# What are engineers trying to maximize when they design for takeoff at MTOW? (It seems to me the only free variable is the OEW.)

For a commercial flight, the payload it carries is the weight of passengers and their baggage. Therefore fixed by seat configuration. Fuel weight is determined by the range of the flight. MTOW or maximum structural take-off weight.

So what exactly are engineers trying to maximize when they design for take-off at MTOW?

MTOW = OEW + PL + FW. - This expression seems to indicate that the only free variable is the operating empty weight, and that's what they are trying to maximize, but obviously that doesn't make sense.

My question is why most planes (and most airlines do it this way) are designed to take off on the MTOW line in this PL-R diagram (2-3), and not on the maximum payload line (1-2) or on the maximum fuel line (3-4).

• – user14897
Aug 18, 2021 at 13:28
• Thanks for the reply. Unfortunately that question seems to address a different issue. Aug 18, 2021 at 13:37
• MTOW as defined in the airport planning manuals, which refers to the structural limit. Aug 18, 2021 at 13:38
• Hi Bianfable. I mean that that is the formula for the MTOW line on the PL-R diagram. My question is why are airplanes designed to take off at this weight and not at MPL or MFW. Most airlines take off at MTOW and never at MPL or MFW. Aug 18, 2021 at 13:45
• @MikeSowsun It's possible to take off at MPL or MFW (see PL-R diagram I uploaded), planes do it under special circumstances. At MPL you reduce the fuel to stay below MTOW, at MFW you reduce payload. My question is more about the motivations of airlines to choose the middle line of this diagram. MLW could be an answer for short routes I suppose, but it wouldn't answer the question for long routes. Aug 18, 2021 at 13:58

Thanks to your drawing it's now clear where the misunderstanding is.

Boeing 787 example

The takeoff weight is not limited to the border you marked 1 through 4.

What you're missing is the gross weight lines shown above.

Above shows two scenarios; same gross weight for both and yet different range/payload:

1. The smaller range (just under 3,000 NM) has more payload.

2. The bigger range (just under 5,000 NM) had less payload (more fuel than scenario 1 since it's the same gross weight for both).

MTOW is maximized to lift a bigger weight off the ground, which means more payload and/or fuel.

For more on how to read/use payload–range diagrams, see this Boeing document (PDF; boeing.com).