Getting to Grips with Aircraft Weight and Balance(109)

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At each tie-down point, a force F is generated in the forward direction with a magnitude proportional to the aircraft deceleration rate. In that case, the load factor in the X direction (nx) becomes greater than 1, and the lashing equipment shall be able to restrain a load equivalent to the piece‘s apparent weight: F = nx.W (with W = piece‘s weight).
Moreover, each tie-down point (single stud fitting) can withstand a maximum load of 900 kg. Therefore, F must be less than 900 kg.

The forces F1 generated by the deceleration at the tie-down point number 1 can be decomposed into three components F1x, F1y and F1z as shown previously. Considering a force F1 of 900 kg, the maximum force components at tie-down point number 1 are:
F1x max = 75% F1 = 675 kg F1y max = 43% F1 = 387 kg F1z max = 50% F1 = 450 kg
6.3.6. Ultimate load 
a) Example 1
Let‘s assume that the previous piece is loaded in the aft cargo hold of an aircraft, COMPARTMENT 3. As shown in the following —Weight & Balance Manual“ extract, the lashing must be done so as to be able to restrain any piece submitted to a load factor of 1.69 in the FORWARD direction.

A. LOADING GENERALITIES

Therefore, if the piece‘s weight is W, the force that has to be restrained by the lashing equipment in the forward direction is : Fx = 1.69 W
As shown in the previous paragraph, the maximum load that can be restrained in the forward direction by one strap fastened to two tie-down points (1 and 2) is : 
Maximum load = Fx max = F1x max + F2x max = 675 kg + 675 kg = 1350 kg
The Ultimate Load represents the maximum weight of a piece or a pallet that can be restrained by a lashing equipment in a given direction. In our example, the ultimate load in the forward direction is : Ultimate load = Maximum Load / Load factor
Ultimate Load  = 1350 / 1.69 = 798 kg
Conclusion : With ONE strap restraining the load in the forward direction, the maximum piece‘s weight must not exceed 798 kg due to the breaking strength of the tie-down points.
b) Example 2
Let‘s assume that the load is now restrained in the forward direction by two strap as shown in the next figure.
Note : It is prohibited to use two straps attached to the same tie-down point and restraining the load in the same direction
Each strap is fastened to two tie-down points, creating a force of magnitude Fn at each tie-down point n.
The force that has to be restrained by the lashing equipment in the forward direction is still: Fx = 1.69 xW
The maximum load that can be restrained in the forward direction by two straps fastened to four tie-down points (1, 2, 3 and 4) is : 
Maximum load = Fx max = F1x max +F2x max + F3x max +F4x max = 4 x 675 kg = 2700 kg
Ultimate Load  = 2700 / 1.69 = 1597 kg
Conclusion: With TWO straps restraining the load in the forward direction, the maximum piece‘s weight must not exceed 1597 kg due to the breaking strength of the tie-down points.
　
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