# Flight time changing according to weight

I flew earlier on a 747-8 from doha to Denver. The flight time would’ve been around 15 hours and according to the weight and balance menu I had enough fuel for 17 hours of flight. I quickly realised that wasn’t the case i had enough only for 11 hours (probably due to my heavy weight). I then thought it would be a good Idea for the flight time to change in the Weight and balance menu according to the weight chosen so that such situations don’t happen anymore.
What do you think?

Flight times do NOT get affected by the weight of an aircraft. It all depends on the programmed Mach number, design & type of aircraft, winds, and cost index of the airline (which actually encompasses most of the formerly mentioned factors).

On the factor of weight, the airline policy decides the allowed range of speed at only the cruise altitude [which is provided in the company given flight plan] which would often only be ±0.01 Mach difference, but here too, the speed will be the final decision of the pilots in charge who can either choose to fly slower to be economical in tailwind while some pilots in the same aircraft type and route may choose to fly slightly faster to make up for a previous delay.

cheers :)

tl;dr = flight time does not depend on weight lol

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oh okay, but how come i only got 11hours of fuel when the sim told me before take off that i had enough for 17 hours?

Because you saw 17 hours BEFORE the engines getting used. As the engines go vroom more fuel is consumed and the pre-flihhgdata might be wrong

The weight-and-balance page uses an arbitrary rate of fuel consumption to estimate endurance. In reality, fuel consumption can be modeled as being equal to (TSFC) x (Total Drag) x (time). With the weight of fuel and TSFC being equal, total drag (D) and endurance are inverse. It is likely you were flying at a higher speed than what the weight-and-balance page is “calibrated” to, or there was something causing excess drag on your aircraft, perhaps a high AOA or extended flaps that went unnoticed. In response to the feature request specifically, more specific endurance times could easily be estimated assuming the aircraft is in a cruise configuration, given a user-inputted cruise speed.

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You’re saying weight doesn’t affect flight times, speed does. Well, weight affects speed. The four principles of aerodynamics are: Thrust, drag, lift and weight. A heavier weight would mean that the autopilot will fly at a higher angle of attack, as it is required in order to generate more lift, resulting in more drag and a slower speed. Furthermore, newton’s second law states that mass is directly proportional to force, and therefore it is incorrect to state that weight does not influence flight time. You have to understand that there is only a certain amount of fuel, and we cannot simply make up for this by applying more and more thrust. Especially in the case of a 15 hour flight.

To this, I mentioned the following solution:

I think you know how step climbing works, “especially in the case of a 15 hour flight”, so I won’t explain that to you :)

And although all your points are valid and not incorrect in any way, weight still only has a minor and indirect [like you explained] effect on flight time. In my opinion, all the factors that weight is induced by is already implemented in the simulator.

You’d find that

are more valid factors affecting flight time. For example, take EWR-DEL as a route. The route is served by UA and AI, both utilising their 77W’s. If you search up on FR24, you’ll find that AI has a significantly longer flight time. Now I know for a fact that AI has a lower load factor which was told to me by an instructor on the dreamliner in Air India who is also a great friend. So why is the flight time so skewed?

1. They depart at different times, hence they face different winds. You can also see that if you scroll the flight playback on FR24 for both the flights on the same day and note the groundspeed at the same altitude and nearly same location. [If you see both the playback as they enter Canadian airspace, they both are at FL290 but the UA 77W is at 570 kts groundspeed compared to AI’s 540]

2. They take different routes. Hence also face different winds at roughly the same point in both their flights.

The way this discussion goes is not me proving you incorrect, but rather just implying that although you are correct, the need for such a topic on the forum makes little to no sense :)

btw I’m glad you mentioned long haul flights while making a comparison! Otherwise, the argument is quite literally, a 747 is faster transatlantic rather than an A321LR but the 747 is heavier sooOO lol 😳

cheers

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The flight time change is affected by the following

• Deviation of track
• Winds
• Total Drag
• Angle of Attack/Thrust Ratio
• Flaps & Slats

Hope this Helps