How refreshing it is that Air New Zealand has decided to maintain services despite a cloud of volcanic ash from an eruption in Chile passing over the country. Jet aircraft have been in commercial operation for about 50 years and it was not until last year that they were grounded by an eruption far away.

European air space was closed for five days by an ash cloud from Iceland, leaving airline executives and passengers angry and mystified. Aircraft were just the same, volcanic ash was no worse than it has ever been. What had changed was the official attitude to safety.

Authorities these days have adopted what is called the precautionary principle, which means a risk does not have to be established to any degree of likelihood before drastic avoidance measures are justified. The possibility of a risk is considered sufficient. Better to be safe than sorry.

The principle originated in the environmental movement, became accepted by bureaucrats and now infects even the corporate sector when it contemplates the worst that could happen. Air New Zealand executives will have thought long and hard about the decision to keep flying after the ash began to drift over the country at the weekend. It decided the cloud was high enough for plans to operate under it, albeit at greater expense since it takes more fuel to fly at lower altitude.

Qantas came to a different decision for itself and Jetstar on transtasman and domestic services, declaring them "not worth the risk". Other airlines have cancelled a limited number of flights. All, including Air New Zealand, will be haunted by the possibility that something could go wrong.

The risk seems to be that hard particles in exploded magma are very abrasive on aircraft surfaces, including cockpit windows, and would melt inside jet engines. The scientists who endorsed the decision of European air traffic controllers last year cited the experience of a British Airways 747 that flew through ash from a 1982 eruption in Indonesia. All four engines failed because of melted ash on engine turbine blades, but the pilot was able to restart three of them after descending.

BA's chief executive described the precaution taken after the Icelandic eruption as a "gross over-reaction to a very minor risk". Others were more dismissive of an invisible danger that caused Europe's largest air traffic close-down since World War II, disrupting the plans of about 10 million passengers worldwide.

Last month, when ash from another Iceland volcano drifted over Scotland, Britain's Civil Aviation Authority gave airlines information about the ash and the cloud density and left them to make the decision. Civil Aviation in New Zealand has done likewise. For the moment it appears to have waived the precautionary principle and decided that airlines' commercial interest can make a safe decision.

This, too, is refreshing if the decision is indeed simply a calculation of the cost of flying under the cloud. But if scientific judgments are required about the nature of the material from this eruption, its drift patterns and likely behaviour in our skies, it should not be a decision for each airline to make in isolation. The public, and airlines, have a right to expect Civil Aviation to make the call solely on grounds of public safety.

If, heaven forbid, Air New Zealand's decision turns out to be wrong, Civil Aviation will not escape blame. Knowing this, it must have decided it is safe to permit airlines to continue to fly if they wish. And its equanimity will have influenced Air New Zealand.

If their common sense causes no harm, it might begin to change the culture of safety, replacing extreme caution with a more balanced weighing of likely risk and people's needs.