Words have a lot of power. Certain words can immediately hurt someone's feelings. Other words can reassure. Some of the most dangerous words provide a false reassurance, and should never be used by a pilot. In a recent FAASTeam Safety Tip (faasafety.gov), Max Trescott, the 2008 National CFI of the year, warned pilots about the use of the word "probably." As he points out, human beings, and especially pilots, are generally optimistic. If you are using the word probably, "there's a good chance that you've overestimated the probability of success, as do the approximately 300 pilots a year who suffer fatal accidents."
I think a lot of the problem has to do with wishful thinking. We want the weather to clear so we say, "It will probably clear up pretty soon." This tendency is even worse if there is a reason for our hope. The weather might be forecast to improve by a certain time, or there may be a local weather phenomenon that usually follows a certain schedule. The problem is that weather forecasts can be very wrong, and a local condition that usually improves at a certain time may not follow that pattern today. Max uses the example of a pilot trying to escape the marine layer in the San Francisco area thinking that he can "probably" make it through one of the mountain passes.
This tendency to hope for the best is even worse if we have gotten away with it before. Max points out that if you have made it through the pass in poor visibility a hundred times before, you are more likely to think you will probably make it through this time. If you have successfully taken off overweight or out of CG limits before, you are more likely to think that this takeoff will be successful too. If you have been able to fly in weather below VFR minimums without an instrument rating before, you are likely to think you can probably make it today too.
There are other words that should set off warning alarms in our heads. The words "usually" and "always" are very dangerous when used in the aviation decision-making process. They are typically brought into action in the situation mentioned above when there is a local weather pattern that often follows a certain schedule. Thus the pilot might say, "The weather usually (or always) starts to improve about 10 a.m." There is nothing wrong with being aware of local weather patterns, but in our "wishful thinking" mode, we are likely remembering all the times the weather followed the typical pattern, and forgetting all the times the fog hung around all day instead of lifting at the prescribed time, or the visibility was not any better on the other side of the pass.
Another dangerous phrase starts with "I'm sure." This can be used in a weather situation to say, "I'm sure the weather will improve soon," or "I'm sure the visibility will be better on the other side of the pass." Pilots will also use this phrase to say, "I'm sure it will be enough fuel to get there," when it actually is not so clear there is enough fuel. "I'm sure" goes a step further than "probably." Instead of the uncertainty that is inherent in "probably," we are introducing the aspect of certainty, when in reality the situation is anything but certain, and the very fact that we are saying "I'm sure" indicates there is some doubt.
My favorite trio of dangerous words to purge are "it must be, it should be, and that has to be." These phrases can be used in some of the situations I have already mentioned. For example, "It should be clear on the other side of the pass," or "The runway should be long enough." However, these phrases are most often used with devastating effect in relation to navigation. It's not so bad for a pilot to say "that should be" a certain town or other checkpoint as long as he continues to search for information that confirms his suspicion. However, a pilot who says "that must be" or "that has to be" the desired checkpoint has already made up his mind and is no longer working to confirm his suspicions. The advent of inexpensive GPS devices has made this error much less likely, but even pilots with a GPS have fallen into this trap and made an approach to the wrong airport because once they made up their minds, they ignored the clear information on their GPS display that they were mistaken.
"It must be" can also be used to ignore or dismiss evidence of a mechanical problem. For example, a pilot who is in a hurry to get going and finds an oil stain under the engine of his airplane might say "it must be" from the last airplane that was parked there to avoid having to find a mechanic to open up his cowling and do a detailed inspection looking for any oil leaks. In my February 2007 article I described how these words were a factor in the near loss of the Air Transat Flight TS236 Airbus A330 when the crew thought that a series of unusual warnings due to a fuel leak "must be" a computer problem.
These words were also a factor in the Eastern 401 accident in the Florida Everglades in the early '70s. When the crew of the L-1011 approaching Miami extended the landing gear, the nose gear down and locked light did not illuminate. They initiated a missed approach and informed the controller they needed time to try to get the light to come on. As they headed west over the Everglades at the minimum vectoring altitude of 2,000 feet, everyone in the cockpit became so focused on the light that no one noticed the autopilot had been knocked off altitude hold and the airplane was slowly descending.
The controller working the flight eventually noticed the airplane was descending. However, it was a clear night, and besides, this was an L-1011, which was only flown by the most experienced airmen. He could not believe that an experienced crew would not be aware their airplane was dangerously low and still descending, so he figured his altitude readout "must be" wrong. He did call the crew, but to ensure he didn't offend them, he only asked them "how's it coming along out there?" This innocuous sounding question was not enough to alert the crew to their dangerously low altitude, and a short time later they crashed into the Everglades, killing 99 people.
If you find yourself saying "it must be," "it should be," "that has to be," "I'm sure it is," "it probably will" or "it usually does," immediately correct yourself. There are actually only two possibilities. If you have done the research and have the data to back up your conclusion, then say "it is." If there is even the slightest doubt in your mind, then say "it might be." Anything else is just wishful thinking.