Boeing's proposal for the P-3 Update IV program designated four categories
for wire segregation and routing. Of these, two were reserved for secure signal
and power ("red wiring") and sensitive navigation equipment (Omega). All other
wiring fell into the remaining two categories.
Identification of the four categories was based on experience gained during
the E-3 AWACS program, where problems with wiring had been solved on a
case-by-case basis over a long period of time. The E-3 AWACS EMC Control Plan
listed six categories: AC power, DC power, discrete commands, digital audio and
RF (secure power and data categories not included). This scheme avoided many
interference problems. Residual difficulties were resolved on a case-by-case
basis through a series of small changes in separation, shielding and routing.
The program schedule allowed these changes without undue disruption to the
overall work flow.
The schedule for the P-3 Update IV program was more stringent. The wiring
design had to be completed before receiving the airplane. No significant lime
remained to work out interference problems associated with wire categorization
after installation. Accordingly, the cost in schedule and budget of the "find
and fix" approach previously used in the E-3 AWACS program was considered
prohibitive and was scheduled out of this project as one of the competitive
cost-saving features of the proposal . The documented lessons-learned in the
previous E-3 AWACS project provided a confidence factor for preventing their
recurrence in this design.
When the P-3 Update IV contract was awarded, the
program EMC staff quickly implemented a wiring categorization scheme with
twelve categories (Figure 5-1
). Two categories were set aside for AC and unfiltered DC power, one for
filtered DC power and low frequency signals, two for higher frequency signal
lines, one each for transmitter output and ordnance, one for "black" audio and
four for "red" lines.
This more disciplined wire treatment was certainly the better approach for a
program with little time for troubleshooting and change during the program's
later phases. Once the new categorization scheme was communicated to
manufacturing personnel, it was easily implemented with little cost to the
A special problem often arises in military derivatives of commercial
airplanes. Commercial wire categorization, though well intended, is seldom
required and often compromised in the manufacturing process. Considerations of
weight, producibility and cost regularly reduce the number of categories and
their shielding and separation requirements. Since commercial airplane EMC
requirements are quite different from military requirements, a commercial
derivative though entirely safe for commercial applications may not perform
adequately in the more rigorous military electromagnetic environment. Thus, when
mission equipment is integrated into the airframe, the commercial airplane's
less disciplined wire treatment may compromise the integrity of the mission
For that reason, EMC personnel MUST consider wire categorization in the
proposal stage and identify potential conflicts between ship's (aircraft's)
wiring and mission wiring Otherwise, significant, costly rework might be
required to achieve the essential compatibility.