What happened
During an instrument landing system approach, the crew encountered erroneous glideslope indications on the aircraft's electronic attitude direction indicators (EADIs). The left EADI displayed a constant on-glideslope indication regardless of the actual flight path, while the right EADI showed similar errors intermittently. Crucially, the system provided no fault or failure alerts to notify the crew of the discrepancy.
Because the localiser indications remained correct and the aircraft began the approach near the glideslope, the crew did not immediately realize the instrumentation was failing. When the autopilot failed to descend the aircraft along the glideslope as expected, the pilot assumed the autopilot was simply experiencing a lag in capturing the signal. To rectify this, the pilot disconnected the autopilot and transitioned to a manual descent.
During this manual phase, the crew performed various altitude and distance checks to reconcile the conflicting data. Believing the aircraft was properly positioned due to the persistent on-glideslope indication, the captain re-engaged the autopilot. However, the aircraft was already descending at a rate exceeding the requirements for a stable approach. The autopilot maintained this excessive descent rate, causing the aircraft to drop significantly below the glideslope. As the aircraft descended below 1,000 ft AMSL, the ground proximity warning system triggered a 'glideslope' alert. At this point, the right EADI began showing correct indications, allowing the crew to recognize the unstable approach and immediately execute a missed approach.
The investigation
Investigators examined the aircraft's instrumentation to identify the source of the false signals. Post-incident testing led to the removal and replacement of the left display processor unit (DPU) after it was found to be faulty. While a teardown of the DPU by the manufacturer revealed failed components, the manufacturer noted that the failure patterns were also consistent with an issue originating from the navigation radio.
Both the aircraft and instrument manufacturers stated that this was an unprecedented event, with no other known failures of this nature recorded for this aircraft type or for aircraft using the Pro Line 4 electronic flight instrument system. The investigation could not definitively conclude whether the DPU or the navigation radio was the primary source of the error.