# 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 (tailwind) and SFO-SIN 17 hours (headwind), 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

• Commented Jan 31, 2018 at 0:23
• I think you have your headwind and tailwind reversed. Commented Jan 31, 2018 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/… Commented Jan 31, 2018 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
Commented Jan 31, 2018 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. Commented Jan 31, 2018 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.

See these posts:

Flight planning is based primarily on time not on ground distance, by this I mean fuel burn is in pounds per hour at a given airspeed, not pounds per mile or miles per pound. Airplanes come with pages of charts showing fuel burn at many combinations of airspeeds, gross weights, altitudes, and temperatures. They also come with fuel and time required to climb to various altitudes and fuel used during startup and taxi.

Modern airline dispatchers use computer programs to calculate the actual time and fuel burn with more precision(interpolating between data points), and importantly with great speed.

A flight leg for fuel planning is a segment between navigation fixes and is commonly 50-300 miles. Even when crossing the ocean there are charted navigation fixes where pilots report to ATC over the radio.

The older manual method was to calculate tailwind or headwind for each leg of the proposed route based on weather forecasts and then calculate time en-route for each leg based on a desired cruise airspeed.(climb and decent are special legs, also included) This time is used for a rough estimate of total fuel burn for the flight to then get a close estimate of gross takeoff weight. The estimate of gross takeoff weight is then used to calculate the fuel consumption for the first leg of the flight, the fuel consumed on the first leg is then subtracted from gross weight to get a new gross weight and rate of fuel consumption for the second leg of the flight. This sequence continues for each leg. Then sum all of the fuel burned on each leg of the route (along with reserve fuel) add this fuel weight to the empty weight and payload weight. Finally check to make sure the flight is within limits by comparing the total fuel needed to the rated fuel tank capacity and checking the total weight sum is less than the allowed maximum takeoff weight of the airplane for the specific departure runway length, climb-out obstacle clearance gradient, and current headwind.

The maximum takeoff weight for a certain set of airport conditions is the common limit on range, the airplane's published maximum structural takeoff weight or fuel tank capacity are less often the limiting factors. Landing is rarely a limit simply because a safe takeoff and climb requires a much longer runway[accelerate plus stop] and a lower obstacle clearance slope [with one engine failure] than is needed for a landing; and you would not purposefully land at an airport that would not also allow a safe departure with a reasonable load. I bring this up because it is possible that if prevailing winds are reliable through the year, and if the physical takeoff limits at SIN are different from SFO, the route could be designed such that a 17 hour flight is only allowable in one direction.