Fire Behavior–Know the Enemy!

By Warren Whitley and John Taylor

Fire behavior training in America is generally limited, but there are signs of improvement here and there. Did you know that the Virginia Department of Fire Programs curriculum for firefighter and officer training requires a grand total of three hours of fire behavior training? That is the sum total from Firefighter 1 through Officer 4. It is almost like sending an Army officer through four years of West Point or Sandhurst with only three hours of infantry training, definitely not good preparation for battle.

In Virginia, the Prince William County Department of Fire and Rescue collaborated with this article’s co-author John Taylor¹ to enhance the knowledge of fire behavior for recruits and officers so that the quality of the risk analysis during size-up at structure fires could be improved. Recruits now receive approximately 50 hours of classroom, laboratory (lab), and hands-on practical instruction. Command officers have received a two-day compressed course to inform them of the instruction the recruits receive. The remainder of the station crews will be receiving similar training over the next year to ensure that everyone has the same baseline of knowledge.

 

LAB WORK

The lab work includes the use of a “doll house” that demonstrates the fire behavior phenomenon on a small scale and emphasizes two important facts: (1) Smoke burns and (2) if you control the air, you can control the fire–Air Track Management (ATM). There is also a backdraft demonstration unit constructed from a 20-foot sea container (photo 1).2 This prop allows students to safely view extreme fire behaviors and observe the positive (over-pressure) and negative (under-pressure) air tracks and the time it takes fresh air to mix with the rich, hot smoke and develop into a backdraft. From there, the students spend time in the fire behavior container that many fire departments erroneously refer to as a flashover “simulator.” We point out that there is nothing simulated about it: Real fire behaviors occur in the containers.

 

 

Prince William County is in the process of constructing a large vent container that will allow practicing the fire attack using an ATM firefighting nozzle and ventilation techniques appropriate for the given fire scenario under study; emphasis is placed on recognizing the fire behaviors as they present during the evolution.

The realistic nature of these ATM firefighting techniques demands the highest levels of safety to facilitate realistic and responsible Compartment Fire Behaviour Training (CFBT). Toward that aim, our ATM instructor courses will from the outset follow the curriculum of the United Kingdom (UK) CFBT Instructors Award.3

 

One of the major additions to the knowledge base of the firefighters in Prince William County involves the extension of the CFBT curriculum–that is the exploitation of the under-pressure or negative air track, a concept covered in detail in Smoke Burns. (1)American firefighters are familiar with the buoyant nature of the hot smoke and fire gases and know about the neutral plane, but they fail to account for the negative path of fresh air coming in to feed the fire. For a fire that has already vented itself through an opening, such as a window, there will be significant clues as to from where the fire is getting its air, and those clues can give a knowledgeable fire officer a wider selection of safer tactics and better information for determining whether to risk an interior attack..

Consider a simple room-and-contents fire during size-up. If the flame front is exiting from only the upper half of the window, physics is telling you that the door into the fire compartment must be closed in these circumstances, because the fire must be taking in the air it needs to burn through the lower half of the window. However, if the door to this fire room compartment were open, the flame front would be exiting from the bottom of the windowsill upward and getting its air from below the windowsill. Therefore, the air feeding this fire must be from the lower half of the open door, because this is below the level of the windowsill, physical and visual evidence that the door to the fire compartment is definitely open.

There are a couple of “thou shall nots” in the American fire service that need debunking and should be replaced with accurate fire behavior knowledge. One is attacking from the unburned side. By understanding the under-pressure or negative air track and how it can be used to carry water fog droplets to the seat of the fire and then be used in tandem with gas cooling to knock down, or even extinguish, the fire quickly, attacking from the unburned side is not always the best option.

The other is conducting an exterior fire attack to hasten an interior entry. In fact, there are many instances when flames are showing from the upper half of a window when introducing water fog droplets from the outside into the under-pressure or negative air track region in the lower half of a window would make the extinguishment faster and the subsequent interior search and overhaul far safer. 

A quick scenario illustrates a point about the under-pressure or negative air track: You are the officer on the first-arriving engine. You start your size-up walk around the building–a 2,500-square-foot rambler (single-story) house in suburbia. You see that the fire has vented itself through the front window and it is blowing out the entire window from the windowsill upward. You notice the front door is open, hopefully, because the occupants have escaped.

 

 

Diagram No. 1.

 

In this case, the fire’s air supply is below the windowsill, coming through the open door to the house. This also indicates that the door to the room involved is also open, allowing the air to flow in from the lower half of the open front door to the fire room compartment. What do you suppose would happen if you shut the front door?

 

Here is what will happen, assuming there are no other openings to the outside at the rear or the side of the building: The under-pressure or negative air track (inflow) to the flame front will have to get its air from the lower half of the window, and the neutral plane (the bottom of the flame front) will rise to approximately half way up the window on over-pressure or positive air track (exit port). If you opened the door again, it would revert to drawing air in through the lower half of the open door and exiting through the window from the windowsill upward. ⁴

 

Diagram No. 2.

 

Think about the common tactic American firefighters employ when extinguishing a fire blowing out of the upper half of a kitchen window at the rear of the building. We open the front door and make things worse! As mentioned above, this is a great opportunity to introduce water fog droplets into the under-pressure or negative air track through the lower half of the window (fireside). The water fog droplets can be carried to the seat of the fire and then continue on the “air track,” following the flames exiting on the over-pressure or positive air track (and catch them as they), catching them up as they exit the upper half of the window, extinguishing these flames as if waving a magic wand—the appliance of science. This can be combined with gas cooling to shrink everything back to the seat of the fire–and all from a safe position.

 In Diagram No. 1, you can see where the over-pressure or positive air track flame front was filling the entire front window, indicating the under-pressure or negative air track was coming from below the windowsill through the lower part of the door opening. The first-arriving officer “read the fire,” noted these signs, and decided to introduce water fog droplets into the lower half of the front door and then attacked from the window as the flame front retreated. (See Diagram No. 3).

 

 

 

Diagram No. 3.

 

He instructed the crews not to do any further ventilation to control the “air track.” These tactics allowed for quick knockdown of the fire with little risk to the crews, who were able to quickly follow up with an entry to conduct their primary search and finish overhaul before the second line in was even pulled and the fire didn’t get any worse after the arrival of the Fire Department. (See Photograph No. 2). 

 

(2) Photo courtesy of the Prince William County (VA) Department of Fire and Rescue.

 

The salient point is that the American fire service can operate more efficiently and safely if emphasis were placed on a better understanding of fire’s behavior and dynamics, including the new concept of ATM. This will lead to a diagnostic approach during the size-up and risk analysis of the fire and the ability to read the stage of the fire on arrival, leading to the selection of correct, safer tactics, which will make us firefighters smarter than our enemy–FIRE!

 

(3. http://www.accreditedqualifications.org.uk/qualification/50028625.seo.aspx/.)

 

3 http://www.edexcel.com/quals/btec-sc/pubserv/lvl3/cfbt/Pages/default.aspx

 

 

WARREN WHITLEY, M.I.Fire.E, CFOD, is an assistant chief with the Prince William County (VA) Department of Fire and Rescue and has more than 28 years in the fire service. He holds a MPA from VA Tech and an MA from the Naval War College.