Aerial Ignition

DAID

When ground ignition techniques are used, the downwind spots will usually coalesce and burn out before the whole block has been ignited. In contrast, aerial firing permits ignition of a block to be completed before the downwind spots have burned out. This does not present a problem at the damp end of the prescribed burning window when actual fine-fuel moisture is near 20 percent. Rapid ignition of a block reduces both flying time and the time needed to complete the burn. However, when using aerial ignition techniques under "traditional" ideal burning conditions for line-backing fires with actual fine-fuel moisture near 10 percent, rapid ignition of the entire area can result in an increase in fire intensity to unacceptable levels. The burn executor have little recourse except to let the area burn out and hope that damage is limited to just a loss in overstory growth.

Some experienced burners start firing early in the day, before the fuel is dry enough to carry fire well. They reduce the distance between spots within a line to less than 2 chains by 2 chains. The increased number of ignitions creates more heat and helps dry the surface fuels, especially when a helitorch is used. The distance between spots must be expanded as the morning progresses and burning conditions improve. Otherwise, the spots will merge laterally forming lines of heading fire that get too intense before reaching the next downwind line of ignition points. The distance between lines can also be increased as necessary to maintain a square ignition grid.

Current aerial ignition techniques can be separated into two major types: the DAID (Delayed Aerial Ignition Device) or ping-pong ball system, and the helitorch or flying driptorch system.

Helitorch in Action

• The ping-pong ball system utilizes small plastic spheres containing potassium permanganate. The balls are injected with ethylene glycol and immediately jettisoned before the chemicals react thermally to produce a flame that consumes the ball. The dispensing machine can be mounted in small airplanes or helicopters. The ping-pong ball system works best in continuous fuels or in areas where a mosaic burn pattern is desired.
• The helitorch is simply a giant driptorch and drum of gelled gasoline mounted or slung under a helicopter. The helitorch is well suited for discontinuous fuels such as those in clearcuts because this system emits a steady stream of burning fuel globs. It is very difficult to effectively regulate the spacing between these fuel globs. At least one gadget that apparently solves this problem is being marketed. Any helitorch not modified to effectively control the timing between the globs of burning fuel should be considered a linefiring device.

Both types of aerial ignition dramatically reduce the time needed for an area to burn out. Although roughly the same amount of smoke is produced, it is emitted over a shorter period and more of it is entrained in the convection column. Thus, the impact of any adverse air quality effects is much reduced.

Factors associated with aerial ignition

• Rapid firing and burnout allows use of a much smaller prescription window.
• Damp, fine fuels are of critical importance. Actual fine-fuel moistures of 15 to 25 percent are preferable.
• Requires an experienced burning boss to make ignition grid adjustments and to determine when to halt ignition due to conditions.
• Although not likely under prescribed fire conditions, too much heat energy released over too short a period will result in a sudden, dangerous increase in fire intensity.
• Large acreages can be safely burned in a single burning period.
• Many widely dispersed tracts can be burned during a single day.
• A contingency plan is essential in the event the aircraft is reassigned or equipment breaks down during operation.

Factors associated with ping-pong ball (DAID) systems

• Best suited for continuous fuels or when a mosaic pattern is desired.
• Ignition spacing within and between flight lines can be easily adjusted.
• A 2x2 chain to 4x4 chain grid (one ignition point every 0.4 acre to one every 1.6 acres) works well in both palmetto/gallberry and Piedmont fuel types.
• When underburning Coastal Plain fuel types, actual fine-fuel moisture should be 20 to 25 percent (even higher in very heavy fuels) and the air temperature should be low, preferably below 50^o F.
• In Piedmont fuel types, actual fine-fuel moisture contents between 10 and 15 percent and air temperature below 55^oF work well.
• Make sure no DAIDs are mistakenly dropped outside the burn as the helicopter turns at the end of each line.

Factors associated with helitorch systems

• Not as safe as the DAID system, but less expensive.
• If the torch and fuel tank is slung under rather than attached to the helicopter, a larger crew will be required.
• Creates disposal problems.
• Very difficult to regulate spacing within a flight line.
• Fuel-mixing viscosity is sometimes inconsistent due to temperature changes which, in turn, further aggravate in-line spacing of ignition spots.
• The most efficient firing technique for large, cleared areas with did discontinuous fuels, including piled or windrowed debris.
• Use extreme caution when underburning Coastal Plain fuel types. Try to keep within-line ignition point interval to at least 2 chains.