# How is the range/fuel computed when there is a 3-hour difference for the same route?

I am not a pilot. I recently took a trip SIN-SFO 14 hours (headwind) and SFO-SIN 17 hours (tail wind), flying United.

According to the Dreamliner Wikipedia page, the 787 range is 7,635 nmi (14,140 km), where the distance SIN-SFO is 8,447 mi.

1. How is the range rating computed? In the example above, there are 3 hours difference in flight time for the same range.
2. What is the difference in fuel consumption on the two trips (rough average)?

If you have a headwind, the longer you stay aloft, the more you are carried back, so you better hurry up. With a tailwind, it helps to slow down because now the wind is helping you to cover even more distance.

*
SFO = San Francisco
SIN = Singapore

• – fooot Jan 31 '18 at 0:23
• I think you have your headwind and tailwind reversed. – Ron Beyer Jan 31 '18 at 0:25
• If you have a headwind, the longer you stay aloft, the more you are carried back, so you better hurry up. With a tailwind, it helps to slow down because now the wind is helping you to cover even more distance. aviation.stackexchange.com/questions/3612/… – blended Jan 31 '18 at 0:47
• @RonBeyer is correct, and the above understanding of headwind/tailwind is confusing various elements. Yes, an aircraft will fly faster into a headwind and slower with a tailwind, BUT not so much as to make the "with tailwind" flight take longer than the "against headwind" flight. Prevailing trans-Pacific winds are west-to-east; the SIN-SFO leg had a TAILWIND and the SFO-SIN leg had a HEADWIND. Until that concept is grasped, discussions of range computations and the multiple factors that affect that are like teaching Algebra to third-graders. – Ralph J Jan 31 '18 at 1:53
• It would help those of us who haven't memorized a list of all the 3-letter airport codes in the world if you would just say where those places are. – jamesqf Jan 31 '18 at 2:49

If there is a 100 km/h headwind, you can't hurry by that much. That will exceed the plane's capability. Likewise for a tailwind, that would be too slow for the plane to stay aloft.

So, the increase and decrease of speed in head- and tailwind does not nullify the wind. The 14-hour flight was the one in tailwind. Think of the wind as a fast wave, with the plane riding it, the faster the tailwind, the faster the plane gets there.

The keyword is air distance. Wind increases (headwind) or decreases (tailwind) the air distance flown. And therefore the 3-hour shorter flight, would have saved 3 hours worth of fuel, roughly.

From Wikipedia, the Dreamliner cruises at 903 km/h. Using the total time in the air, you can arrive at a rough estimate of how long the air distance was.

From the table here 'For a B787 in cruise, what is the altitude, speed, and angle of attack?', we are looking at roughly 15 tonnes of fuel saved on the 14-hour flight.

The distance between SIN and SFO is 7340 in nautical miles, always use the same units. The plane's range you found is for a typical payload, if there are fewer passengers, the plane flies for longer, as it is lighter in payload and can carry more fuel. And with the wind accounted for, the actual air distance is then known.

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