Min Inflight Weight – allow Reserve Fuel or Not?

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  • #1649
    Greg Kappen
    Participant

    All,
    Question for the group on AFM minimum inflight weights. Do you add ballast fuel to bring any aircraft BOW up to min inflight weight or do you allow reserve fuel to be included in the min inflight weight calculation. Here is my question into Boeing and their subsequent response which implies we cannot use reserve fuel and operationally treat min inflight weight as min zfw by adding fuel that must be held for the whole flight.

    Atlas to Boeing,
    We have a 767 that has a minimum inflight weight of 168,000 lb per AFM. The BOW of the aircraft is 167,000 lb.
    Since our minimum planned landing fuel at the alternate airport is 45 mins + our FAR reserve fuel we’ve not added 1000 lb of ballast fuel for this jet and our inflight weight always exceeds 168,000 lb

    A previous Boeing response seems to indicate that the Minimum Inflight Weight is actually a Minimum ZFW even though it’s not called that. In our flight planning system, we have an option for both minimum flight weight and minimum zfw and they perform two very different functions.

    Min flight weight option = adds ballast fuel, but uses FAR Reserve fuel as part of the calculation to get to the min flight weight since FAR Reserve is never planned to be burned. This fits the definition in the last paragraph of the previous Boeing response “The minimum inflight weight is a certified minimum weight below which the aircraft should not be planned for operation,… takeoff to landing.”

    Min ZFW option = adds ballast fuel all the way to make the ZFW equal to in this case 168,000 every flight that we have less than 1000 lb payload. Not such a big deal on this tail, but some of our 747s have much more impact on the amount of fuel to carry to meet a minimum ZFW limitation.

    Can Boeing please confirm if “minimum inflight weight” “minimum flight weight” “minimum flying weight” (I’ve seen all these terms used) means minimum zero fuel weight?
    GREG KAPPEN

    Boeing RESPONSE:

    Dear Greg,

    Please find below the requested Ref /A/ information to clarify if the “minimum inflight weight”, “minimum flight weight”, “minimum flying weight” mean minimum zero fuel weight.

    The minimum inflight weight is the minimum flight weight analyzed for certification. Typically, the highest load factors occur on the airframe at the minimum flight weight. This weight is included in the AFM in compliance with FAR 25.25 and 25.1519 and 25.1583.

    Each major/minor airplane model has a minimum flyable weight defined by weights (mass properties) and analyzed by loads (structures). The Minimum Flight weight term varies between airplane programs. It is listed in the AFM as Minimum Flight Weight for 747-400s and Minimum In-Flight Weight for 767s, 777s, and 757s. The 737 is the only currently produced airplane without a minimum flight weight supplied in the AFM Weight Limitations page.

    The Zero Fuel Weight (ZFW) is constrained by loads and fatigue analysis necessary to comply with certification. ZFW is the weight of the airplane fully loaded with all required flight-related aircraft components (Basic Empty Weight) + Mission required operational items + Passengers and Carryon materials + Checked Baggage + Cargo. In short, ZFW is the weight of the airplane without mission-related fuel loaded.

    So, the ZFW and the “Minimum Flight Weight” (or “Minimum Inflight Weight” or “Minimum Flying Weight”) are not the same. The Minimum Flight Weight represents the minimum weight allowed at any time during flight. It can typically be calculated by adding the ZFW and the loading fuel (this fuel must be held for the whole flight).

    However, as explained on Ref /B/ (service request TLS-TLS-16-0287-02B), the possibility exists where the Minimum Flight Weight can be lower than the ZFW. So, on a Boeing airplane, Minimum Flight Weight would rarely be an operational concern other than possibly on a short flight with minimum crew or in cases where an airplane has been stripped off of its interior items. If the ZFW for a given flight is going to be lower than the published Minimum Inflight Weight, ballast must be carried. This can either be hard ballast or fuel that is designated as unusable for that particular flight.

    The airplane must not be operated in flight at a weight less than the minimum flight weight. This limitation will be met provided the planned landing weight is always equal to or greater than the minimum flight weight.

    Fuel that is loaded in order to bring the ZFW up to the AFM Minimum Flight Weight is ‘additional’ fuel that is loaded over and above all planned mission plus reserve fuel. The mission and reserve fuel may also need to be increased in order to provide the additional fuel that will be burned to carry the fuel used to reach the minimum flight weight. However, as this is an operational decision, it is the responsibility of each airline operator to determine the appropriate total fuel load and the airline’s mission rules to meet regulatory operational requirements.

    #1762
    John
    Participant

    Although apparently not a problem for Boeing aircraft, the limit could also relate to Vmca considerations due to the 5 deg bank limit on establishing Vmca.

    #1771
    Greg Kappen
    Participant

    Atlas ended up increasing ballast fuel for all aircraft to come up to the min inflight weight if payload doesn’t do it for us. done automatically through flight planning system.

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