I’ll Buy the Band-Aids
Maintainability is not necessarily accessibility
By Jack Hessburg
Airplane designs are governed by contrasting requirements. The Aerodynamics group wants an airplane with low drag. The Weights group wants the lightest possible airplane. The Performance group wants the highest possible payload, long range, minimum fuel burn and shortest possible takeoff and landing distance. The Structures group wants a structure that looks like the underside of the San Francisco Bay Bridge. Bookkeepers don’t like to spend money. They want low initial cost and even lower cost of ownership. Manufacturing wants low cost, efficient, easily produced devices. Everyone wants high reliability. Mechanics become focused upon accessibility. Understandably, they don’t like working blind or grinding down their tools to fit. They value their knuckles. They want things to be easily accessible. Emphasis here is on the word easily.
The politics of design
Whether designing a desk chair or an airplane, decisions must be made when a project is initially established. The first two questions are:
1. What is it you want the design to do?
2. What do you want it to do best?
You can’t have the best of all these worlds. Thus, engineering design is somewhat like politics — it is the art of compromise. A designer is always trying to optimize all of the contrasting requirements. When designing the airplane and its systems, high on the list of his report card are low drag, high thrust, specific fuel consumption, light weight, and low cost of manufacture. Unfortunately, easy accessibility many times will suffer to meet these items. There is, contrary to conventional tribal wisdom, no conspiracy among engineers to make things difficult to access. Like other factors in a design, accessibility has costs and benefits. Compromises must be made.
Design for accessibility is extremely easy. It is well-defined and documented in numerous "cookbooks" – textbooks, handbooks and design standards. Contemporary designs are greatly enhanced by computer-aided design (CAD) programs that include accessibility criteria. They provide the ability to check tool sweep volumes. And yes, they do include the mechanic’s hand attached to the tool. They can insert human models directly into the design. I know of one CAD program that includes the human dimensions covering males and females between the ages of 16 and 64 and numerous racial groups. For example, a designer today can insert the physical characteristic data of a 37-year old male, petite Thai mechanic (or a rotund German mechanic like me) into the design to see if a person with that makeup could operate effectively. The more advanced programs even allow you to clothe the mechanic in a bulky arctic parka and mittens.
Design "give and take"
All the items in a good design will be accessible, but many times the easy feature must be subordinated. For example, consider that increasing the volume in the nacelle to make it easier to change an IDG increases the frontal area of the nacelle. This correspondingly increases drag. It, of course, also results in increased structure and thus increased weight. High drag and weight increase fuel burn and reduce available payload. This is not a small matter over the 30 plus years of an airplane’s economic life.
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