of operational delays.

• Repeated Fuse Tripping

If the defect is a short circuit on the output of the EPSU on A320/A330/A340 family aircraft, then the EPSU must be replaced. If the cause of the short circuit is not repaired, this will immediately damage the fuse of the replacement unit EPSU. This fuse is only resettable on EPSU used for A350XWB and A380 aircraft. The fuse will repeatedly trip to disconnect the current at the EPSU output and this causes the fault to reappear until the defect is rectified.

An Emergency Light System fault message only indicates a Fault Isolation Number (or FIN) for the affected EPSU, but that EPSU is usually not the direct cause of the defect. It is more likely that a component or light connected to that EPSU is defective and is the reason for the fault message. 

(fig.4) Emergency Light System test and fault detection

There are 3 ways to identify faults or lights not illuminating in the Emergency Lighting System: 

1. Visual Inspection 

A thorough visual check of all of the emergency lights by Cabin Crew or Maintenance Personnel should find any lights that are not illuminating. These should be reported by Cabin Crew, as per the CCOM Ground Check, or recorded by maintenance for troubleshooting or replacement in accordance with TSM & AMM/MP 33-51-00. 

2. System BITE Monitoring 

On A350XWB and A380 aircraft, a continuous system monitoring compares the electrical loads at the outputs of the EPSU with a stored reference value during its previous Teach-In process. If the measured electrical load deviates from the stored reference value, a maintenance message (or “dispatch message” for A350XWB aircraft), will appear to alert Flight crew and maintenance of a defect in the aircraft’s Emergency Lighting System (fig. 4). In this case, refer to the Troubleshooting procedure in TSM & AMM/MP 33-51-00 and DO NOT use the EPSU Teach-In process to clear the fault.

3. System Test 

Emergency Light System test is carried out by maintenance personnel in accordance with AMM/MP 33-51-00. This will manually launch the system monitoring described above (for System BITE monitoring). The LED indicator on the EPSU will advise what components to check to find the cause of the fault.

The Emergency Lighting System Test can be performed locally on each EPSU by pressing the TEST button (fig. 5). Caution: On A350XWB pressing the test button for more than 5 seconds launches the Teach-In function for that EPSU.