AOG Details

Location: Luton Airport

MEL status: Grounded

Total downtime: 4 days

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Brief summary

What at first looked like a simple electrical fault quickly turned into a complex and time-consuming investigation. Over four days, the team dealt with three AC Motor Pump (ACMP) replacements, conflicting system behaviour, and a fault that refused to sit neatly in either the electrical or hydraulic system. In the end, the cause was not a defective part but mislabelled terminals inside the AC distribution cabinet.

When a relay was replaced and wired according to those incorrect labels, the three-phase supply feeding both the ACMP  (AC motor pump) and the Transformer Rectifier Unit (TRU) was reversed. This reversal explained every symptom, from noisy pumps to inconsistent displays, and highlighted the importance of trusting diagrams over labels.

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Initial assessment

The issue was first reported when the Engine Indicating and Crew Alerting System (EICAS) showed an electrical fault. The engineering team was called to Luton to investigate. Once connected to the maintenance diagnostics system, the fault pointed toward a latching relay inside the AC cabinet.

Relays play a central role in an aircraft’s electrical system. They are heavy-duty, electrically controlled switches that direct power to subsystems such as hydraulic pumps, avionics, lighting, and rectifiers. This particular relay carried three-phase AC, used for high-load items like ACMPs and TRUs (transformer rectifier unit). A latching fault can cause a pump or rectifier to receive unstable or no power, so replacing the relay was the obvious step.

A new relay was ordered, installed, and tested. The installation appeared perfect: the relay latched, the systems energised as expected, and no further faults were logged. With everything apparently in order, the aircraft was released back to service.

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Hydraulic anomalies

Less than 24 hours later, the aircraft returned with a different complaint. This time the crew reported hydraulic irregularities during their pre-departure checks: one pump was unusually noisy, fluid levels were shifting unexpectedly, and the synoptic display was showing inconsistent information.

On inspection at Luton, engineers traced the symptoms to an ACMP. These electric pumps supplement or replace engine-driven pumps, particularly during taxi, take-off, landing, and in standby situations. The affected pump was running loudly and showing behaviour inconsistent with healthy operation.

Since there were no electronic fault messages, the problem was assumed to be mechanical. The pump was replaced with a repaired unit the following morning. Yet once the aircraft was powered up, the symptoms were back almost immediately: the same noise, the same inconsistent pressure.

The odds of two bad pumps in succession were extremely low. Still, the team had no clear reason to doubt the diagnosis. An inspection of the installation of the relay that was replaced was carried out and found to be fitted correctly.

The repair notes from one of the removed pumps stated “No Fault Found,” which raised the possibility that the unit only showed issues when installed in the aircraft rather than on the test bench.

The relay wiring was inspected visually and compared with the labels on the cabinet. Everything appeared correct. The team considered whether the replacement pumps could have been faulty on arrival, but this seemed implausible after two in a row.

A second replacement pump was fitted due to this ambiguity, but the issue repeated again. By this stage, the assumption that the fault lay with the pumps themselves was no longer credible.

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Reassessing the situation

With three pumps all behaving the same way, attention turned back to the electrical supply. The previous day’s relay replacement now seemed highly relevant. According to the aircraft’s wiring diagram, that relay fed both the affected ACMP and a TRU.

At this point the investigation shifted from a narrow focus on the pumps to a wider view of the entire circuit. Something systemic had to be causing the repeatable behaviour.

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Transformer rectifier checks

As part of a broader inspection, the TRU fed by the same relay was checked. TRUs convert three-phase AC into stable DC power and are fitted with cooling fans. During the first round of testing, fan operation had been confirmed by checking airflow with a piece of paper near the housing. The fan had appeared to be running.

On closer comparison with the other TRUs, however, a difference was noticed: the fan on this unit was spinning in the opposite direction. The airflow test had shown movement but not direction, and until now this detail had gone unnoticed.

This observation was critical. Three-phase motors, unlike single-phase devices, rely on correct phase sequence to determine direction of rotation. Reversing two of the phases makes the motor spin the opposite way. For a cooling fan this is enough to reduce effectiveness, but for an ACMP it meant the pump was turning backwards.

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Understanding the symptoms

Once this was recognised, the symptoms began to make sense. The ACMP was not broken, it was simply running in reverse. Hydraulic pumps are designed to pressurise fluid in a specific direction, and when reversed they produce noise, vibration, and very poor performance. The system still showed pressure at times because accumulators and pre-charged reservoirs masked the loss of flow during short tests. That is why the pumps seemed serviceable during basic checks before deteriorating under extended load.

Electrically, everything appeared healthy because the relay was energised, the correct voltages were present, and the circuit was closed. What those checks did not reveal was that the order of the phases had been reversed. That reversal affected not just the ACMP but also the TRU fan.

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Root cause analysis

With this new understanding, the team returned to the AC cabinet. The wiring was examined once more, this time with particular care. The relay had been wired according to the labels provided on the panel. Unfortunately, those labels were wrong.

The mislabelling meant that when the new relay was fitted, its terminals were matched to incorrect positions. This caused two of the three AC phases to be swapped. Any equipment powered by that relay,  including both the ACMP and the TRU was therefore receiving reversed phase supply.

The relay was rewired to match the aircraft wiring diagram rather than the panel labels. After this correction, the ACMP ran smoothly, the TRU fan rotated in the right direction, and extended operational tests under load showed no anomalies. The cabinet was re-marked properly to prevent a repeat.

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Technical reflection

This AOG event demonstrates the importance of cross-checking work against approved diagrams rather than relying solely on labels or assumptions. Mislabelled terminals led to the relay being wired out of phase, which in turn caused motors to run backwards. Because voltages and continuity appeared normal, no fault codes were generated, and the problem was masked during short-duration ground tests.

Aircraft systems are built with redundancy, which can sometimes conceal failures instead of exposing them. In this case, hydraulic accumulators held pressure for long enough that the pumps seemed to function correctly during initial checks. Only after longer operation did the reversed rotation become obvious. This contributed to repeated misdiagnosis and unnecessary component changes.

The repeated pump swaps illustrate how misleading symptoms can be when the underlying cause is systemic. Each replacement ACMP operated in exactly the same way, because each was subject to the same reversed supply. With no fault indications generated, it was easy to blame the pumps rather than reconsider the supply.

The TRU fan proved to be the key indicator. By checking direction of rotation against other units, the team discovered the phase reversal that had been overlooked in earlier tests. This emphasises the need to observe details beyond simple function checks. A fan that moves air may appear serviceable, but without checking direction, a critical fault can go unnoticed.

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Conclusion

Over four days at Luton, a relatively simple relay replacement developed into a drawn-out investigation. Three ACMPs were replaced unnecessarily, system behaviour was misinterpreted, and valuable time was lost. The true cause (mislabelled terminals in the AC cabinet) was straightforward once recognised, but it took persistence and detailed checking to uncover.

This case shows how important it is to challenge initial assumptions, especially when multiple components show the same fault. It also reinforces the value of careful observation: the reversed fan direction on the TRU provided the crucial clue. Above all, it highlights that documentation must take priority over panel labels or visual cues. Once the wiring was corrected according to the diagram, all systems returned to normal, and the aircraft was released back into service without further issues.

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