Extracts from Wood (1973)

On the basis of local research into the use of controlled burning to reduce fuel accumulations under safe conditions, and a recognition of the fact that devastating wildfires would continue to occur unless a radical new approach was taken, a policy of large scale scientific application of fire has been introduced. An essential aim is to avoid possible damage to growing stock and soil structure. This can be achieved by burning under carefully prescribed conditions which ensure that an unconsumed layer of material remains. Such conditions arise when the lower layers of the fuel bed are moist, resulting in what is termed a "cold burn".

On this basis, it has become an operational procedure in south eastern and Western Australia to carry out prescribed burning over large tracts of the Australian bush at frequencies from 4 to 8 years.

Since the 1920's aircraft have played an important role in forestry in Australia. In the early years they were principally used as aerial platforms for observers, and due to a lack of availability from civil sources, were almost exclusively hired from the Royal Australian Air Force.

It was not until the 1960's, which heralded great expansion in the agricultural aviation, charter, and airwork industries, that foresters were in a position to utilise light aircraft to develop the many and varied specialised airwork applications that collectively comprise forestry aviation. Today, the Forests Commission of Victoria engages light single and twin engined aircraft and helicopters for some seventeen functions on a routine basis, ranging from forest establishment by aerial seeding to management with sophisticated photographic methods, and protection by fire-bombing, control of insects and diseases, and a variety of other techniques.

Once the need for large scale controlled burning was recognised as an essential objective, it became evident that the key to treatment of large areas during the brief periods of suitable weather was the use of incendiaries dropped from aircraft.

Two separate developments ensued - a helicopter technique introduced by the Forests Commission of Victoria, and a fixed wing technique developed by the Commonwealth Scientific and Industrial Research Organisation in collaboration with the Western Australia Forests Department.

Both techniques are used in Victoria, where the annual area fuel reduced is increasing towards 1 million acres.

A fundamental of both techniques is that throughout the area to be fuel reduced, sufficient ignition points must be established to enable each indidual fire to join with its neighbours overnight following the day of lighting. Too few fires will mean that unburnt areas will be subjected to possibly more severe weather the following day, too many will expose parts of the area to high fire intensity at junction zones where two fires meet, during the afternoon of the day of lighting, rather than in the cool conditions of night.

Helicopter Technique

Helicopters are well suited to aerial ignition because excellent visibility allows easy recognition of irregular boundaries in rugged terrain, slow flying permits accurate placement of incendiaries, and excellent manoeuvrability at low speed enables a high degree of control over the lighting pattern even in the steepest terrain.

This technique is widely used in Victorian mountain forests whew variable aspect and fuel conditions demand precision lighting to ensure that fire behaviour is within prescribed limits.

The incendiary used, known as (Delayed Action Ignition Device), is a 7 inch length of 30 second safety fuse coated for half its length with modified match head compound. This is covered with a layer of high melting point wax to prevent accidental ignition by friction. The other end of the fuse has a small quantity of match head compound for striking and igniting the incendiary.

The length of the fuse allows a seventeen second delay between striking the small end, and intense flaming of the large end, which lasts for 40 seconds.

Fixed-Wing Technique

This method employs a semi-automatic incendiary dispensing machine installed in light twin-engined aircraft. The machine ejects capsules on a time interval, resulting in a geometric grid pattern of ignition points throughout the designated area. This technique does not permit precise placement of incendiaries on specific targets along a flight line, but it is a much faster method than the helicopter technique and is generally cheaper. It is ideally suited to areas where the fuel type is reasonably homogenous, and the terrain is not mountainous.

The machine occupies the space of a conventional passenger seat, and is powered from the aircraft electrical system. The incendiary capsules are small polystyrene vials pre-loaded with 5.5 gm. of potassium permanganate crystals, and capped with a tight fitting plastic lid. These are conveyed in a metal cabinet adjacent to the priming machine. Capsules are fed by hand into a rotary turntable on the machine. An automatic cycle processes each capsule by injecting 1.3 ml of ethylene glycol and dropping the capsule through a venturi tube out of the hull of the aircraft.

The two chemicals combine in an exothermic reaction after approx-imately 30 seconds, by which time the capsule has fallen to ground. The chemical reaction causes intense flaming and combustion of the capsule and contents. The rate of ejection of the capsules from the aircraft can be varied to suit the required overall lighting pattern but a rate of 2 seconds is common in Victoria, resulting in a line of ignition points about 5 chains apart for a flying speed of 100 mph. By flying parallel flight lines 10 chains apart, a grid pattern of one ignition point per 5 acres can be established. Under suitable conditions, a grid of fires at this spacing will slowly join together before the following morning, thus achieving the theoretical all-over burn.