Brief Summary

An aircraft grounded in Friedrichshafen reported a fault with its fuel tank inerting system. This safety system plays a vital role in reducing the risk of explosion by replacing the oxygen-rich air inside fuel tanks with nitrogen-enriched air. It is particularly important when tanks are partially empty and filled with fuel vapour.

Initial suspicion centred on a valve known as the PRSOV (Pressure Regulating and Shut-off Valve), which regulates the airflow into the system. A replacement valve was already being arranged, but as the fault developed, the issue proved to be far more complex. What followed was a day of detailed diagnostics, an unexpected ferry crossing, and the eventual discovery of a loose connector buried within an electrical control unit. The aircraft was returned to service later that night without the need for any parts to be replaced.

‍

Travel

At first, it looked like a routine trip. The engineers flew from London to Zurich, picked up a hire car, and began the drive towards the airport. But after a wrong turn east, courtesy of Google Maps, they found themselves in an all-too-relatable situation: stuck in a queue with no way to reverse, being slowly ushered onto a ferry without quite knowing where it was going.

Ever been in one of those moments where you're just along for the ride? Google Maps clearly had a plan, the team just weren’t briefed. Fortunately, the ferry crossed Lake Constance and dropped them on the other side, surprisingly close to their final destination.

It turned out to be an unexpected detour that offered a rare moment of calm before the more demanding part of the day began.

‍

Initial Assessment

The fault was hard and consistent rather than flickering, which pointed towards a definite failure rather than a momentary glitch. The team’s first focus was the PRSOV, a valve responsible for delivering regulated airflow to the inerting system from the bleed system. It is a key part of ensuring the system receives a stable air supply to generate nitrogen-enriched air.

Engineers disconnected the valve’s power plug and found no 28-volt direct current present. That result came with some relief. Replacing the PRSOV would not have been easy due to its location, which is difficult to access and would have made the job significantly more time-consuming. With no power at the connector, attention shifted upstream towards the aircraft’s power distribution system.

‍

Diagnostic Approach

First things first: was the circuit breaker in? It would have been a long way to come just to reset a CB. But alas, it was in, and the fault ran deeper.

Using the aircraft's wiring diagrams, the team traced the power supply back to the SPDA, or Secondary Power Distribution Assembly. This unit functions as a central hub for managing and distributing electrical power to the aircraft’s various systems. The specific line feeding the valve was controlled by SPDA Card 3.

To help isolate the issue, engineers decided to swap SPDA Cards 3 and 4. These cards are identical in hardware but feed different systems. The goal was simple: if the fault followed the card into the new position, the card was likely the problem.

Instead, the swap created an entirely new situation. Eight separate systems that were normally fed by SPDA Card 3 suddenly reported faults. At this point, several attempts were made to reload the software onto both cards, as software corruption can sometimes trigger cascading system failures. None of the reloads were successful.

With all eight faults linked to systems that relied on SPDA Card 3 position, the suspicion grew that the card had been damaged during handling. Electrostatic discharge, or ESD, is a known risk when working with sensitive electronics. Even a small build-up of static on clothing or tools can be enough to harm internal components during removal or reinstallation.

However, before concluding that the card needed replacement, a full physical inspection was carried out. That is when the real issue was found. One of the rear connectors feeding into the SPDA housing was slightly loose. It had not been visibly disconnected, but it was not making solid contact either.

Once properly secured, both SPDA cards were reinstalled, and the aircraft systems powered up. All eight fault messages cleared instantly.

With that, the shipment for the new PRSOV and the SPDA card was cancelled, phew! The engineers confirmed the original valve was not at fault and, more importantly, avoided the complex task of removing and replacing it.

‍

Communication and Planning

Once it became clear that the fault was more than a valve replacement, the airline was kept informed with regular updates. This helped manage expectations as the scope of the issue grew beyond the initial diagnosis.

‍

Logistics and Installation

No spare parts were needed in the end. All diagnostics were performed using existing tools and on-board documentation. The SPDA cards were swapped purely for troubleshooting purposes, not because there was a spare on board. Card 4 was already in service, powering a separate group of systems. The swap was made only to test whether the issues would follow the card, which would have indicated a card fault.

After the faults cleared, the order for the PRSOV was quickly cancelled, saving both time and unnecessary labour. This avoided the need to work in a confined and awkward area of the aircraft where the valve is located.

Testing and Return to Service

With the connector secured and the SPDA cards reinstalled, full operational checks were carried out on all eight systems affected during the fault. This included the fuel inerting system, which originally triggered the call-out. All systems passed their tests with no residual issues, and the aircraft was returned to service that same evening.

‍

Technical Reflection

This case shows that fault codes are often only the beginning of the story. It is easy to follow the data trail and assume a specific component has failed, especially when the messages point in that direction. In this instance, the reported fault led directly to the PRSOV, which appeared at first to be the most likely cause.

However, the real issue lay in a connector at the back of an electrical housing, slightly loose and barely making contact. When the SPDA card was disturbed, that weak connection failed entirely, which in turn affected every system fed by that card.

The sudden appearance of eight new faults raised suspicions of electrostatic damage, but the answer turned out to be far simpler and more mechanical.

It also served as a clear example of how physical checks are just as important as software diagnostics. A card swap, a suspected discharge event, and several reload attempts all pointed towards hardware failure. But in the end, the solution involved reseating a connector that had moved less than a millimetre.

‍

If you enjoyed this case study, you might also like our other AOG Diaries entries, including real-world scenarios like AOG Diaries #1 – Right-Hand Windscreen Heat Failure and AOG Diaries #4 – APU Failure. These provide further insight into the diverse challenges faced in keeping aircraft airworthy.