Air France Airbus A330-200 did not have Pitot Tube Sensor Upgrades

Post date: Jun 9, 2009 8:42:33 PM

Special thanks to Robert Wall/Paris from Aviation Week and Space Technology for the following report.

Search teams have recovered 17 bodies so far, and wreckage from the Atlantic Ocean off the coast of Brazil from Air France Flight 447, an A330-200 that accident investigators have revealed had not yet undergone a pitot tube upgrade the airline was introducing throughout its fleet.

Air speed sensors have come into focus early in the investigation of what happed to AF 447 when it crashed a week ago on its way from Rio de Janeiro to Paris with 228 persons on board.

The pitot tube is drawing attention because some of the 24 Acars (automatic communications and reporting system) messages point to inconsistencies in speed measurements on the aircraft just before all contact was lost at 2:14 UTC. The 24 messages were received during a four minute period, with 14 received in the last minute alone.

The Acars messages point to a discrepancy between three of the aircraft's speed data inputs of more than 30 knots within a period of less than a second. As a result, the system reported a fault and other subsystems - relying on the speed data - also triggered fault warnings, including the Air Data Inertial Reference Units.

Paul-Louis Arslanian, the head of the French air accident investigation office, BEA, says a pitot tube failure has not been confirmed, merely that the Acars messages point to speed sensors disagreeing with each other. However, he confirms that the aircraft had not yet undergone a pitot tube upgrade the airline had underway.

In a statement defending its actions, Air France says only since May of last year has it seen increased loss of air speed data associated with icing in the pitot probes on A330s and A340s and argues it has been instrumental in driving the upgrade program.

The airline says it upgraded pitot probes on A320 narrow bodies following Airbus's advice in September 2007 to do so; it didn't do so on the Airbus wide bodies because at the time water ingestion did not appear to be a problem and no recommendation had been made to do so. Air France insists it asked for a fix once it started seeing anomalies on its A330s and A340s, but that Airbus at first said the A320 issue was different. The carrier says Airbus only let it be known that the A320 improvement could also benefit the A330 and A340. Once it heard that, Air France says it launched the A330, A340 pitot tube upgrade program on April 27 without waiting for a recommendation for Airbus.

The pitot tube upgrade Airbus developed is designed to improve measurement capability of the system.

While stressing the pitot sensor has not been implicated, Arslanian says on any aircraft these probes can be contaminated and fail. He added that the upgrade program does not imply the old device was faulty.

Industry officials and pilots note that the aircraft should still be flyable even if the speed sensors malfunctioned, suggesting if the anomaly turns out to be related to the crash, it would likely be only one in a series of events that brought down AF 447.

However, flying the aircraft would be more challenging with such a system anomaly, particularly in the potentially hazardous conditions the A330 was traversing. One of the Acars messages indicates the autopilot was turned off, possibly automatically. Meteorologists supporting the BEA have identified a severe cumulonimbus cloud in the area the A330 was flying, but have not been able to pinpoint exactly the conditions the aircraft had to pass through.

After the accident, Airbus sent a reminder to pilots of all its aircraft types on maintaining proper speed and on other procedures when flying with ambiguous speed information. Arslanian notes such a message to operators is routine.

He also points out that the regular Acars reports on aircraft location and speed indicate a routine flight up to the incident. However, he also notes that the Acars messages have so far not allowed investigators to draw conclusions about aircraft speed around the final minutes of the flight. And, he adds, the data from Acars may be insufficient to ever establish that information.

"For the time being, we are working with what we have, which isn't much," Arslanian says.

A French vessel, two American ships and a French nuclear submarine will try to track down the cockpit voice and data recorders. The ships will tow microphones to detect the 37.5 kHz signal. However, Arslanian reiterated concerns the recorders may not be found and notes the pinger emitting the signal could have detached.

Meanwhile, the French navy's oceanographic office will try to gain a better sense of what the sea-bed looks like, to aid recovery operations, says Laurent Kerleguer, from the oceanographic department. The ocean in the region can be up to 4,600 meters deep, but is very mountainous leading to elevations where the ocean floor is only 860 meters. The Pourquoi Pas vessel has been dispatched to undertake the research.

The aircraft departed Rio de Janeiro at 10:03 UTC on its way to Paris. The A330 took off with a weight of 233 metric tons and 68 metric tons of fuel. It was climbing to 37,000 feet, but was still at 35,000 feet at the time of the mishap, says Alain Bouillard, who leads the accident investigation team.

Photo credit: Airbus

Section of the vertical stabilizer of the Airbus A330-200 being recovered by Brazilian Navy personnel.