What happened
On 9 June 2019, a Boeing 747-436, registration G-BNLN, was operating a commercial passenger flight from London Heathrow to Phoenix, USA. Upon reaching its cruising altitude of FL3/30, the crew encountered sudden and conflicting airspeed indications. The aircraft's Engine Indication and Crew Alerting System (EICAS) triggered multiple warnings, including overspeed, altitude disagreement, and airspeed low alerts. Simultaneously, the crew noted a change in cabin air pressure.
As the right Air Data Computer (ADC) began reporting an increasing airspeed, the autothrottle automatically reduced engine thrust. This caused the actual airspeed to drop, creating a dangerous divergence between the left and right data sources. While the crew worked to implement the unreliable airspeed procedure from the Quick Reference Handbook (QRH), the aircraft's stick shakers activated. The crew performed a stall recovery manoeuvre, which involved reducing the pitch to 1° nose-down. During the process of stabilizing the aircraft at the required pitch and power settings, the stall warning repeatedly reoccurred, forcing the crew to adjust the pitch multiple times until the warnings ceased.
The investigation
The AAIB investigation utilized data from the aircraft's Quick Access Recorder (QAR) to reconstruct the event. The data confirmed that while the left ADC showed a steady decline in airspeed, the right ADC showed a rapid increase from 300 kt to 407 kt.
Upon inspection of the removed right ADC at the operator's workshop, investigators discovered that the non-volatile memory containing critical fault codes had been accidentally erased during the initial inspection process. Subsequent functional testing by the manufacturer could not replicate the failure. However, investigators identified that a non-mandatory modification designed to prevent periodic malfunctions had not been applied to this specific unit. This lack of modification likely caused the unit to supply an incorrect Mach number, triggering the erroneous stall warning system.
Findings
- The primary cause of the airspeed discrepancy was a fault within the right Air Data Computer (ADC).
- The activation of the stick shakers was likely due to the right ADC providing an incorrect Mach number to the stall warning system.
- The crew's decision to perform a stall recovery was influenced by the fact that the QRH procedure did not explicitly state that stall warnings could be erroneous during an unreliable airspeed event.
- The loss of vital diagnostic information was due to the accidental erasure of fault codes during the post-incident inspection of the ADC.
Safety action
- The aircraft manufacturer is updating the QRH procedure for the B747-400 to specifically mention that stall warnings may occur erroneously during unreliable airspeed events.
- The operator is implementing new procedural changes in their avionics workshop to prevent the accidental deletion of fault data during troubleshooting.