Air Tankers and Suppression of Wildfires

One of the most dramatic sights on a wildfire is the vision of an air tanker releasing its red payload of retardant near the fire. Behind this picturesque vision is a long history of science in the development of a valuable tool for firefighters - the use of fire retardant. Today retardant is used when appropriate to help suppress wildland fires.

Aerial drops of retardant have come a long way since the first recorded water drop in 1930, when a Ford Tri-Motor airplane used a wooden beer keg filled with water. Now air tankers can drop 500 to 2000 gallons of retardant at a time to help suppress fires. Helitankers, which are helicopters with built-in tanks, can drop up to 2000 gallons; retrofitted DC-10s have an 11,000 gallon capacity, and Boeing 747s 20,000 gallons.

Fire retardant is just one of many tools in the arsenal of firefighters. Like any other tool, it must be used under the right conditions and for the right job in order to be effective. Firefighters consider many factors in deciding which tools to use at a particular location and time to suppress a fire. Characteristics of the terrain, weather variables such as temperature, humidity, wind direction and force; the types of vegetation in the fire area; proximity to homes and other buildings; and the first priority of safety of the public and firefighters are all factors in choosing the right tools for the particular circumstances.

Retardant drops are most often used in extreme fire conditions. The retardant is usually dropped just ahead of the advancing edge of the fire and the flanks of the fire. This cools and so slows the fire, helping firefighters on the ground. It reduces the rate of spread and the intensity of fires, and slows larger, more damaging, and therefore can reduce the cost of fires. Often, using retardant to fight fires is the most effective and efficient method of assisting firefighters in protecting people, resources, private property and facilities. The remoteness of many wildland fires can delay the arrival of firefighting ground forces. Retardant drops can rapidly reduce the intensity and spread of the fire until firefighters can safely take action. Topography in the fire area and windy conditions are some of the factors limiting the effectiveness, and therefore the use, of retardant.

Although fire retardant is most often colored red or orange, sometimes a colorless retardant is used. The colored variety allows both the pilot and the firefighters on the ground to see where retardant has been applied. This helps the pilots in aiming their next drop and firefighters to know where to position themselves. The colorless variety is often used on roadside locations where color is not needed or desired or in the wilderness.

In 1956, water drop tests showed that conditions had to be near perfect for water to reach the ground and be effective. So specialists began to mix in chemicals to make the drops have the desired effect. Over 50 years of retardant use, research and development have evolved into the current products and practices. Today long term retardant is most commonly used in heavy vegetation and in some critical fire situations needing immediate response. This type of retardant consists of 85 percent water, 10 percent fertilizer salts ammonium phosphate or ammonium sulfate (not sodium chloride), and 5 percent minor ingredients such as colorant or fugitives (iron oxide -rust-, or a color that fades with exposure to sunlight), thickener (natural gum and clay), corrosion inhibitors, and dispersants. Foams and gels are also used, often in areas of lighter vegetation, on buildings and other structures such as fences or other resources. Foam is 99 percent water with 1 percent wetting agents, foaming agents, corrosion inhibitors and dispersants. Water enhancers absorb large amounts of water and dry slowly. They are good for protecting structures and for mop-up operations. Water enhancers consist of 95 to 98 percent water, and the balance a mixture of thickeners, stabilizers and other minor ingredients.

The fertilizer in the long term retardant may help in re-sprouting of vegetation if other conditions are favorable. Conversely, excessive fertilizer may cause a temporary "burn" on exposed vegetation. Retardant use can be a more cost-effective and readily available tool than some other methods in remote locations, and lighter on the land than a dozer line.

Retardant products that are used must meet strict specifications and are tested against a prescribed set of health and safety protocols. Those who apply fire retardant undergo extensive training in its safe and proper use. The USDA Forest Service has guidelines for the use of fire retardant near aquatic areas and habitat of Threatened and Endangered species to avoid or minimize any negative effects. Aerial application of retardant is avoided within 300 feet of waterways visible to the pilot. The effects of the aerial application fire retardant on human health and safety have been analyzed and evaluated and it has been determined that it does not pose a risk to the health and safety of the public or firefighters.

The use of fire retardant is a valuable tool in the firefighting arsenal. Fire managers evaluate the many variables of a particular fire and the resources and values that are threatened. Training, experience, and pre-planning allow them to choose the best response to the fire and the appropriate tools to use.