AAIB investigation to Boeing 747-436, G-BNLN
Unreliable airspeed indications, in flight from London Heathrow Airport to Phoenix International Airport, USA, 9 June 2019.
Summary:
On reaching top of climb the aircraft experienced unreliable airspeed indications resulting in overspeed warnings and activation of the stall warning system. In recovering, the crew carried out the unreliable airspeed procedure but also carried out the stall warning procedure, which was not required.
The problem was believed to have been caused by a fault with the right Air Data Computer (ADC), although this could not be replicated.
As a result of this incident, the aircraft manufacturer is providing additional information as part of their published unreliable airspeed procedure. The aircraft operator is also reviewing its maintenance procedures due to the accidental erasure of fault codes on the right ADC as part of the post-incident inspection process.
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