What happened
On January 24, 2007, an Airbus A320 was climbing through FL120 en route from Nuremberg to London-Stansted when the crew heard a loud bang on the right side of the aircraft, just below the cockpit window. Immediately following this noise, the aircraft's Electronic Centralised Aircraft Monitor (ECAM) displayed ADR1, ADR2, and ADR and ADR3 faults. This triggered a cascade of system failures, including the loss of both autopilots, the autothrust system, and both flight directors. The flight control laws transitioned from Normal Law to Alternate Law.
The crew noted that the three airspeed indicators were providing conflicting data, with values differing by as much as 30 knots. During the climb, the pilot in command maintained manual control, leveling off at FL130. The crew performed a holding pattern near Erlangen to analyze the situation and manage the procedures. Due to the unreliable airspeed information and the deteriorating weather conditions, the crew decided to divert to Frankfurt/Main. During the approach and landing, the extension of the landing gear caused the flight control laws to transition into Direct Law. The aircraft landed safely without any injuries to the 52 passengers or 7 crew members.
The investigation
The BFU investigation focused on the cause of the airspeed discrepancies and the subsequent system failures. Investigators examined the aircraft's maintenance records, the flight data recorder (FDR), and the cockpit voice recorder (CVR). The investigation noted that while the wings and tail had been de-iced with Type-II fluid prior to departure, the fuselage had not been de-iced. Post-flight inspections of the aircraft revealed visible ice and snow residue on the fuselage nose, specifically near the static ports and Pitot probes.
Findings
- The investigation determined that the contamination of the Pitot-static system was the primary cause of the unreliable airspeed indications.
- The loud noise heard by the crew was likely caused by the separation of ice plates from the aircraft fuselage.
- Because the fuselage was not de-iced, snow and ice remained on the nose. As the aircraft climbed through an inversion layer (between FL100 and FL120), melting ice likely allowed water or slush to enter the Pitot probes and static ports.
- The resulting pressure variations led to the disagreement between the three Air Data Reference (ADR) units, which prevented the system from performing a plausibility check and triggered the transition to Alternate Law.
- The replacement of ADR2 following the flight was determined to be unrelated to this specific incident, as the component had shown errors on previous days.