Qantas' flagship QF1/QF2 Sydney - London - Sydney flights normally make a pitstop in Singapore. It makes a long-haul flight between the two countries even longer. While most passengers don't mind the opportunity to stretch their legs, Qantas doesn't stop in Singapore for passenger comfort reasons. The airline stops over because its aircraft cannot make the flight in a single hop.

Aircraft cannot make the distance carrying normal payloads

It is 3,907 miles (6,228 kilometers) between Sydney and Singapore and 6,765 miles (10,888 kilometers) between Singapore and London, adding up to a 10,672 mile (17,116 kilometer) flight. That's before any planned or unplanned changes to the most direct tracking.

Qantas has four long-range aircraft types available - the Airbus A380-800, A330-300, A330-200, and Boeing 787-9. The Airbus A380 can fly 9,445 miles (15,200 kilometers) with a commercial payload. A Boeing 787-9 Dreamliner has a range of 8,464 miles (13,621 kilometers) when normally loaded.

Qantas hasn't a history of sending A330s into Europe or the United Kingdom, but their A330-300s can fly 7,021 miles (11,200 kilometers), and their A330-200s can cover 8,326 miles (13,400 kilometers).

These ranges factor in average passenger and cargo payloads. However, extra heavy payloads and environmental conditions can reduce the range. In any case, none of the Qantas' long-range aircraft can fly between Sydney and London nonstop while carrying a profitable payload.

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The Boeing 787-9 test flights were successful as only 50 people were onboard, and the pilots wore EEG monitors and had their melatonin levels measured during the flight by having urine samples analyzed. Photo: Qantas

Flight can be done - in a near-empty plane

It's not to say these aircraft can't fly the Sydney - London city pair nonstop. Qantas first made the nonstop flight in 1989 when their first-ever Boeing 747-400 made history by flying between the two countries in one hop for the first time.

In 2019, a Boeing 787-9 Dreamliner operated a nonstop Project Sunrise research flight between London and Sydney. More recently, a Qantas A380 flew from Dresden to Sydney nonstop (Dresden being a mere stone's throw from London).

But none of these flights operated with commercially viable payloads. There was no cargo and few, if any, passengers. Loaded up, the planes won't make the distance.

The Singapore pitstop allows the aircraft to refuel. It also allows a crew change and passengers to get off the plane temporarily. On the downside, the Singapore stopover increases the length of the overall flight.

When the A380 is back operating QF1/2 by the middle of 2022, it will take nearly 24 hours to complete the Sydney - Singapore - London sectors. Flying down to Sydney clocks in at just over 23 hours.

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Singapore has long been a popular pitstop for Qantas aircraft. Photo: Getty Images

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It makes sense for Qantas to send QF1/2 through Singapore. It is usually Qantas' busiest offshore port, and the airline has good infrastructure there. Changi Airport (and Singapore in general) is also popular with Qantas' Australian customer base.

But Qantas hasn't always sent QF1/2 through Singapore. For a long time, the flights operated via Bangkok. In 2012, Qantas axed the Bangkok - London leg of its Sydney - Bangkok - London service. The A380 operated Sydney - Singapore - London service (QF31) and return London - Singapore - Sydney service (QF32) was rebadged as QF1 and QF2.

In the latter part of the last decade, the daily Melbourne - Perth - London Boeing 787-9 Dreamliner flights complemented QF1/2. This year, the Dreamliner flights via Perth remain suspended, and as a temporary measure, Qantas is re-routing QF1 and QF2 through Darwin instead of Singapore. However, QF1 and QF2 should head back to Singapore later in the year.

Qantas does want to fly the Sydney - London city pair nonstop. They plan to do that in the future with modified Project Sunrise Airbus A350-1000 aircraft. But Project Sunrise, if it goes ahead, is still a few years off. In the meantime, once Darwin is dropped, a stopover in Singapore beckons for passengers on QF1 and QF2.