NO-ONE BENEFITS when accident-investigation agencies clash over the cause of an air crash. The arguments may be based on genuine grievances, but they only serve to deflect attention from the wider issues at stake.

It has happened this week because the US National Transportation Safety Board (NTSB) has placed blame for the October 1994 American Eagle ATR 72 crash jointly on the manufacturer, the French DGAC certification authority and the US Federal Aviation Administration. France's investigative authority, the Bureau Enquetes Accidents (BEA), rejects the findings of the NTSB, and makes the telling point that "major disagreement" between the two agencies may lead the aviation community to discount the safety recommendations resulting from the crash, because they are based on an "arguable report".

The NTSB and BEA agree on the physical reason why the ATR 72 crashed. There is no doubt that ice formed behind the de-icing boots, which resulted in aileron hinge-moment reversal, leading to a loss of control. Thereafter, the agencies diverge dramatically.

The NTSB accuses ATR of failing to disclose previous icing problems , the DGAC of failing to provide the FAA with information on those problems, and the FAA of failing to oversee the continued airworthiness of the ATR 42 and 72.

The BEA firmly blames the flightcrew, stating that the aircraft was flown for a prolonged period in icing conditions well beyond the ATR 72's certification envelope, at a flap setting not provided for in the manual and with the crew failing to "-exercise proper situational awareness".

Given the highly charged atmosphere surrounding air safety in the USA today, anyone could be forgiven for accusing the NTSB of political motivation, particularly as its savage treatment of the FAA is in stark contrast to its gentle handling of the flightcrew (on the basis that the flight attendant was able to visit the cockpit, and the captain to visit the toilet, while holding in icing conditions, because this was not against airline rules).

The crux of the disagreement is whether several previous ATR icing incidents included the aileron hinge-moment reversal phenomenon responsible for the Roselawn crash. The NTSB says that they did (and that ATR and the DGAC failed to pass the information on to the FAA). The BEA says that they did not, and that the phenomenon was only discovered during icing tests after the crash.

The BEA says that ATR had warned operators that icing could cause roll-control problems. The NTSB argues that flight-manual information on the aileron-reversal phenomenon could have averted the disaster. It seems sensible that any reference in the manual to control problems in icing, however specific, should have guaranteed the crew's full attention. Even the NTSB acknowledges some of the potential for blame, by recommending that the need for a "sterile" cockpit environment, while holding in icing conditions, be evaluated.

All this may be lost, however, in the fracas which will follow the NTSB's call for a review of how the FAA monitors the airworthiness of foreign aircraft. The Board is careful to say that the ATR 42/72 does not appear to be improperly certificated, despite the FAA's "limited involvement". At issue is "continued airworthiness" and the information flow from the foreign certificating authority to the FAA during an aircraft's life.

This is bound to have implications for efforts to harmonise US and European certification standards, and for negotiations under way on bilateral airworthiness agreements with countries such as Russia. Bilaterals exist because the regulatory authorities, and the manufacturers, do not have the resources to undertake a complete certification programme for each country. That should not be changed on the basis of one disputed accident report which contains many other valuable safety recommendations.

Source: Flight International