Simon Calder: The Man Who Pays His Way

Airline alliances are expanding: is this the Finn end of the wedge?

Every day at around 8.30am, an extraordinary manoeuvre takes place over the North Sea. British Airways flight 6031, an Airbus A320 flying from Helsinki to Heathrow, overtakes British Airways flight 795 - also an Airbus A320 flying from Helsinki to Heathrow.

The faster plane left Helsinki Terminal 2 at 8am, a quarter-hour later than the slowcoach. It is scheduled to reach Heathrow Terminal 1 five minutes ahead of BA 795 - the first London-bound plane of the day to depart from the Finnish capital, but not the first to arrive.

Passengers with flexible tickets who dawdled en route to the departure airport are rewarded with a faster trip than the conscientious souls who turned up on time. They have a wider range of "legal" connections. And they get to spend 20 minutes less on an aircraft.

Any traveller who consults the British Airways timetable can acquire all this information, apart from the bit about the overtaking manoeuvre: that is my inference from this curious piece of scheduling. It is odd that identical aircraft flying between identical points should have such different journey times. Stranger still, though, is the mystery of why a pair of BA flights should leave from a beautiful but marginal European capital just 15 minutes apart, with no further flights for another six hours.

The answer is another of those great aviation misnomers: code-sharing. Many airline terms involve slack use of language (most of us would like our non-stop flight to stop at least once, at our final destination). With "code-sharing", it's the plane that is shared, not the code. Every flight between Heathrow and Helsinki carries both a BA and a Finnair flight number. The faster plane each morning is operated by the Finns, reflecting the national passion for speed.

You may need to be reminded why code-sharing exists. A grumpy old cynic (see picture, left) would argue that it stifles competition, thus allowing fares to rise. But the airlines insist it allows them "to offer their customers more services and benefits than any of them can provide on its own". That is the rationale given by Oneworld, the alliance to which both BA and Finnair are affiliated.

In practice, this seems to involve despatching two flights inside a quarter of an hour. There is then a six-hour gap until the next London-bound service, when two more planes take off just 15 minutes apart; on this occasion BA 6079 narrowly fails to overtake BA 793.

Travellers heading in the opposite direction are equally spoiled for choice. The penultimate departure to Helsinki leaves Heathrow terminal 1 at 6.05pm. If you miss it, don't worry, because there will be another one along in five minutes.

British Airways' comfortable synchronicity with Finnair has been going on for six years, during which two other airlines have tried and failed on the route: Flying Finn recently followed Buzz on the flightpath to oblivion after failing to make money between Stansted and the Finnish capital. With the route between London and Helsinki to themselves, I have struggled to find a fare on BA or Finnair lower than £160 return. True, this is slightly less than a decade ago, but to destinations elsewhere in Europe airfares have halved since the onset of the no-frills revolution.

My mistake has been to start in the wrong place. Anyone able to travel to Birmingham will find fares lower, thanks to the new arrival of Duo (0871 700 0700, www.duo.com). You can get a return flight, with frills, between the West Midlands and Helsinki for £150. And if you watch carefully while flying over the North Sea, you might just see the duel of the BA Airbuses.

WHY YOUR TRIP COULD INCLUDE SOME UNPLEASANT SURPRISES

Code-sharing is enshrined in the 1929 Warsaw Convention, the anti-consumer legislation which, 75 years on, still governs air travel within the EU. It says airlines can do more or less what they like. When you buy a ticket you agree to let them "substitute alternate carriers" any time.

Take British Airways' promise of "complimentary catering and bar service" with a pinch of inflight salt if you find yourself on a "BA" flight operated by one of its partners in the Oneworld alliance. It is increasingly likely that you will have to cough up for coffee and anything else you drink or eat on board.

For the past six months, economy passengers on BA-coded flights operated by Swiss have had to pay for refreshments; now Iberia is following suit. "Choose what you want to have onboard", sounds a tempting proposition from the Spanish airline, "with our great value-for-money 'Tu Menu' à la carte service in economy class." Anyone who infers that "great value for money" means gratis inflight catering is sadly deluded. Passengers on half the BA flights from Heathrow to Barcelona and Madrid will discover there is no such thing as free tapas.

It gets worse. Never heard of JetStar? If you book yourself aboard a Qantas flight such as QF5624 from Sydney to Queensland's Sunshine Coast you soon will. JetStar, which starts flying on 25 May, is the low-cost offshoot of Qantas - the national carrier's no-frills response to Sir Richard Branson's highly successful Australian venture, Virgin Blue. Yet Qantas is flogging flights on JetStar to overseas travellers as though they were the "real" thing, even though familiar Qantas facilities such as allocated seats, complimentary meals and baggage transfer are unavailable.

In its choice of destination, JetStar has more in common with Ryanair than does Virgin Blue. Hitherto Avalon may have put you in mind of the location of Glastonbury or a song by Van Morrison. But it is also the name of an airfield on the edge of the Victorian town of Geelong, and is described by its owners as a "base for heavy jet pilot training" (they do not specify if the pilots or the jets are overweight).

Avalon airport is convenient for the start of the Great Ocean Road, but not for the city of Melbourne, 55km away; the main airport, Tullamarine, is much closer. But if you buy a JetStar flight from Brisbane or Sydney to Melbourne (at www.jetstar.com), Avalon is where you will end up. A little patch of Somerset is set to become the Frankfurt Hahn of Australian aviation: a field far from the city it purports to serve.