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.