Figure 7—Use this graph to estimate the speed needed to produce
the longest line of gum-thickened retardant at various coverage levels.

The line length graphs predict line length (in feet) as a function of air speed (in knots). The tables are constructed by selecting the drop producing the longest length of line (on the ground) at each coverage level. Either the graphs or tables may be used to estimate the air speed required to produce the maximum length of line for a given coverage level. The tables show an ideal case, while the graphs represent an average.

To select the proper helicopter speed, 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 the average fire intensity for each fuel model. The required coverage level can be adjusted up or down depending on the actual fire intensity. Once the required coverage level is determined, the air speed can be found. Use the graph for the material dropped (water, foam, or gum-thickened retardant) to find the speed that produces the longest line for the desired coverage level. The same information can be found in the appropriate drop table.

For example, if a fire is burning in NFDRS Fuel Model C (Fire Behavior Model 2), represented by conifer with grass, Table 1 shows that a coverage level of 2 is required. The graph for water shows that for coverage level 2, an air speed of about 36 knots produces the longest line.

The ground drop characteristics for the Griffith 100-gallon helibucket were derived through controlled drop test procedures on flat ground (Figure 8). This information is to serve only as a guide in assisting field personnel to determine the proper drop height and air speed for delivering water, foam, or gum-thickened retardant. Actual coverage may vary depending on terrain, wind, weather, and pilot proficiency.

Figure 8—Drop test of the Griffith 100-gallon helibucket.