What happened
On 6 August 2021, an Airbus A319-131, registration G-DBCF, departed Edinburgh Airport for London Heathrow. During the takeoff roll, the commander noted a loud bang as the aircraft passed over what felt like a centerline light. Shortly after climbing through 1,500 ft, the crew observed a brief GPS primary lost message.
Upon reaching FL340, the autopilot and autochrust disconnected, and the flight directors disappeared from the displays. The crew noted abnormal Inertial Reference System (IRS) position readings, with values fluctuating significantly. Although the crew managed to re-engage the systems after five minutes of manual flight, the autopilot and autothrust disconnected again during the approach to Heathrow at approximately 4,000 ft. The crew transitioned to a manually flown raw data approach and landed the aircraft safely with no injuries to the 5 crew members or 101 passengers.
The investigation
The AAIB investigation focused on the cause of the IRS drift and subsequent system disconnections. Analysis of the Quick Access Recorder (QAR) data revealed that during the takeoff roll at 120 kt, the nose landing gear (NLG) weight-on-wheels signal toggled three times within two seconds, indicating a significant jolt from the runway surface.
Subsequent inspections of the aircraft found that the NLG shock absorber was over-extended by 0.6 inches. This condition reduced the gear's ability to dissipate energy, allowing vertical forces to be transferred directly into the airframe and the avionics rack housing the ADIRUs. The investigation also examined the runway at Edinburgh, where a slightly uneven patch repair was identified as the source of the initial shock.
Findings
- The primary cause of the incident was severe vertical shock loads transferred from the runway to the aircraft's inertial reference system.
- An uneven patch repair on the Edinburgh runway triggered the initial vertical jolt during takeoff.
- The nose landing gear shock absorber was over-extended, which increased the amount of vibration and shock transmitted to the avionics rack.
- The specific ADIRU units installed on the aircraft were sensitive to vertical accelerations that fell outside their environmental qualification envelope.
- While the manufacturer had previously issued guidance to maintenance teams regarding similar issues, no specific operational guidance had been provided to flight crews regarding managing such drift events.