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Ground Pattern Performance of the CDF S–2T
Turbo Tracker Airtanker

Greg Lovellette, Physical Scientist, and
Ann Suter, Statistician

The Technology and Development Aerial Delivery Program tests a variety of fixed- and rotary-wing tankers to determine the parameters for optimal coverage over a wide range of fuel and fire conditions. The California Department of Forestry (CDF) S–2T Turbo Tracker airtanker (figure 1) is one of a family of Type II airtankers that are used for initial attack during wildland fire suppression.

Figure 1—The California Department of Forestry
(CDF) S–2T Turbo Tracker airtanker.

The CDF constant flow system (designed by Marsh Aviation, Inc.) contains a single compartment that is certified by the Interagency Airtanker Board to carry 1,200 gallons. The tank has bulkhead dividers to minimize fluid movement from front to back and to provide structural integrity. Two opposing doors, which run the length of the tank, control the fluid flow. The system controller interprets fluid height information from sensors in the forward and aft compartments and adjusts the door opening to produce a constant rate of flow from the tank.

The flow rate and volume for any individual drop can be selected using rotary switches on the drop control panel in the cockpit. Any fraction of the load also can be released by holding down the drop button. When the operator lets go of the button, the doors will close.

Drop tests included airspeeds from 108 to 135 knots and drop heights from 142 to 291 feet (measured from the bottom of the tank to ground). Drops were made with water, gum-thickened retardant, and a water-enhancing gel.

The materials were dropped over an array of plastic bowls that were 6 inches in diameter and 3 inches deep. The quantity of material in each bowl was measured and the data were used to determine the drop pattern.

Flow rate, drop height, volume, and airspeed affect the drop pattern. Increasing drop height gradually widens the drop while decreasing the coverage levels. This effect is modified by the ambient wind. Increasing windspeed widens the drop, decreasing coverage levels. Increasing airspeed increases the line length while reducing the coverage level.

Because this airtanker has variable volume and flow rates, it can produce coverage levels needed for various fire suppression missions. Figures 2, 3, and 4 show the effect of three fire-retarding materials (water, a gum-thickened retardant, and a water-enhancing gel) dropped at a coverage level setting of 3.

Figure 2—Drop pattern characteristics for the CDF S–2T Turbo Tracker airtanker using 1,200 gallons
of water, a coverage level setting of 3, an airspeed of 118 knots, and a drop height of 195 feet.

Figure 3—Drop pattern characteristics for the CDF S–2T Turbo Tracker airtanker using 1,200 gallons
of gum-thickened retardant, a coverage level setting of 3, an airspeed of 129 knots, and a drop height of 184 feet.

Figure 4—Drop pattern characteristics for the CDF S–2T Turbo Tracker airtanker using 1,200 gallons
of water-enhancing gel, a coverage level setting of 3, an airspeed of 111 knots, and a drop height of 179 feet.

The proper amount of fire-retarding material to drop, expressed as coverage level in gallons per 100 square feet, differs depending on the fuel type. Table 1 shows the coverage needed for specific fuel types using both the National Fire Danger Rating System (NFDRS) and the Fire Behavior Fuel Models. The results of drop tests allow managers to estimate the line length of the most effective coverage level required for a given fire intensity based on airspeed, drop height, and flow rate.

Tables 2, 3, and 4 were constructed from actual drop test data and indicate the longest measured line length (on the ground) of each coverage level using water, gum-thickened retardant, and water-enhancing gel. The graphs in figures 5, 6, and 7 were derived from the total drop test data set and indicate trends of line length production for a wide range of coverage levels over the tested range of flow rates using water, gum-thickened retardant, and water-enhancing gel.

Figure 5—Use this graph to estimate the flow rate needed to produce the longest line of water at
various coverage levels. GPC stands for gallons per 100 square feet.

Figure 6—Use this graph to estimate the flow rate needed to produce the longest line of gum-thickened retardant at various coverage levels. GPC stands for gallons per 100 square feet.

Figure 7—Use this graph to estimate the flow rate needed to produce the longest line of water-enhancing gel at various coverage levels. GPC stands for gallons per 100 square feet.

To select the proper flow rate, first use table 1 to determine the coverage level required by the NFDRS or Fire Behavior Fuel Model. The coverage levels in table 1 represent the coverage level required for average fire intensity for each fuel model. The required coverage level can be adjusted up or down depending on the actual fire intensity. Use the table for the material dropped (water, gum-thickened retardant, or water-enhancing gel) to find the corresponding flow rate, coverage level setting (on the aircraft drop controller), and longest line produced for the desired coverage level. Similar information can be found in the appropriate graphs.

For example, a coverage level of 3.0 (gal/100 ft²) is required if a fire is burning in light slash (NFDRS Fuel Model K/Fire Behavior Fuel Model 11— see table 1). Table 3 for gum-thickened retardant shows that a coverage level setting of 3 produces the longest line (595 feet), when dropping a full 1,200-gallon load.

The ground drop characteristics for the CDF S–2T Turbo Tracker airtanker were derived through controlled drop test procedures on flat ground (figure 8).

Figure 8—A drop test of the CDF S-2T Turbo Tracker airtanker

This information is to serve only as a guide to help field personnel determine the proper airspeed, drop height, and flow rate for delivering water, gum-thickened retardant, or water-enhancing gel. Actual coverage may vary depending on terrain, wind, weather, and pilot proficiency.

About the Author

Greg Lovellette is a physical scientist for the Forest Service's Technology and Development Aerial Delivery Project. He has worked with the technical tank and gating aspects of aerial retardant delivery systems since joining the Forest Service in 1989.

Ann Suter is a statistician for the Missoula Technology and Development Center's wildland fire chemical systems group. She joined the Forest Service in 1997 after serving 2 years as a Peace Corps volunteer in Jamaica, where she worked on reforestation and soil erosion control. She has a master's degree in international development from the American University.

Additional single copies of this document may be ordered from:

USDA Forest Service, MTDC
5785 Hwy. 10 West
Missoula, MT 59808
Phone: 406–329–3978
Fax: 406–329–3719
E-mail: wo_mtdc_pubs@fs.fed.us

Electronic copies of MTDC’s documents are available on the Internet at: /eng/t-d.php

For additional information, contact Greg Lovellette at MTDC.

Phone: 406–329–4815
Fax: 406–329–4763
E-mail: glovellette@fs.fed.us

Forest Service and Bureau of Land Management employees can search a more complete collection of MTDC’s documents, videos, and CDs on their internal computer networks at: http://fsweb.mtdc.wo.fs.fed.us/search

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