While I feel comfortable saying that, some might take me to task. I once wrote that many experts act like "we know more than we do" and was roundly criticized. The truth is computers have allowed designers to calculate equations to 15-decimal places that were likely derived with a bucket and shovel.

I was reminded of that recently when calculating the diameter of a propeller shaft. Now the shaft size calculation is straightforward with most variables well defined. The biggest question mark is the factor of safety used. The safety factor at its simplest is a ratio adjusting for the unknowns in the analysis. For propeller shafts on heavy diesel boats that factor is supposed to approach 10. Now those with an engineering mind-set will appreciate that a safety factor of 10 is an admission that there’s a lot of uncertainty. The less charitable might call it fixing a "wild ass guess".

In our industry’s defense it’s not laziness. It’s just an example of how little is known about the true forces involved. The dynamic forces are easy. What happens, however, when the prop encounters a fish, an oil drum or a panic reverse? They can be examined, but only if you know which you’re planning and are willing to pay for the lengthy review. NASA would factor in all the imaginable loads since the risk would be too great.

Aviation in general often works to safety factors of between 1 and 2. They can because they have much more data, both real and experimental, that define the requirements. Their success in doing so is well proven by their impressive safety record. Still there are always unknowns. This was highlighted earlier this year when a few birds did battle with the engines of a 140,000-lb. airplane resulting in a heroic landing in the Hudson River. Every trip is an adventure.