Perimeter Solutions Director of Research and Development Melissa Kim and Agency Liaison Ron Raley look at safe fire retardant drops

When asked to comment on the proliferation of wildfires in recent years, a recurrent theme among leaders in fire management over the past 12 months has been that we no longer have a wildfire season, but a wildfire year. In 2021 alone, more than 8,500 wildfires took place across North America with fires burning through the end of December into January 2022. And conditions aren’t expected to get any better.

 Earlier this year, a new report from the United Nations forecast that the total number of wildfires around the world will increase 50% by the end of the century. With that expected growth in activity and considering that developers are continuing to build homes and businesses within the wildland-urban interface (WUI), there will be increased dependency on aerial attack to suppress and stop the spread of wildfires in order to save lives and protect property. Whenever pilots take to the skies to battle wildfire, they are not only putting their lives at risk to protect others, but also firefighters on the ground working to extinguish the fire.

Pilots have to deal with the heat from the fire, turbulence, reduced visibility from smoke, and a host of other dangers inherent with fire. The act of dropping fire retardant is one of the most significant threats faced by both pilots and fire suppression crews on the ground. This article will outline years of innovation in fire retardant formulation that have helped to improve drop safety for pilots and firefighters.

Challenges with drop safety

The predominant safety issue when applying retardant on active fires is “low drops” – when retardant is released by aircraft from an altitude that is not sufficiently high to ensure safety. Low visibility from smoke can block obscure trees or mountainsides from view, creating safety issues for pilots. For those on the ground, the retardant itself presents a major safety issue.

In order to understand the safety issues associated with low drops it is important to first understand the dynamics of a safe and effective drop.  As the aircraft doors open and the retardant is released from the tanks it forms into a mass of fluid.  The retardant is moving at the same forward airspeed as the aircraft, which can be anywhere between 120 to 150 knots, depending on the airtanker type. This forward-moving mass of fluid is immediately subjected to winds equivalent to the forward airspeed.  It begins to erode by wind shear and break apart into smaller droplets. As these smaller droplets are formed, their forward airspeed begins to decline to a point where they reach terminal forward velocity and begin entering the fuel vertically.

When the aircraft is at an altitude in which this dynamic occurs, it is considered a safe and effective drop height. These safe and effective altitudes are contingent upon the type of tank door system, forward airspeed, and volume of retardant. The United States Forest Service (USFS) has issued guidance requiring drop heights to be no lower than 150’ above ground level (AGL). In actuality, it is the altitude above the top of the target fuels or vegetation that is important. Higher safe drop heights have been established for Very Large Airtankers (VLATs). 

A mixed solution of retardant weighs roughly nine pounds per gallon. If a 3,000-gallon airtanker drops 2,000 gallons of retardant on a fire, that means 18,000 pounds of retardant are coming out of the plane all at once. Drops released at less than the safe and effective drop heights do not have sufficient time to erode and reach terminal velocity. These drops will impact terrain, vegetation—and many times—firefighters with this tremendous weight and at full force. (See figure 1 below.) When this happens, the results can be devastating, as was the case during the Mendocino Complex Fire that burned in Northern California during the summer of 2018.

The Mendocino fire burned nearly 460,000 acres before it was completely contained. During the firefight, an airtanker was cleared by the Air Tactical Group Supervisor to drop and inadvertently released the retardant load below the established safe drop height. An 87-foot tree was toppled and, unfortunately, killed a firefighter and injured three others.

The minimum safe drop height for a VLAT like a 747 or DC 10, is at 250 feet AGL. As mentioned earlier, the USFS has established a minimum safe drop height of 150 feet for large air tankers (LATs), while there is a minimum safe drop height of 90 feet AGL for single-engine air tankers (SEATs) according to the Bureau of Land Management.

While minimum drop heights are established for different aircraft, calculating the optimal drop height for each aircraft is challenging, primarily because each type of aircraft has a different gating system, different volumes of product it can safely transport, drop speeds and more. The sweet spot for each aircraft must be established, which is why drop testing and immediate pilot feedback is critical.

Improving Safety with Fire Retardant

Even after determining the sweet spot for any given aircraft, reduced visibility and unfamiliarity with surrounding geography will continue to present challenges to aerial firefighters. The use of highly advanced fire-retardant formulations today plays a role in improving drop performance and safety.

Recovery is an important parameter when dealing with long-term fire retardant. Recovery is a measure of the amount of solution that actually reaches the target compared to the quantity dropped.  It is represented as a percentage.  Gum thickeners are critical to drop performance, allowing for drop effectiveness at much higher altitudes while increasing recovery. 

Typically, gum-thickened retardants will have recovery rates over 85 percent while those without gum thickeners may have as little as 50 percent. The gum thickeners increase viscosity, droplet size, provide cohesion and elasticity, which combine to result in less evaporation and drift, and therefore higher recovery rates. Elevating drop heights provides for a much higher level of safety for both firefighters and pilots.

How effective the retardant drop is—regardless of altitude—is heavily dependent on properly mixing the solution. Whether you’re 150 feet in the air or 850 feet in the air, retardant mixed improperly can clump together, creating a mass of retardant that weighs thousands of pounds and is falling through the air at up to 140 miles an hour.  Thoroughly mixed solutions of fire retardant improve safety for aerial firefighters. Current day advanced retardant mixing systems have nearly eliminated these issues providing for a higher degree of safety.

Visibility of retardant is another advancement that has improved performance. When fire retardant solutions are applied, successive drops are often made by the aircraft to form a fire line. When creating the line, it is essential for the aircraft’s pilot to visually determine where the preceding loads were dropped and ensure that the next load sufficiently overlaps the previous drop. This will provide a continuous line with no breaks, which helps prevent the active wildfire from burning through any gaps in that line.

Since fire retardant components may be colorless or include colors that do not contrast well with the ground or vegetation, coloring agents are added to give fire retardant solutions their distinct red color. This color provides significant contrast with the hue of the ground vegetation, enhancing the ability of the pilot to determine where the last loads of fire retardants were dropped.

One of the less obvious ways retardants helps improve drop safety is through improved solutions that inhibit corrosion and damage to aircraft parts.  When the US Forest Service created the first modern specification for long-term fire retardants in 1986, it covered a broad range of issues relating to efficacy and use of fire-retardant chemicals, including “uniform and intergranular corrosion”. These new formulations of retardant have essentially eliminated corrosion on aircraft as a critical safety issue.

While wildfire season continues to grow in length and intensity the demands on aerial firefighting will continue to increase. Responsible fire-retardant manufacturers like Perimeter Solutions are working with leaders in the fire management industry to continually introduce innovations that improve environmental footprint, help pilots do their job more effectively, and make it safer for them to protect themselves and firefighters.

This article was originally published in the March edition of IFSJ. To read your FREE digital copy, click here.