Sometime last year a very nervous crew boarded a top secret Airbus A340 flight in a remarkable human guinea pig experiment.

Airbus, which is the second biggest aircraft manufacturer in the world behind Boeing, had decided to literally fly into the heart of the Y2K problem - the "year 2000 bug" - to see what would happen to airplanes flying over the millennium.

As the A340 craft flew across the international dateline, the pilot set all the plane's clocks forward to midnight on December 31st of this year. There were no reported problems with both plane and crew landing safely a few hours later.

The problem, if it still needs reiterating, is that the very first computers in the 1960s began abbreviating dates (1968 became 68) in order to save what was then valuable disk memory.

Thus computer and computer chips may fail to recognise the two zeroes of next year as a new year, or interpret it as 1900 and subsequently crash.

Most everything we take for granted in our everyday life has the potential to be adversely affected. But whereas it is one thing being concerned about your banklink card not working or your VCR recorder not recording your favourite programme, it is quite another thing if you are suspended a couple of thousandmiles above the Atlantic relying on computer technology to get you safely back down onto the ground.

The Y2K prophets of doom who envisage global blackouts, world economic recession and planes falling out of the sky may be substantially wide of the mark but even the most sanguine commentators are advising caution when it comes to transport,particularly airline transport.

It is not just that even as we speak, most insurance companies are writing exclusion clauses into next year's policies to cover potential problems arising from the date change, it is also that senior executives within the airline business are expressing doubts.

"I don't believe everything will work perfectly," said Mr Scott Nathan, vice-president of information technology at American Airlines (one of the biggest carriers in the world).

"I think we will see doors that don't open, problems with utilities, telephone systems that don't work and power failures. What we don't know is whether these will amount to a nuisance or something more serious."

Britain's largest charter airline, Britannia airlines is still decidingwhether to fly over the millennium with a spokesman saying "there are risks of power failures, disruption to ground transport, to signal and traffic failures and even critical personnel being stuck in lifts".

KLM's chief information officer, Mr Max Rens said: "In answer to the question of whether I'll be flying over the millennium, I reply "yo" - that's a combination of yes and no . . . I'm fearful that there will be delays and detours. Planes will stay on the ground and within half a year some airlines will be facingbankruptcy."

The problem does not lie with the airlines themselves - most US and European carriers have been working on the problem since the mid 1990s to "de-bug" every single computer chip liable to be affected - BA alone has 3,000 technicians working on Y2K and expects its total "debug" bill to be more than £100 million.

Aer Lingus is similarly "clean", mainly because it started its Y2K project with an inventory in 1996 and has allocated considerable resources to the project.

With more than 70 million people around the world claiming Irish heritage and with two million Irish people living in Britain, there will be massive pressure on Aer Lingus and on Irish airports with people travelling home for the period. The company will be using its Y2K state of readiness as a marketing tool over the next 11 months.

It's more of a problem of "connectivity": this refers to the fact that computer-controlled air traffic control and radar surveillance systems must be "clean" of the bug as should those engaged in flight scheduling and passenger ticketing.

Similarly the software that manages cargo, aircraft fuel, baggage, routing, security, ground transport and catering must also pass the test.

And it doe not end there: even if an airline, an airport and all its myriad support and supply systems prove themselves to be Y2K compliant, there's a whole new series of questions to be asked about the airport to which you are flying.

The US government is already considering a selective ban on flights to countries that have not yet shown they are fully Y2K compliant (Russia, South America, the Middle East, Africa and parts of Asia are frequently mentioned) with African countries causing the main concern.

KLM has already announced that it will ground flights to countries where aviation authorities can not show they have made adequate preparations.

At a meeting of the International Civil Aviation Organisation last year, 22 African countries were called to a special meeting to hear concerns being expressed about their lack of readiness.

US President Bill Clinton has told African states they will be offered technical help but not financial aid. The African countries causing extreme concern are those whose air-traffic control systems were "donated" by either the CIA or the KGB as bribes during the Cold War - neither of these organisations offer after-sales services.

All principal airlines and US and European airports are already well-prepared for Y2K. The Irish Aviation Authority (IAA), which regulates and monitors the safety and efficiency of aircraft in the air and on the ground here, has initiated a comprehensive Y2K testing programme.

With effect from January 1st this year, the IAA will not be renewing certificates of airworthiness for Irish-registered aircraft unless the owner or operator provides satisfactory evidence that an appropriate programme has been or is being implemented to address Y2K, and planes belonging to Irish aviation companies could be grounded unless they comply with the IAA's strict millennium guidelines.

Remember though that this is the largest software project in history and addressing a conference on Y2K last year, the US Deputy Secretary of State, Mr John Hamre said: "We have to expect that we will not get everything fixed and there will be `known unknowns' - known problems with unknown solutions and outcomes. And there will also be `unknown unknowns'. . ."