Ground Pattern Performance of
the Marsh Turbo Thrush

The results of drop tests allow managers to estimate the air speed, height, and door opening that provide the retardant coverage level required for a given fire intensity represented by the applicable fuel model. Table 2 can be used to estimate the door opening of a water drop to obtain the maximum line length at the desired coverage level. Table 3 can be used to estimate the door opening of a foam drop to obtain the maximum line length at the desired coverage level. Table 4 can be used to estimate the door opening of a gum-thickened retardant drop to obtain the maximum line length at the desired coverage level.

Table 2—Water tests producing the longest line at various door openings.

Table 3—Foam tests producing the longest line at various door openings.

Table 4—Gum-thickened retardant tests producing the longest line at various door openings.

The tables are constructed by selecting the drop producing the longest length of line (on the ground) at each coverage level from the drop tests. The tables may be used to estimate the door opening required to produce the maximum length of line for a given coverage level.

To select the proper door opening, 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. Once the required coverage level is determined, the ideal door opening can be found. Use the table for the material dropped (water, foam, or gum-thickened retardant) to find the door opening that produces the longest line for the desired coverage level.

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 table for water shows that a door opening of 60 percent produces the longest line length for coverage level 2.

The ground drop characteristics for the Marsh Turbo Thrush were derived through controlled test drop procedures on flat ground (Figure 5). This information is to serve only as a guide in assisting field personnel to determine the proper drop height, air speed, and door opening for delivering water, foam or gum-thickened retardant. Actual coverage may vary depending on terrain, wind, weather, and pilot proficiency.

Figure 5—Drop test of the Marsh Turbo Thrush.