How do you become a millionaire?
Start with a billion pounds then launch an airline.
The joke attributed to Sir Richard Branson, of Virgin Atlantic fame and fortune, goes some way to convey how difficult it is to run a profitable airline.
Between oil prices, delays and disruption and a fiercely competitive marketplace, air travel may seem like big business but it is notoriously tough.
That said, it's not all scraping a living. New analysis by aviation experts OAG has highlighted the specific routes on which some of the world's biggest airlines make their money – and it's a lot of money.
The single route that earned the most revenue in the 12 months to March 2019 was British Airways' flights between London Heathrow and New York JFK, banking more than $1.15billion ($A1.7bn): that's nearly 5 per cent of the entire annual revenue of IAG, which owns BA. The route was the first ever to earn more than a billion dollars for an airline, and remains alone in the achievement.
The sum is more than twice the GDP of Tonga.
OAG says more than 30 per cent of the seats on the route are in either first or business class, making it clear where the value lies.
The second most lucrative route was a domestic flight: Qantas' services between Melbourne and Sydney, earning the Australian airline $861m a year. The sum is all the more remarkable given the 90-minute flight is flown on small 737 planes, with only 174 seats, just 12 business class.
In third place, raking in $796m in revenue a year for Emirates is the airline's services between London Heathrow and Dubai.
Singapore Airlines is the only carrier to have two routes in the top 10, one between London and Singapore, earning $735m, and the other between Singapore and Sydney, banking $549m. Five of the top 10 were domestic services, with three in the US.
"For every airline there are a small selection of lucrative routes where either competitive advantage, market circumstances or limited competition make for very attractive revenues; they are the routes that are 'protected' at all costs," said analyst John Grant.
OAG said the top 10 this year is unchanged from last.
Grant said: "Our analysis suggests that there is unlikely to be any great movement either in or out of these tables in the next few years until capacity becomes available and then of course at both ends of the route which for many of these airlines will ensure their status for some time."
Does this mean that British Airways' Heathrow to JFK is the busiest route in the world? Far from it. According to analysis by OAG earlier in the year, the flight doesn't make the top 10.
The most crowded flight path on Earth is actually the 280-mile hop from Seoul Gimpo to Jeju International, for the popular holiday destination of Jejudo, South Korea's answer to Hawaii.
A staggering 76,460 flights travelled between the two airports in 2018; by comparison, just over 14,000 flew between London Heathrow and New York's JFK. Seven airlines ply the route: Asiana Airlines, Jeju Airlines, Korean Air, Jin Air, T'way Air, Eastar Jet and Air Busan.
Melbourne-Sydney was, however, second busiest - as well as the second highest earning - with 54,102 departures. Europe's busiest service is Barcelona-Madrid, with 18,812 flights last year; Latin America's is Lima-Santiago (10,369).
The most oversubscribed long-haul route is New York JFK to San Francisco, with 15,587 flights in 2018.
Revenue earned on route, by airline, in millions ($), 2018/19
JFK to Heathrow (BA) - 1,159
Melbourne to Sydney (Qantas) - 861
Dubai to Heathrow (Emirates) - 796
Heathrow to Singapore (Singapore Airlines) - 735
San Francisco to Newark (United) - 689
Los Angeles to JFK (American) - 661
Heathrow to Doha (Qatar) - 639
Hong Kong to Heathrow (Cathay Pacific) - 604
Sydney to Singapore (Singapore Airlines) - 549
Toronto to Vancouver (Air Canada) - 541
The Telegraph, London