BA1441 - Incident: British Airways A319 near London on Aug 6th 2021

A British Airways Airbus A319-100, registration G-DBCF performing flight BA-1441 from Edinburgh,SC to London Heathrow,EN (UK), was just about to level off climbing out of Edinburgh when the aircraft climbed slightly above the assigned FL350 (up to FL353). The crew reported avionics issues causing the autopilot and autothrust to disconnect. The crew took manual control of the aircraft, descended to FL350, continued to London and performed a raw data approach for a safe landing on Heathrow's runway 27R about one hour after the avionics issues were detected.

The AAIB rated the occurrence a serious incident and opened an investigation. The aircraft suffered avionics issues including unreliable position information.

On Jun 16th 2022 the AAIB released their final report concluding the probable causes of the serious incident were:

The aircraft experienced severe navigation position drift in flight. The drift was caused by abnormal vertical shock loads being transferred through the overextended NLG shock absorber to the ADIRU. The abnormal shock loads were initiated by an uneven patch repair on the runway. The NGC ADIRU is particularly sensitive to sudden vertical shock loading outside its environmental qualification envelope.

The AAIB analysed:

The aircraft suffered multiple severe IR drift events in flight which caused the autopilot and autothrust to disconnect. The events were successfully managed by the flight crew and the aircraft continued to its planned destination.

The incident was caused by a chain of events, each of which was necessary to cause the eventual outcome:

1. A slight irregularity in the runway surface induced sudden vertical shock loads into the NLG.

2. An over extended NLG shock absorber reduced its absorption effectiveness, creating an increase in the vertical forces transferred to the airframe and avionics rack housing the ADIRUs.

3. The NGC ADIRUs fitted to this aircraft (fitted to approximately 14% of the fleet) were sensitive to vertical acceleration forces outside their environmental qualification envelope which induced severe drift.

Runway surface

Following these events, the airport authority found a slightly uneven runway surface patch repair which they believe may have caused the problem. It has now replaced the repair patch.

Nose landing gear leg servicing

Information provided by one of the operators indicated that the improved servicing task and revised maintenance check introduced by the manufacturer was effective in preventing their IR drift issues. However, further occurrences with other operators indicates that the issue has not been completely resolved. The development of a dedicated LANCE to improve the servicing of the NLG shock absorber is intended to reduce potential errors during maintenance activity.

NGC ADIRU

All three of the ADIRUs were tested after the event and no faults were found. During the occurrences, the air data information remained accurate throughout the flight, ruling out ADR faults. The pilots reported that aircraft attitude displays also remained accurate indicating that the IRS gyroscopes were functioning correctly. The severe positional drift experienced was probably caused by IRS accelerometer anomalies from abnormal vertical shock loads transferred to the airframe and avionics rack.

Whilst the NGC ADIRUs performed within their qualification envelope, the shock loads encountered during operation occurred outside their Cat B qualification standard. The aircraft manufacturer commented that in hindsight, the inclusion of Cat C environmental qualification criteria during aircraft design may have avoided these severe position drift issues.

Information to flight crew When this event occurred, no specific information was available to flight crews describing the possibility of multiple IR drifts, the possible indications or how to manage the situation. The aircraft manufacturer decided that publishing such information would be detrimental.

Related Flight: BA1441, Twitter: #BA1441, British Airways News