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An average of 15.8 million gallons of fire retardant has been used in firefighting each year. Most of this retardant is released from the air. Aircraft are used to transport firefighting chemicals at the appropriate height and speed. The types of aircraft include:

  • Fixed-wing multiengine airtankers.
  • Fixed-wing single-engine airtankers.
  • Helicopters with fixed tanks.
  • Helicopters with suspended helibuckets.

The fire-retarding chemicals typically used in wildland firefighting are short-term and long-term retardants, foam, and water. The retardants may include a gum thickener.

The 10 principles for proper retardant application are:

Figure 1-Figure 1—Airtanker establishing an anchor point.
Figure 1—Airtanker establishing an anchor point.

  1. Determine tactics, direct or indirect, based on fire characteristics and available resources.
  2. Establish an anchor point and work from it (figure 1).
  3. Use the proper drop height.
  4. Apply the proper coverage levels.
  5. Drop downhill and away from the sun.
  6. Drop into the wind for the best accuracy.
  7. Maintain an honest evaluation and effective communication between the ground and air.
  8. Use direct attack when ground support is available or it is feasible to extinguish the fire.
  9. Plan drops so that they can be extended or intersected effectively.
  10. Monitor retardant effectiveness and adjust its use accordingly.
Knowing the characteristics of the ground pattern obtained from a specific aircraft is an important component of proper retardant application (principles 1, 2, 4, and 9). Factors that influence the ground pattern of a retardant drop include:
  • Drop height and drop speed.
  • Flow rate of the liquid as it exits the tank.
  • Volume of the liquid released.
  • Tank geometry and venting.
  • Gating system (the tank doors and release mechanism installed in an aircraft to release retardant).
  • Rheological properties of the fire chemical (the flow characteristics of a fluid).
  • Wind speed and direction.
  • Temperature and relative humidity.
  • Fuel type.
  • Topography.
  • Safety concerns of aircraft and ground personnel.
  • Pilot proficiency.
Figure 2—The Dromader spray aircraft dropping water over a test grid.
Figure 2—The Dromader spray aircraft dropping water over a test grid.

Since the 1950’s the Forest Service has used a procedure known as drop testing to quantify ground patterns. The procedure involves dropping fire chemicals from an airtanker flying over open cups arranged in a regularly spaced grid (figure 2). The cups are weighed before and after the drop to calculate the amount of retardant deposited in gallons per hundred square feet (gpc). These values are plotted onto a map of the grid. Points between cups are estimated, usually by an interpolation method that assumes uniform change between cups. Contour lines are made by connecting all points of equal coverage level. The length of each contour, referred to as line length, is calculated from observed and estimated data.

During a drop test, drops are made at varying drop heights, drop speeds, flow rates, volumes, and with different retardant materials to obtain a graphical and numerical picture of the ground patterns produced by the airtanker. Examining ground patterns provides information about the factors that influence the distribution of the drop (figure 3).

Figure 3—Drop pattern characteristics for the Marsh Turbo Thrush with a drop speed of 80 knots and a drop height of 31 feet.
Figure 3—Drop pattern characteristics for the Marsh Turbo Thrush with a drop speed of 80 knots and a drop
height of 31 feet.

Some factors in a drop can be controlled, such as height, speed, flow rate, tank and gating system, and rheological (flow) properties of the retardant. Wind speed, wind direction, temperature, humidity, fuel type, and topography are among the factors affecting the ground patterns that cannot be controlled (Newstead and Lieskovsky 1985). Drop tests allow different tank and gating systems to be compared under similar conditions. Ground patterns can help managers learn the capabilities of an airtanker by determining the intervals between trail drops (figure 4). A trail drop is when door opening times are staggered to produce a long stream of retardant. An accurate set of ground patterns from an airtanker provides data to predict the time between releases needed for a successful trail drop.

Figure 4—Computer simulation of a trail drop.
Figure 4—Computer simulation of a trail drop.

This report will discuss factors affecting the ground pattern and give some historical background of the cup-and-grid method used to measure ground patterns. Additionally, new statistical tools will be presented that may improve the current cup-and-grid method.

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This page last modified July 30, 2001

Visitor since July 30, 2001