Here’s a ‘Secret’ Way to Beat High Cost of Flying : Airlines: Believe it or not, the cheapest way to get from London to Paris might be on a carrier based in Bahrain. It pays to know your ‘fifth freedom rights.’

Earlier this year, I was in Bangkok and needed to fly to Hong Kong. There were some obvious airline choices. Thai Airways flies the route. So does Cathay Pacific, the Hong Kong-based airline. A check with each airline showed the one-way coach fare at $229.

A friend who lives in Bangkok, however, suggested an alternate flight. For only $140, I bought a ticket from Bangkok to Hong Kong on Air India Flight 332.

“The flight will be half empty,” he told me, “because no one ever thinks Air India when booking between Bangkok and Hong Kong.”

Indeed, the flight--on a spacious airbus--was half empty and my ticket was cheaper.

And last month I was flying between Nice and London. Both British Airways and Air France had regularly scheduled flights. But after my Bangkok-Hong Kong experience, I decided to check all airline flight schedules between the two cities.

There were no fare deals to be had (with short notice, all carriers charged about $473 round trip for the ticket).

But I flew to London in style, on Flight 865. The plane was more than half empty--only 55 people on an L-1011. There was great service and no delays. The airline? Air Canada.

The only delay occurred when I arrived. Friends who were meeting me in London went to the British Airways terminal at Heathrow Airport, convinced I couldn’t possibly be flying from France on Air Canada.

To be sure, no one thinks about Air India when flying between Bangkok and Hong Kong, or Air Canada as the carrier of choice between Nice and London. Yet, with a little advance planning, passengers choosing these routes stand a reasonably good chance of being pleasantly surprised.

Welcome to the world of “secret” flights. With few exceptions, these flights are not heavily promoted or advertised. But they are scheduled services. In many cases, because so few people know about them, fares on these flights are also cheaper.

Flying from London to Frankfurt? Instead of a cramped 737, try a 747 . . . on Philippine Airlines. How about New York City to Vienna . . . on Royal Jordanian Airlines. Los Angeles to Tokyo . . . on Varig, the Brazilian airline.

The routes and city pairings of the secret flights are nothing less than staggering.

It’s all part of something called “fifth freedom rights,” destinations that are added on as part of deals that have been made between countries as a way to conclude international air route agreements.

In Europe, London is the starting point for dozens of secret flights.

For example, Aerolinas Argentinas flies between London and Madrid. Gulf Air, based in Bahrain, flies between London and Paris. Kenya Airways flies from London to Athens, then on to Nairobi. Air China, based in Beijing, flies between London and Zurich.

And, in some cases, due to some bizarre foreign government protective regulations, the “secret” airlines operating these flights are forbidden to sell more than half the available seats on each flight.

For example, Air Canada actually operates Flight 865 between Nice and Montreal, with a stop in London, but can carry passengers between Nice and London.

Still, for competitive reasons, the French government strictly limits Air Canada. It prohibits the airline from selling all the seats on Flight 865. As a result, passengers flying Air Canada between Nice and London are virtually guaranteed empty seats next to them.

There are also some bizarre secret flights in North America. If you’re in Dallas and want to fly to Mexico City, try Lufthansa Flight 494. It operates on Mondays and Thursdays--and it’s a 747. (Every other competing airline only operates 727s on that route.)

Flying from Montreal to New York City? LanAir Chile flies the route, using a Boeing 767. So does Royal Air Maroc, using a 747. Air France Flight 47 flies between Houston and Mexico City. And, once a week, a Lufthansa plane flies between San Juan, Puerto Rico, and St. Maarten. Flying between Anchorage and Tokyo? You can take Swissair Flight 164 three times a week.

In Central America, you can fly between Panama City and Guatemala on a DC-10, which belongs to KLM.

In Asia, the secret flights also abound.

Swissair, for example, has 30 separate sectors that qualify as “fifth freedom rights” flights. From Bangkok alone, Swissair flies to Abu Dhabi, Bombay, Hong Kong, Jakarta, Karachi, Manila and Singapore. Japan Airlines flies from Bangkok to Athens. Air France flies between Bangkok and New Delhi.

Between Dubai and Singapore, and Singapore and Melbourne, try KLM. And between Bombay and Singapore, try Air Canada Flight 888.

Many of the secret flights are “positioning flights,” segments of longer legs. For example, Philippine Airlines Flight 731 travels between London’s Gatwick Airport and Frankfurt, then continues on to Karachi, Pakistan and Bangkok before arriving in Manila.

Most of these secret flights--such as Flight 837--only operate once or twice a week, but they can be excellent travel deals. Chances are good, for example, that since few people know you can fly between London and Copenhagen twice a week on Varig, the Brazilian carrier, there will be many empty seats on the flight.

As a result, and despite official denials from the airlines involved, there are substantial discounts available. In London, many travel agents and ticket consolidators (also known as bucket shops) sell discount seats for these flights.

For example, the cheapest official advance-purchase, round-trip fare on British Airways between London and Frankfurt (for a ticket purchased in London) is approximately 96 (about $172 U.S.).

But a discount ticket on the same route bought on Philippine Airlines through a discount agent in London--with no advance purchase--runs 70.

In Asia, where ticket dumping by airlines--especially those flying fifth freedom routes--is ongoing, the same types of deals are available. Virtually every travel agent in cities such as Taiwan, Bangkok and Hong Kong acts as a sort of commodity broker for discount seats on many secret flights.

On the Bangkok-Hong Kong run, if a discount ticket isn’t available on the Air India flight, other fifth freedom airlines on the route include Swissair, Canadian, Gulf Air, China Airlines, Air France, Alitalia and Air Lanka. More often than not, at least one of these airlines will discount their seats on the route.

Many U.S. travelers can buy discount seats in advance from one of these travel agents by phoning ahead and using credit cards. But it’s not necessary to buy many of these seats in advance. Most travelers wait until they get to a country and buy them locally.

One note of caution: When buying a discount ticket on one of the secret flights, you may only receive a receipt from the travel agent, which will then be redeemed for the actual ticket and boarding pass at the airport. The ticket you receive will often be heavily endorsed with restrictions.

Chances are it will reflect the full “official” fare (even though you paid less), will be good only on that airline, only on that flight and only on the day you chose to travel. It will also be nonrefundable.

Why the listing of the full fare? It’s the factor of “credible deniability.” With full fares listed on discount tickets, the airlines can then officially deny that they sold discount tickets.