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Retardant effectiveness and appropriate drop heights. The safety of the pilot and the ground firefighters remains paramount. We understand that no one willfully endangers themselves or the other firefighters during fire suppression actions.
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The safety of the pilot and the ground firefighters remains paramount. We understand that no one willfully endangers themselves or the other firefighters during fire suppression actions.
The following two slides are just a review of information presented at the National Training Course (the annual Boise training) during the flight safety class. The purpose is to highlight an evident problem that can arise during the drop sequence.
“The Problem”, a review. Note the elevator deflection during the drop sequence. Should the gate not open to counteract this action, the aircraft will begin a descent towards the ground, and must be recognized by the pilot very quickly . This condition needs to be reversed to avoid contact with the ground. The greater the distance from the surface, the more time the pilot has to react.. (Refer to the “Time to Impact” chart from the “Operational Flight Briefing” presented by Phil Oppenheimer.)
“With Safety being paramount, the completion of the mission shall never take precedence over your personal safety!” As professional Aerial Firefighters, it is understood that performing the mission as safely as possible must always be the primary target of our efforts. With this understanding, we shift our focus to the secondary issue. Mission Effectiveness.
In order to enable you to perform in the most effective and efficient manner, an understanding of how retardant works is essential. In the following slides we will try to dispel some common misconceptions about the best way to apply retardants and suppressants.
The subject of what determines an effective delivery of retardant centers around the appropriate drop height. Misconception: “The lower you drop the more effective the retardant.” Although it is true that the lower you drop the more accurate you may become, it is not true that the retardant becomes more effective.
Misconception:“ Accuracy is the most important factor in making an effective retardant drop.” Accuracy is important. However, effectiveness is more important. A drop that is accurate but leaves gaps or “Shadows” the fuels, is not an effective drop. It may even provide the ground firefighters with a false sense of security. Every experienced firefighter confirms that they would rather have an effective drop that was a little off target than an accurate drop that was ineffective.
Appropriate Drop Heights. Not to be confused with the “Minimum Descent Altitude”of 60’ above the vegetation. The AT802s’appropriate drop height for retardant effectiveness has proven to be an altitude of between 80 and 100 feet above the vegetation. These target elevations of the flight envelope, provides for a “Sweet Spot,” an area where a more consistent and effective application of the retardant is assured.
*SEAT drop from 60’. This is an example of a retardant drop from approximately 60’ above the vegetation. Note how the retardant column still maintains forward momentum as it descends to the surface. Although it is easier to be more accurate from this drop height, the retardant loses it’s effectiveness because of the “Shadowing” and matting of fuels. This leaves gaps and untreated fuels which provides an opportunity for the fire to burn through the line. *Extensive testing has shown that retardant drops from 60’ are only marginally effective.
Examples of “Shadowing”These are photos of a SEAT drop from an altitude of 60’ above the vegetation.
SEAT drop from approximately 90’ above the vegetation. Note how the retardant column’s forward momentum has slowed and has begun to drop in a more vertical direction just prior to hitting the surface. This will facilitate the proper application of the retardant to the fuels and creates a “Raining Down” condition that will apply a uniform coverage to the vegetation.
Examples of retardant application without “Shadowing.”These photos were taken after a SEAT drop from 90’ above the vegetation.The photo at left shows an area with a “Coverage Level” of approximately “8”.
SEAT drop from approximately 120’ above the vegetation. Notice how the retardant has begun to disperse and lose it’s cohesion and density. Although the retardant does lose it’s momentum and “Rains Down” onto the vegetation, it has become too widely dispersed and will become less effective on the fuels. The coverage levels from this height reduce drastically. Retardant effectiveness degrades rapidly.
SEAT drop from approximately 120’ above the vegetation. Note the “light” coverage of retardant on the ground, and the very wide line of application. Drops from this height, although providing consistent overlay of the materials, presents problems in accuracy, drift, and coverage levels.
Retardant/Bucket Operations Evaluation Scalloping: Uneven coverage within the retardant drop resulting in weak areas the fire can burn through. Shadowing: Retardant coverage on one vertical side of the fuel, but not the other. Minimum Drop Heights: Heavy tankers: 150 feet SEAT: 60 feet (802 wingspan = 60 feet)
The presented “Effective Drop Height “tests were conducted primarily with retardants. All drops were made from a AT802 with a constant flow gate system, set at coverage level 4 with 600 gallons on board, and equipped with a radar altimeter to assist the pilot in maintaining a consistent altitude above the vegetation. The weather conditions at the test site were: winds at 10 to 17 mph with some gusts to 20mph, and at a 45 degree angle from the right . Temperatures were in the 60s f.
The results of previous tests, with other types of aircraft (Turbine powered Dromaders and Thrushes) utilizing fire suppressants and retardants, do not significantly change the most effective drop heights. For example, with the Dromader, dropping 500 gallons of retardant the most effective drop height was found to be between 70 and 95 feet above the vegetation. The most effective application of “Suppressants” (Foam, Gels, and water) remains at the 70’ to 90 ‘ level.
