Airspeed sensor failures

WASHINGTON — On at least a dozen recent flights by U.S. jetliners, malfunctioning equipment made it impossible for pilots to know how fast they were flying, federal investigators have discovered. A similar breakdown is believed to have played a role in the Air France crash into the Atlantic that killed all 228 people aboard in June.

The discovery suggests the equipment problems are more widespread than previously believed. And it gives new urgency to airlines already scrambling to replace air sensors and figure out how the errors went undetected despite safety systems.

The equipment failures, all involving Northwest Airlines Airbus A330s, were brief and were noticed only after safety officials began investigating the Air France crash — on a Rio de Janeiro to Paris flight — and two other recent in-flight malfunctions. The failures were described by people familiar with the investigation who spoke only on condition of anonymity because they were not authorized to comment publicly.


Air pressure readings
While a car’s speedometer uses tire rotation to calculate speed, an airplane relies on sensors known as Pitot tubes to measure changing air pressure. Computers interpret that information as speed. A car’s broken speedometer might be little more than an inconvenience, but many airplane control systems rely on accurate speed information to work properly.
Like the fatal Air France flight, the newly dis- covered Northwest incidents and the two other malfunctions under investigation all involved planes with sensors made by the European electronics giant Thales Corp.

The Air France crash called into question the reliability of the sensors and touched off a rush by airlines to replace them.

Many companies, however, simply replaced them with another Thales model. As it became clear the problem was more widespread, Airbus and European regulators told companies to replace at least two of the three sensors on each plane with models made by North Carolina-based Goodrich Corp. The planes are allowed to continue flying while the switch is made.

Thales officials declined to comment. The company has previously said its sensors were made to Airbus specifications.


Data found in merger
The Northwest incidents were discovered when Delta Air Lines, which merged with Northwest last year, reviewed archived flight data for its fleet of 32 Airbus A330s, the people close to the inquiry said. All the planes involved landed safely.
Aviation experts said the discovery could provide clues to what caused Air France Flight 447 to crash into the Atlantic June 1, and what might be done to prevent future tragedies.

French investigators have focused on the possibility that Flight 447’s sensors iced over and sent false speed information to the computers as the plane ran into a thunderstorm at about 35,000 feet.

Three weeks after the crash, the U.S. National Transportation Safety Board began investigating two other A330 flights that experienced a loss of airspeed data.

The most recent was on June 23, when a Northwest flight hit rain and turbulence while on autopilot outside of Kagoshima, Japan. According to an NTSB report, speed data began to fluctuate. The plane alerted pilots it was going too fast.

Autopilot and other systems began shutting down, putting nearly all the plane’s control in the hands of the pilot, something that usually happens only in emergencies.


by the associated press