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.
The devil’s in the details
Let’s look at some ways that accessibility becomes significant. Consider the racks in the lower electronic compartment of the 777. Now the volume available in this location is limited after accommodating the nose landing gear. Ten pounds of equipment must be fitted into this five-pound bag. Thus, the designers who had to fit the electronic racks into this location were desirous of placing the racks as close as possible to the fuselage skin and frames. This makes sense as it maximizes use of the available volume – including a little more room for the mechanic to work because the crawl space can be a little larger. But consider that the structure behind these racks must be accessed to inspect for cracks, deformation and corrosion. Placing the racks too close to the structure would severely inhibit this inspection as it requires the removal of some of racks to do the inspections. Now doing this will disturb several aircraft systems. This means, that to verify the integrity of the disturbed systems, additional checks must be accomplished when they are reinstalled. This increases maintenance costs, time out of service, exposes systems to a greater probability of maintenance error, and drives up manufacturing cost (for example properly fitting insulation blankets).
If an LRU has shown itself to be unreliable in previous designs, easier accessibility to it may be in order. If a device cannot be qualified on the minimum equipment list, then accessibility must become a higher priority in the design. No go items must be highly accessible so as to minimize out of service time. On the other hand, if the device is highly reliable and only requires inspection every five years for example, the designer can "bury it." It will upset one mechanic every five years but burying it gives the designer a little wiggle room in the available volume to make other more frequently touched items easier to access.
Mechanic’s input appreciated
Having mechanics involved in the design does not assure easy accessibility. It does, however, assure that when design trades are made, the impact upon accessibility is included. Mechanics can help the designer to find alternative locations or positions for a component. They bring their culture, working environment and manual dexterity to the design table. Consider the case of pressure regulating and shutoff valves (PRSOV). They are notoriously unreliable and are frequently changed. I am familiar with one design in which the access hole in the pylon for the valve was incorrectly sized. The result? When changing the valve, mechanics can put their hands thru the access hole. They can put the valve through the hole. However, they cannot put their hands the valve through the hole at the same time. Mechanics, being sensitive to such issues, could have avoided this dilemma without changing the size of the access hole (and thus the weight of structure). Had the installation been slightly tilted, hands and valve would fit. Changing the valve would remain difficult but access certainly more acceptable.
Remember, mechanics are trained to work blind. If you don’t like working blind, putting grease on your fingertips to hold a nut, or wearing Band-Aids — take a bench job. The amount of skin remaining on a mechanic’s knuckles is not the determining issue when defining accessibility. Like it or not, if higher drag results when trying to save knuckles, the knuckles will lose. Band-Aids are cheaper than 30 years of extra fuel.
I’ll buy the Band-Aids.
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