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Image of the Drop Guide header for this pub.

Ground Pattern Performance of the
Ayres Turbo Thrush with Standard Fire Door

Gregg Johnson, Project Leader,
and Cammie Jordan, Project Assistant

The Wildland Fire Chemical Systems (WFCS) program tests a variety of fixed- and rotary-wing tankers to determine the parameters for optimal ground pattern coverage over a wide range of fuel and fire conditions. The Ayres Turbo Thrush, operated by Pierce Aviation, is an aircraft designed for use as an agricultural sprayer. It is converted for fire suppression by using the standard fire door.

MTDC tested the Ayres Turbo Thrush (Figure 1) with a series of drops over an array of plastic bowls much like Cool Whip containers. The quantity of material in each bowl was measured and the data were used to determine the drop pattern.

Photo of the Ayres Turbo Thrush.
Figure 1—View of the gate on an Ayres Turbo Thrush.

The mechanically operated standard fire door used on the Ayres Turbo Thrush provides only one flow rate with a maximum volume of 450 gallons, all of which is released at one time. Tests included airspeeds from 79 to 100 knots (91 to 115 mph) and drop heights from 15 to 34 feet from the bottom of the door to the ground. The drops were made with three different materials: water, foam, and gum-thickened retardant.

Flow rate, drop height, and air speed all have an effect on the drop pattern. Because this type of airtanker is normally used over a narrow range of heights and speeds and has a single flow rate, information about an average drop is presented (Figure 2). The figure represents the drop pattern from a water drop with a full flow rate.

Image of the dop pattern for the Ayres Turbo Thrush with Standard Fire Door.
Figure 2—Drop pattern characteristics for the Ayres Turbo Thrush with
an air speed of 96 knots (110 mph) and a drop height of 21 feet.
The contour lines are at coverage levels of 0.5, 1, 2, 3, 4, 6, 8,
and 10 gallons per 100 square feet.


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USDA Forest Service - Missoula Technology & Development
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