In summary, only four cost drivers in Group 1 – size, material, structural-design concept, and manufacturing philosophy – are required to establish the cost of component manufacture and assembly. The other four cost drivers in Group 1 can be evaluated similarly for nacelles that differ in geometry, technical specifications, functionality, and manhour rates.

16.4.2 Nose Cowl Parts and Subassemblies

The build–work breakdown of the two nacelles from start to finish is grouped in six stages, as shown in Table 16.7; however, the cost of their parts is different. Nacelle A is an existing design and its cost is known. Nacelle B is a later design with a lower parts count and assembly time, achieved by superior structural and manufacturing considerations through DFM/A studies. The nose cowl consists of pure structures (STRs), minor parts (MPs) (e.g., brackets and splices), engine-built units (EBUs) (e.g., anti-icing units and valves), and aircraft general supply (AGS) (e.g., fasteners, rivets, nuts, and bolts). EBU costs are studied separately and not herein.

The expensive components are the STR and the installation of EBU parts. Clearly, these costs are reduced to almost half, thereby saving the cost of the Nacelle B nose cowl even with a larger engine. Assembly hours are also reduced to nearly half. AGS is not expensive but there are numerous rivets, nuts, bolts, and so forth.

16.4.3 Methodology (Nose Cowl Only)

The author points out that this seemingly simple algebraic procedure with elementary mathematics becomes a complex workout. Newly initiated readers may find it difficult to follow. It will require the instructor’s help and industrial data to understand the coursework for their project.

The methodology generates the factors and indices from existing Nacelle A, the cost data for which are known. Based on the similar geometry, these factors and indices are then adjusted using the DFM/A considerations and applied to Nacelle B. The conceptual design phase outlines the basis of the manufacturing philosophy under the DFM/A, relying heavily on the Nacelle A experience. Table 16.8 lists the necessary factors and indices for the eight cost drivers (i.e., the data from the industry). The table is followed by expanding the eight cost drivers.

Note:

Primary cost driver.

The shop-floor learning characteristics are an important factor in cost consideration. Initially, parts fabrication and their assembly take longer (actual manhours) than when it is a routine task with a stabilized time frame of standard manhours, which initially is the target time. If actual manhours do not reach standard manhours, the investigation is required to change the standard manhours. The faster people learn, the greater is the savings for taking fewer manhours to manufacture. The number of attempts required to reach the standard manhours varies, and the DFM/A study must consider this aspect. In this case, Nacelle B has a faster learningcurve factor, with fewer parts.

1.Ksize: Geometric details of the nacelles and engine parameters are listed in Table 16.5 to estimate Ksize.

2.Material Cost: Material is classified in two categories: (1) raw materials (e.g., sheet metal, bar stock, and forging), and (2) finished materials (e.g., lipskin, engine ring, and some welded and cast parts acquired as subcontracted items). The weight fractions of both nacelles are listed in Table 16.9. The unit cost for each type varies, depending on the procurement policy (see notes in the table). The next part of the table lists details of the raw-material weight fractions. The last column provides various material costs per unit weight, normalized relative to the aluminum sheet-metal cost. The AGS consists of various types of fasteners including blind rivets (more expensive) and solid rivets; they are classified as raw materials because it is impractical to cost each type separately.

3.Cost of Manufacture: The core of the manufacturing cost buildup considers the cost drivers of geometry, technical specifications, manufacturing philosophy, functionality, and manhour rates. For this study, only the evaluation of the manufacturing philosophy is required, as discussed in the next two subsections.

Raw material weight fraction (finished material not included)

Notes:

There is no composite in the nose cowl of Nacelle B, but it is used in the core cowls of both nacelles. The subscript “T” stands for total weight of nose cowl; A and B stand for each nacelle.

Cost of Parts Fabrication

Table 16.10 lists the cost of parts fabrication in a nondimensional form from the manhours involved. Actual manhours needed to manufacture parts for each of the six Nacelle A stages can be obtained from the shop-floor engineering process sheets. Factored indices for Nacelle B can be established through DFM/A studies at the conceptual design stage.

At each stage of parts manufacture, manhours are given in fractions of the total manhours for all parts; manhour rates are invariant. The Nacelle B learning-curve factor for parts fabrication is about the same as for Nacelle A but not for the assembly. Nacelle B has fewer parts, thereby saving on costs.

The rate and factor for Nacelle A are 1; details for Nacelle B are in Table 16.7.

Subassemblies

Table 16.7 lists details for the Nacelles A and B nose cowl subassembly in six stages of processing. Table 16.10 lists subassembly costs in a nondimensional form in fractions of the total assembly manhours for all stages. Costs of the pure structure of the nacelle mould lines are separated from those of all other nonstructural components (e.g., anti-icing ducting, linkages, cables, and accessories) that are part of the complete EBU fitment for a nacelle that is ready for a turbofan engine. The costs of installing EBUs in the assembly process are considered but not the actual EBU cost. The assembly cost is expressed as follows:

assembly cost = rates × manhours × learning-curve factor

The rate and factor for Nacelle A are 1; details for Nacelle B are shown in Table 16.11. Savings are realized through the DFM/A study.

4.Cost of Support: Certain additional costs are incurred when a product fails to adhere to the desired quality during inspection. In that case, reworking and/or design concessions are required to salvage the product from rejection as scrap. These are the support costs – generally minor but difficult to determine. A flatrate of 5% of the cost of material plus parts manufacture plus assembly is added as the support cost. DFM/A studies attempt to ensure design and manufacturing considerations that minimize support costs by making the product right the first time (i.e., the Six Sigma concept).

Cost of Amortization of the NRCs

Table 16.12 shows the two types of NRCs in nondimensional form. Amortization is performed for more than 200 aircraft – that is, distributed over 400 nacelle units.

Table 16.12. Nonrecurring costs