By Bob Mason, Contributing Writer
Four engineers learn a lesson about scoping things out in advance when they realize man-hour requirements for the task at hand will reach into the hundreds of thousands of hours.
My first job out of college was for a military electronics company. We built those metal boxes full of electronics that went into aircraft and space vehicles. The boxes were covered with keyed circular connectors, both plug and socket types. We went to great lengths to make sure that only the intended connectors could possibly be mated. We varied the sizes of shells and whether a given connector was plug or socket. If we had two shells of the same size, we made sure that each had unique and incompatible keying configurations.
So, naturally, an aircraft maintenance technician mated two incorrect connectors. Somehow, he managed to mate two plug-type connectors together, despite all of the keying and other preventative measures we had designed in. The result of this configuration was a large flash and BANG! when he powered up the aircraft. The electronics unit was destroyed, of course, as were other circuits in the aircraft.
While we were impressed with his ingenuity and the sheer dogged perseverance needed to accomplish this feat, the officers in his chain of command were not amused. (Perhaps their views were jaundiced by the cost of the required repairs.) But, rather than address the problem by teaching the technicians to not forcibly mismate connectors, the general wanted to make the equipment “idiot proof.” He ignored the fact that this was what we had already tried to do.
The first order he gave us was to determine what would occur if ANY connector signal on the box were to be connected to ANY OTHER connector signal on the box, regardless of whether it was physically possible to mate the connectors. After that, we were to make whatever circuit changes were necessary to make such a mismating innocuous.
Our management, duly chastened after their visit to the general’s office, set to work immediately. A task force was put together to start the cross-connection analysis. People were pulled off existing projects to staff the task force. Each member of the task force got a set of connectors to analyze. To ensure good communication among task force members, they all moved to a common office. The work began.
The task was enormous. The box had 20 connectors, with about 30 pins each. When we began, it took several hours to analyze a signal pair. As we progressed that time dropped to about 1 hour. But at this point we were only at the analysis phase; the hour interval didn’t include the redesign phase. After several days of this, and the realization of how little progress we had actually made, we decided to determine the scope of the task facing us.
The math was not hard. 20 connectors of 30 pins each is 600 pins. The combination of 600 items taken 2 at a time is 179,700 separate combinations. At 1 hour each, that would take 179,700 man-hours, or 86.4 man-years. The four people on the team could count on spending the next 21+ years doing these analyses. The redesign time was still unknown.
We presented our figures to management. They didn’t say much, but some eyebrows rose. They took the figures to the general. We never heard what the general said, but the next week we returned to our interrupted projects.
Lessons learned: Always define the scope of a job in the beginning.
Contributing Writer Bob Mason “Bob Mason has 35+ years of design experience and thankfully has moved off of this assignment. He currently designs industrial control equipment and lives in Wake Forest, NC.
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