fuel imbalance
 

Hello,

Would someone please explain to me (in a simple way) how an 'exceedence of a fuel balance limitation' affect the structural life of the airframe and landing gear? The FCTM also says "Lateral control is not significantly affected when operating with fuel beyond normal balance limits".

thanks

You didn't specify what aircraft or any details, but fuel loading by tank concerns not only aircraft control but wing bending moment. Fuel load may be attached to structural and operational limitations, such as being limited to certain speed ranges at certain altitudes, at certain fuel states or with certain tanks filled or not filled. One may be required to maintain a greater fuel difference outboard than in, in some aircraft, for wing bending moment purposes. Some of these restrictions won't necessarily affect aircraft control, but will affect the stresses places on the airframe (specifically the spar, and associated load-bearing structure).

thanks for your reply SNS3Guppy

Sorry for not mentioning the a/c type, I'm talking about Boeing 737-800.

From your answer I understood that an out of limit fuel imbalance could cause the wing/tank that has the low fuel might bend. However I am still not sure if this will happen with the center tank fuel is empty. (in the 737 we use fuel from the center tank fuel first untill it becomes zero). Also I still do not understand how this could affect the landing gear?

1. Loads are calculated assuming certain parameters for weight distribution, one of which is the expected maximum imbalance. If you have more than that amount, the airframe loads will be (slightly) out of line with the design assumptions. For example, at a given AUW, the designers will have catered for various fuel states, but always more-or-less balanced. When you have much more fuel in one wing than another you have a situation where an applied lift force has much less "inertia relief" in one wing than another (that's why you're using centre fuel first - weight in the wings is actually better from structural viewpoint that weight in the fuselage) so the loads on the underweight wing willl be more transferred through the root - in effect the airload on the light wing is having to help support the heavier wing, and that means loads transferring through the wing root which are higher than normal. Hence a possible fatigue effect.

2. The LG situation is similar. LG is designed for certain impact speeds for certain masses sat at the top of the gear. Simplistically, each main gear is assumed to carry a certain % of the landing weight - more than 50%, to account for small imbalances and the like. But if you have a lot more fuel in one wing than the other, even if your overall weight and touchdown conditions are "in limits", the gear under the heavy wing is seeing a lot more load than normal. Hence possible gear issues. (If you took the extereme and had one wing full and one empty, your aircraft is probably under max landing weight - but the gear under the heavy wing is seeing conditions akin to a MTOW overweight landing)