How to Simplify and Improve Schematics
Example: Boeing 727 Electrical Power Distribution

Figure 1. Original Boeing electrical power distribution for B-727 (Flight crew system training)
Boeing 727
Boeing 727 electrical power distribution schematic for flight crew training.

This is an example where the designers did not fully understand the technical backgrounds of their intended target audience, pilots. The original schematic is part of Boeing 727 electrical system ground school course for pilots to familiarize flight crews to the various electrical systems on B-727. The left side of the original schematic is clear and self-explanatory. That part of the illustration is almost identical to the switches and gauges on an actual flight engineer's panel. It is the right side of the schematic that can be confusing to many pilots. Perhaps the designers wanted to produce a simplified version of the actual B-727 electrical power distribution while maintaining the schematic "electrically correct", i.e. correct symbols, etc. That's where the real problem lies. Unless a person is familiar with relevant electrical components and the corresponding symbols, the right side of the schematics is quite difficult to understand. Pilots in general do not have, nor is it required to have, that kind of background. It can be very difficult to follow the power distribution from one bus to another. The symbolic representation of relays can hinder students from following the various paths the power is supplied and distributed under normal conditions and in emergency situations such as loss of all generators where battery supply (standby) is the only power available.

At first glance our improved schematic looks even more cluttered that the original one. There is an explanation for it. Before we simplify any schematic, we'll make sure that there aren't any errors or anything pertinent missing from the schematic. In this case there were several parts missing that we felt needed to be included (Generator Control, XFER 28VAC, MAIN 28VAC). Therefore, there are now more system parts on our schematic. Also, we have corrected some errors that we found on the original schematic. We have increased the size of some of the buses (boxes) to emphasize the importance of those components.
The biggest improvement was accomplished by substituting relay symbols with more simple switch symbols that create less confusion. Now it is easier to follow the power distribution from bus to bus and to see which relays (switches) are open and which are closed. Reorganizing some of the buses has made an improvement to clarity of the schematic. We used dash lines to indicate some of the many relays that will be effected when Standby is selected at the Essential Power Selector.

By utilizing these simple techniques, we have achieved a schematic that has improved students' learning speed and the level of understanding of the B-727 electrical power distribution.

Figure 2. Simplified and corrected B-727 electrical power distribution (Flight crew system training)
Improved and simplified Boeing 727 electrical power distribution schematic for flight crew system training.