What happened
On February 22, 1999, a Dornier 328-100 was operating a scheduled flight from Berlin-Tempelhof to Mannheim City. While cruising at FL 260, the aircraft descended into IMC conditions between the ERFURT and WÜRZBURG VORs. During this descent, the aircraft encountered moderate to heavy icing conditions within cumulonimbus clouds.
At approximately 16:27 UTC, the aircraft experienced a complete loss of all airspeed indications on both Primary Flight Displays (PFD) and the standby indicator. The vertical speed indicator also failed, and the autopilot disconnected due to a miscompare with the Air Data Computer. The crew found themselves without reliable airspeed data for approximately five to six minutes. To maintain safe flight, the crew manually managed the descent and flight path using pitch attitude and engine torque settings. The aircraft eventually reached VMC at FL 200 and all instruments returned to normal by FL 140. The flight concluded with a safe landing at Mannheim City at 16:49 UTC with no injuries to the three crew members or five passengers.
The investigation
The BFU investigation, conducted alongside the manufacturer Dornier Luftfahrt GmbH, focused on why three independent pitot systems failed simultaneously. The investigation noted that the operator had not initially reported the incident to the BFU as required by law. While the flight data recorder was unavailable for analysis, the BFU examined Air Data Computer logs, weather reports, and conducted wind tunnel tests and theoretical studies.
Findings
- The primary cause of the instrument failure was the blockage of the pitot tubes by ice accumulation.
- Despite the pitot heat being active and functioning correctly, extreme meteorological conditions—specifically the presence of supercooled water droplets within cumulonimbus clouds—led to ice buildup on the probes.
- The aircraft's design allowed for the blockage of all three independent pitot tubes under these specific severe icing conditions.
- At the time of the incident, neither the Aircraft Flight Manual (AFM) nor the Aircraft Operating Manual (AOM) provided any emergency procedures for a total loss of all airspeed indications.
- The investigation found that the aircraft's design did not guarantee pitot tube functionality during accidental encounters with severe icing, even with heating enabled.