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.
Band-Aids® is a registered trademark of the Johnson & Johnson Corporation.
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