Quantcast Choosing Spare Parts

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MIL-HDBK-1023/4
and would, therefore, require a substantial quantity of various spare parts. When establishing a
spare parts inventory, two questions must be answered: (1) Which parts should be stocked, and
(2) How many of each part?.
3.8.1
Choosing Spare Parts. To answer the two questions posed above, several factors must
be considered, including failure rate, part availability, and effect of the part failure.
a) Failure Rate. The failure rate (or replacement rate) is the product of the expected
life of an item and the number of that item in the system. For instance, if a bulb is expected to
last 6 months, and we have 100 bulbs in the system, then an average of 100 bulbs will be
replaced every 6 months, or approximately 4 per week. The failure rate may be determined from
the maintenance records, which should be compiled according to the instructions in par. 3.3.
b) Part Availability. Part availability refers to the time it takes to secure a replacement
part (procurement lead time). If a part can be readily procured from shelf stock of a local
supplier, it might not be necessary to add the part to the spare part inventory; it could be
purchased when needed. However, if there is a 6-week lead time required by the supplier, then
six times the weekly failure rate (24 bulbs in the example above) should be stocked. There are
methods of obtaining parts which may reduce the effect of a long lead time. These include
substitution (the use of a functionally equivalent part from another manufacturer),
cannibalization (replacing one of a pair of adjacent failed bulbs by borrowing a bulb from
elsewhere in the system), and temporary fixes (such as the use of portable lights in place of the
fixed light installation) while awaiting corrective maintenance.
c) Effect of the Failure. The effect of the failure of a particular spare part depends on
how important the part is to the equipment it is installed in, and how vital the equipment is to
airport operation. For example, the failure of a lamp in an edge light would not lead to any
system downtime, but the failure of a circuit board in a constant current regulator would cause
the loss of the entire lighting circuit that it powers. The equipment manufacturer will give
guidance on recommended spare parts. As experience is gained with a system, other parts may
be added or deleted from the inventory. The impact of a part's failure should be considered
when building a spare parts inventory.
3.8.2
Part Identification. An important part of maintaining a spare parts inventory is
accurately cataloging the parts on-hand by manufacturer's part number. This is important to
ensure that the correct part is used in a broken piece of equipment, since many optical parts are
visually similar but vary significantly in performance. The use of the manufacturer's part
number is also vital when reordering; if a part is ordered by its generic name, the manufacturer
may send a later version of the part which is incompatible with the existing system. It is
extremely important to maintain manufacturer's data which reflects the facility equipment;
describing the type, model, number, and serial details.
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