AAIB Report: Boeing 747-436, unreliable airspeed indications
On reaching top of climb, a Boeing 747-436 (G-BNLN) experienced unreliable airspeed indications resulting in overspeed warnings and activation of the stall warning system, 9 June 2019.
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 investigation safety action has been taken by the aircraft manufacturer to provide 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.