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Our article about adding to the list of common denominators of fatal fires led someone to ask in a comment, “When and why was Wilson`s 5th Common Denominator dropped ?”

In 1976 four firefighters were entrapped on the Battlement Creek Fire, killing three near what is now Parachute, Colorado. Following the tragedy, Carl C. Wilson, who at one time was the Chief of Forest Fire Research at the U.S. Forest Service’s Pacific Southwest Forest and Range Experiment Station, published a paper titled Fatal and Near-Fatal Forest Fires: The Common Denominators.

In developing his paper, Mr. Wilson studied 67 fires that occurred during the 61-year period from 1926 to 1976 on which a total of 222 firefighters were killed from “fire-induced injuries”. He also evaluated 31 other “near-fatal” fires, searching for common themes or causes of the deaths in all of the fires. His results were considered ground-breaking. Since then his lists of Common Denominators have been republished, quoted in fatality reports, and included in many standard publications that are very familiar to firefighters.

When we listed his Common Denominators in the January 29 article we used the four that are seen in all of the recent and semi-recent publications that we looked at, including the last paper version of the Fireline Handbook (2004), the 2014 Incident Response Pocket Guide, and the report authored by Dick Mangan, Wildland Firefighter Fatalities in the United States: 1990-2006. The only revision of the Fireline Handbook since 2004 was completed in 2013 and was renamed Wildland Fire Incident Management Field Guide (PMS 210). It does not include the Common Denominators. The 2004 and 2006 editions of the Incident Response Pocket Guide also include the four-item list.

After a great deal of searching we found that there is another list of Common Denominators attributed to Mr. Wilson that has similar but different wording, and has five instead of four. The five-item list was in a 2011 paper by by Martin E. Alexander and Miguel G. Cruz and also in the 1998 version of the Fireline Handbook.

Finally we found a copy of Mr. Wilson’s paper, Fatal and Near-Fatal Forest Fires: The Common Denominators, that was published in 1977 in The International Fire Chief. It has two different lists.

The first, the five-item list, is printed on the first page just below the heading “Common Denominators of Fatal Fires”.  Here is the text just below that heading:

“Based on personal knowledge and information obtained from reports and reviewers, the following generalizations can be made about the fatal fires in Tables 1 and 2 [tables 1 &b 2 are fatal fires]:

1. Most of the incidents occurred on relatively small fires or isolated sectors of larger fires.
2. Most of the fires were innocent in appearance prior to the “flare-ups” or “blow-ups”. In some cases, the fatalities occurred in the mop-up stage.
3. Flare-ups occurred in deceptively light fuels.
4. Fires ran uphill in chimneys, gullies, or on steep slopes.
5. Suppression tools, such as helicopters or air tankers, can adversely modify fire behavior. (Helicopter and air tanker vortices have been known to cause flare-ups.)”

A key to that list is that it only applies to the 67 fatal fires he studied, and not the 31 that were near-fatal.

Toward the end of the paper in the “Conclusions” section, Mr. Wilson wrote:

“There are four major common denominators of fire behavior on fatal and near-fatal fires. Such fires often occur:

1. On relatively small fires or deceptively quiet sectors of large fires.
2. In relatively light fuels, such as grass, herbs, and light brush.
3. When there is an unexpected shift in wind direction or in wind speed.
4. When fire responds to topographic conditions and runs uphill.” 

We’re not sure why Mr. Wilson broke down the common denominators into fatal and near-fatal fires. I don’t know that it adds value, but can, and has, produced a little confusion when two different versions of the lists are floating around.