Boeing plans to update the 747-400 flight crew operating manual to include stall warnings notice where they were previously only presumed. The announcement came from the UK’s aviation authority investigating a “serious incident” involving British Airways Boeing 747.
On June 9, 2019, British Airways Boeing 747-400, carrying 320 passengers and 18 crew members, took off from London Heathrow Airport (LHR), the UK. The aircraft was operating a scheduled passenger flight and was en route to Phoenix International Airport (PHX), the U.S.
Shortly after the takeoff, when the aircraft had just reached the top of the climb at FL330, the jumbo jet’s pilots encountered a problem with the aircraft avionics: unreliable airspeed indications provoked overspeed warnings and activation of the stall warning system, the British Civil Aviation Authority’s (CAA) Air Accident Investigation Branch (AAIB) outlined in the incident report, issued on April 30, 2020.
The pilots managed to recover by applying the unreliable airspeed procedure and the stall warning procedure ‒ although the latter was not required, the investigators pointed out. Afterward, the crew continued the journey to the original destination, where the aircraft landed with no further incidents.
The authority believes that the problem was caused by a fault with the right Air Data Computer (ADC). However, the non-volatile memory (which includes faulty codes) of the ADC unit was later “accidentally” deleted at the operators’ avionics overhaul base. The company has since taken action to implement procedural changes, to avoid losing troubleshooting and fault data.
While Boeing’s Quick Reference Handbook noted that “overspeed warnings and AIRSPEED LOW alerts may occur erroneously or simultaneously”, it did not mention stall warnings at the time. The aircraft manufacturer assumed that pilots would understand this was included, but the incident showed that the crew “considered they must react to the stall warning when it occurred”, the investigators found.
So now, Boeing is planning to update the Handbook procedure to specifically include stall warnings as part of the note. The revision to the B747-400 Flight Crew Operating Manual is expected in April 2020, and similar action is considered with other relevant types, according to the AAIB.
Delivered to British Airways in July 1990, the Boeing 747 (registration number G-BNLN) was 29-years old at the time. Following the incident, the plane was sent for storage later the same month.
