What happened
On May 19, 1999, a Dornier 328-300 was conducting a ferry flight from Dresden to Oberpfaffenhofen under instrument flight rules (IFR). The aircraft was operated by Dornier Luftfahrt GmbH and carried a crew of two. During the descent, the flight crew experienced significant malfunctions on both the left and right Primary Flight Displays (PFD). Specifically, the indicated airspeed (IAS), flight level (FL), and outside air temperature (OAT) became unreliable.
As a result of these discrepancies, the autopilot automatically disconnected, and various warning and status messages appeared on the Engine Indicating and Crew Alerting System (EICAS). At the time of the malfunction, the aircraft was flying through clouds and experiencing light to moderate icing. Despite the disturbances, which lasted approximately one minute, the crew managed to land the aircraft safely in Oberpfablehofen without injuries or damage to the aircraft.
The investigation
The BFU, in collaboration with the manufacturer, conducted a technical examination of the aircraft. Investigators analyzed the flight data recorder (FDR) and focused on the pitot heat system and the ice detection system. The investigation confirmed that the pitot heats had been operational since the engines were started in Dresden and had shown no failure indicators during the flight.
Meteorological analysis performed by the DLR (Institute of Physics of the Atmosphere) revealed that the aircraft had flown through an extensive area of convective clouds over southern Germany. This convective activity involved strong upward transport, which created conditions where significant amounts of supercooled water could exist even at higher altitudes with temperatures below -20°C.
Findings
- The primary cause of the airspeed display failures was the blockage of the pitot tube openings due to ice accumulation.
- Although the pitot heat system was functioning correctly, the design of the pitot system was unable to prevent ice buildup during brief encounters with extreme meteorological conditions.
- The presence of significant amounts of supercooled water at high altitudes was a critical contributing factor.
- At the time of the incident, the Aircraft Flight Manual (AFM) and Aircraft Operating Manual (AOM) lacked specific emergency procedures for a double or triple failure of the airspeed indicators.