Air France 447 - Despite No Firm Link, Pitot Tubes Spotlighted in Flight 447 Investigation
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By Robert Wall and Jens Flottau
There is growing anxiety within aviation circles that a failure to pinpoint the cause of the crash of Air France Flight 447 could leave a shadow hanging over the Airbus A330 and, possibly, the airline.
No matter the outcome of the accident investigation, not finding a probable cause would leave the door open to rampant speculation about potential shortcomings in the aviation system, and this is troubling to industry officials. As one equipment maker points out, at least if a hardware fault is found it can be fixed.
What is prompting fears of such a scenario are statements made by investigators that they may not be able to retrieve the cockpit voice and data recorders. And, so far, wreckage recovered and data on-hand have provided no clear indication of what may have transpired on the night of May 31-June 1.
The early focus of the investigation has been on assessing the meaning of 24 Automatic Communications and Reporting System (Acars) messages to the A330-200 transmitted in a 4-min. period prior to all contact with the aircraft being lost at 2:14 UTC; 14 of the messages were received in the last minute alone.
The fault warnings indicate a range of problems, from unreliable airspeed indications - which triggered a series of further alerts from subsystems relying on that data - to a drastic change of cabin pressure.
Pitot tubes are particularly under the microscope. These three sensors are pressure measurement instruments used to gauge fluid flow velocity and they are intrinsic to airspeed indications. If blocked with ice or debris, false readings can appear in the cockpit and could lead pilots to operate at inappropriate airspeeds either risking stall or, on the other extreme, causing the aircraft to exceed design limit loads.
The speed sensor anomaly occurred at 2:10 UTC after other fault warnings had already appeared. But it has driven particular attention to a potential fault in the pitot tubes. Air France was upgrading the Thales sensor (Goodrich also provides A330 pitot tubes) but Flight 447's A330 had not yet been upgraded, says Paul-Louis Arslanian, head of the French air accident investigation office (BEA).
In a message to operators, Airbus states that the new Thales pitot tube "has been developed to enhance water drainage encountered during heavy rain conditions on takeoff or landing phases." But the aircraft maker insists that the old design can continue to be operated safely and points out that the new device is not entirely immune to icing either.
Air France says it launched the upgrade of the pitot tube on Apr. 27 after starting to see an increase in pitot tube failures last year. The airline says that, at the time, Airbus indicated a pitot tube upgrade recommended for the A320 would not benefit the A330/A340 family. Further research indicated there would be a benefit from the upgrade, prompting Air France to launch the program. It has not been mandated by safety authorities, however.
Air France chief executive, Pierre-Henri Gourgeon, says the airline initially planned to replace failed tubes with the new version. The first ones ordered had arrived just days before Flight 447 crashed with 228 persons on board. The airline has now accelerated the upgrade program, but Gourgeon insists there is still no indication the pitot tubes are linked to the accident.
The Air France pilots union (SNPL) says the carrier made a commitment as an interim measure to fly only A330s and A340s that have at least two of the upgraded devices until all three are at the newest standard; the upgrade program should be complete by the end of the month.
The European Aviation Safety Agency last week issued a safety bulletin reminding operators to ensure that air crew are properly trained to handle aircraft if unreliable airspeed is encountered. EASA also said the unreliable speed information can be caused not just by extreme environmental conditions, but also by inappropriate maintenance, contamination by small objects or other causes.
Industry officials and pilots note that the aircraft should still have been able to operate even if the speed sensors malfunctioned, although in degraded flight control modes with reduced flight envelope protection. That suggests that even if the anomaly turns out to be related to the crash, it would likely be just one in a series of events that brought down Flight 447.
One A330 pilot points out that the key is to maintain basic pitch attitude and power settings and avoid sudden changes. The flight manual spells out specifically how to try to operate the aircraft in such conditions.
However, flying the aircraft would be more challenging with such a systems anomaly, particularly in the potentially hazardous weather conditions the A330 was traversing. Meteorologists supporting the BEA's conclusions have identified a severe cumulonimbus cloud in the area the doomed aircraft was flying, but have not been able to pinpoint what conditions the aircraft had to pass through.
Another A330 pilot points out that during such turbulence the aircraft is best flown on autopilot, but the Acars messages indicate that the autopilot was turned off at 2:10 UTC, either by the pilot or automatically. The latter can occur if the aircraft speed exceeds limits either at the low or high end.
After the accident, Airbus sent a reminder to pilots of all its aircraft types regarding maintaining proper speed and other procedures when flying with ambiguous speed information. Arslanian notes that such messages to operators are routine.
He also points out that the regular Acars reports transmit aircraft location and speed every 10 min., and in this case indicate a routine flight up to the time of the incident. Nevertheless, 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.
The series of events spelled out by the Acars messages also suggests a far more complicated series of problems than mere pitot tube icing.
Around 2:10 UTC, not only was the autopilot disabled, the Acars messages indicate the aircraft defaulted to its alternate law flight controls, which doesn't provide the full automatic envelope protection. Failure messages also indicate that the Traffic Alert and Collision Avoidance System (TCAS) or the rudder travel limiter failed, followed by those for the Air Data Intertial Reference Units (Adiru). Then, the alerts point to a fault in the primary and secondary flight computers ("F/CTL PRIM 1 FAULT" and "F/CTL SEC 1 FAULT"). The final message, from the Electronic Centralized Aircraft Monitor, signaled a change in cabin altitude at a rate greater than 1,800 ft. per min., Airbus states, adding that it "remains to be explained."
Airbus also notes that data so far do not suggest loss of electrical power, loss of instrument display, or a fault with the Adiru.
Flight 447 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 ft., but was still at 35,000 ft. at the time of the catastrophic incident, says Alain Bouillard, who leads the accident investigation team.
Meanwhile, work is underway to locate the cockpit voice and data recorders. On June 10, the French nuclear-powered attack submarine Emeraude arrived at the search zone, which is located several tens of nautical miles west of the debris field. The Emeraude can cover an area of 20 sq. naut. mi. per day down to a depth of 3,600 meters (2.23 mi.). Given the 1,500-meter range of the recorder transponders, that should be enough to pick up a signal, according to French military officials.
Early this week, the search for recorders should be stepped up. A pair of French supply ships fitted with U.S.-provided microphones to detect the recorders' pingers are expected to arrive in the crash zone. They could be joined by buoys deployed by French Atlantique 2 and U.S. Navy P-3 marine patrol aircraft when the debris search winds down. The actual retrieval of the recorders, if located, will be undertaken by deep-diving submarines on the French research vessel Pourquoi Pas.
But officials are not optimistic. A French navy officer points out that even in the crash of a Flash Air 737 near Sharm El Sheikh, Egypt, in 2004 it took two weeks to recover the so-called black boxes, and there the wreckage was near the shore. This time, the sea-floor is more than 4,000 meters deep in some areas, and the ocean bed is described as extremely varied in depth.
With Michael A. Taverna in Paris.
