What happened
On September 20, 2017, an Airbus A320-214, registered EC-HTD, departed Zurich (LSZH) for Barcelona (LEBL) as a scheduled commercial flight. Shortly after takeoff, the aircraft experienced a significant technical malfunction. At approximately 18:44 UTC, while the crew was transitioning to a new radio frequency, the autopilot and autothrust systems automatically disconnected, accompanied by a master warning and the characteristic 'cavalry charge' alert.
In an attempt to resolve the issue, the crew performed a computer reset. During this process, the circuit breakers for both Flight Management Guidance Computers (FMGC) were pulled almost simultaneously. This action resulted in the total loss of the flight plan and navigation data on both the Multipurpose Control and Display Units (MCDU) and the Navigation Displays (ND).
Unable to follow an instrument approach due to the lack of navigation indications, the captain requested radar vectors for a visual approach to runway 14 at Zurich. During the descent, the aircraft's Enhanced Ground Proximity Warning System (EGPWS) issued several false 'terrain ahead, pull up' warnings due to the corrupted positioning data. The aircraft landed safely at Zurich, and the passengers disembarked without injury.
The investigation
SUST examined the aircraft's flight data recorder and maintenance reports. The investigation established that the initial discrepancy began at takeoff, when the Air Data Inertial Reference Units (ADIRU) reported a disagreement. This was likely triggered by the nose gear bouncing over runway lighting, which caused vertical accelerations that disrupted the sensors.
As the flight progressed, the FMGCs began rejecting data from at least two ADIRUs, leading to the automatic disconnection of the autopilot and autothrust. The investigation also reviewed the crew's actions during the reset procedure and their communication with Zurich Air Traffic Control.
Findings
- The primary cause of the loss of navigation capabilities was the simultaneous pulling of the circuit breakers for both FMGCs, an action that violated manufacturer procedures and caused the irreversible loss of the flight plan.
- The crew's task sharing and Crew Resource Management (CRM) were deficient; the captain made the decision to return to the airport without consulting the copilot.
- Communication with Air Traffic Control was inconsistent, as some of the captain's transmissions were not broadcast due to failure to properly engage the push-to-talk button.
- The EGPWS warnings during the landing phase were false alerts caused by the lack of reliable positioning data.
Safety action
No specific safety recommendations were issued by SUST, as the investigation concluded that no further findings were necessary for accident prevention beyond the summary provided.