Well, there were a number of specialized forms of the slide rule used in the Air Force a few years back. Their function was to help flight crews calculate aircraft weight and balance data. Each type of bomber and cargo airplane came with its own unique slide rule calculator, also known as slip sticks because the central element could slide or slip along the outer portion. A sliding cross-hair piece was also included to permit accurate readings and assist in aligning the scales.

The slip stick that was the bane of my existence was the one for the B-58 Hustler bomber. Calculating the center of gravity (CG) position involved use of that special slide rule. The user could compute the location of the CG for any condition of fuel loads, bombs attached and even the amount of chaff and ammunition aboard. And since the B-58 carried an external bomb pod with separate fuel tank attached, even the weight and specifics of the bomb pod could be factored in to determine CG.

The problem, and the reason for some grief to Defensive System Operators (DSO's) responsible for computing CG in flight and during pre-flight, was the two-sided nature of that darn sliding part. Because the B-58 carried either of two bomb/fuel pods, the slipstick was printed and marked on both sides. You had to match the proper slipstick side with the type of attached pod to make an accurate CG calculation. It could be a little confusing.

Now it should be a very simple thing for the DSO to remember which side of the slipstick slider part to use, but it didn't always work out that way. Especially during check rides or when a flight examiner pressured DSO's in the simulator, some DSO's inevitably worked from the wrong side. Now in the simulator it may be more understandable, because the simulator didn't have a bomb or fuel pod to see and be more obvious. But even in the airplane some DSO's were known to goof up and attempt to calculate CG using the slipstick slider that didn't match the airplane's actual configuration. As they say today, "been there and done that".

Was that serious? You bet. It could cause the DSO to mis-calculate CG by enough error to make flight unsafe. CG calculations were required in flight every hour during subsonic cruise and every 15 minute when supersonic. CG was that critical. Fortunately, many DSO's recognized their error and corrected for it by reversing the slipstick slider to properly match the bomb/fuel pod type carried. It was mostly in the simulator that errors persisted and some DSO's flunked their check "rides".