Stall warnings rang out in the cockpit of a packed British Airways jumbo jet flying to the US, a report revealed today.
The Boeing 747 was travelling from London Heathrow to Phoenix, Arizona, with 338 passengers and crew aboard when the pilots were given unreliable speed readings – triggering an alarm system.
The plane dropped 2,800ft as the crew fought the problem.
No-one was hurt and the flight continued to Phoenix.
The “problem was believed to have been caused by a fault with the right air data computer (ADC),” according to the Air Accidents Investigation Branch, an arm of the Department for Transport.
There was known to be a potential fault with the systems and a modification had been developed.
But it had not been fitted to the jet, the report says.
Outlining the mid-air drama, which unfolded 91 minutes into the flight last June, the report says the 29-year-old plane had just hit its cruising height of 33,000ft “with the right autopilot and auto-throttle engaged, when the master warning activated”.
“On reaching top of climb the aircraft experienced unreliable airspeed indications resulting in overspeed warnings and activation of the stall warning system,” it adds.
The left air data computer was telling crew the plane was travelling at 306pmh while the right system was saying the aircraft was flying at 391mph.
The crew spoke by radio to BA's maintenance office which confirmed the right ADC had failed.
Once the plane landed safely it was “grounded and permanently removed from service”, the AAIB says.
“It is likely that the false warnings had been generated erroneously as a result of an incorrect Mach number being supplied by the right ADC,” the report reveals.
“This would then also have caused the stall warning system to operate erroneously at a safe airspeed.”
While the investigation does not name the airline, it gives the plane's registration number, G-BNLN – showing it was operated by BA.
When the device was sent to BA for analysis by technicians, it was accidentally wiped, the report says.
The AAIB concludes: “Since the event, the operator has taken steps to identify the process shortcomings that permitted the loss of the fault codes to occur following arrival of the ADC in their avionics workshop.”
Boeing is updating its emergency handbook to “specifically include stall warnings as part of the note”, it adds.
A BA spokesman said: "Safety is always our top priority and our highly skilled pilots are trained to deal with these situations.
"The aircraft continued to Phoenix as normal and landed safely.
"The aircraft was thoroughly inspected by our engineers and had no further incidents."
