What happened
On April 5, 1998, an Airbus A320-200 operating a scheduled flight from Lyon to Frankfurt experienced a significant flight instrument failure while in a holding pattern near Frankfurt/Main. During the approach, the airspeed indications on both Primary Flight Displays (PFD) and the standby instrument failed simultaneously for a short period. This loss of data triggered the disconnection of the autopilot and auto-throttle systems, while the Electronic Centralized Aircraft Monitor (ECAM) issued various warning and error messages.
The pilot in command (PIC) immediately took manual control of the aircraft from the pilot trainee. After stabilizing the aircraft at 10,000 feet using pitch and power settings, the airspeed indications returned to the displays. As a precaution, the PIC manually activated the pitot/probe heat. At the time of the incident, the aircraft was flying through instrument meteorological conditions characterized by heavy icing and turbulence.
The investigation
The BFU investigation involved a review of flight data recorder (FDR) records, maintenance logs, and interviews with the crew and maintenance personnel. While the FDR showed the autopilot was interrupted for 59 seconds and System 1 speed data was lost for 14 seconds, the crew reported that the loss of airspeed on both PFDs was significantly longer and nearly identical in duration. The investigation also included an analysis of the aircraft's maintenance history and communication with the aircraft manufacturer and the pitot tube manufacturer.
Maintenance inspections following the event, conducted according to Service Information Letter (SIL) 3ary-047, found no blockages or residues in the pitot tubes, and the aircraft was returned to service. However, the BFU examined the pitot tubes and their associated computers to determine the root cause of the intermittent signal loss.
Findings
- The simultaneous failure of all airspeed indications was caused by the blockage of pitot tube pressure holes due to ice accumulation.
- The investigation confirmed that the design of the pitot tubes made the aircraft vulnerable to airspeed fluctuations and failures during heavy rain and severe icing conditions.
- There was a discrepancy between the FDR data and the crew's experience regarding the duration of the failure, likely due to the limitations of the FDR's recording capabilities for specific system status changes.
- While the aircraft manufacturer had previously issued communications regarding similar issues in the A320 family, the investigation found that the existing maintenance procedures and the aircraft operating manual did not provide specific limitations for flight in such severe icing conditions.
Safety action
- The BFU recommended that pitot tube specifications be modified to ensure unrestricted flight in heavy rain and severe icing conditions, specifically advocating for the mandatory installation of improved pitot tubes across the affected Airbus models (A320, A321, A330, and A340).
- The BFU recommended that airline operating manuals or maintenance instructions should mandate special inspection procedures following the simultaneous failure of multiple redundant systems to ensure the aircraft is safe for return to service.