Sources of Data

Actual Burnover Cases

A search of the scientific literature and popular press yielded numerous reports of bushfire-fighter deaths extending back as far as 1939. The older reports, however, commonly related to "wet sack" firefighters and rarely distinguished untrained civilians from regular volunteers. This review is confined to burnover incidents involving regular organised firefighters occurring after 1979. Thirty-seven incidents were identified, 19 in Australia and 18 overseas. The Australian burnovers occurred as early in the season as September, and as late as April, and whilst a number occurred at major fires, others occurred during quiet times at small fires or burnoffs. Whilst the number of burnovers identified may seem disturbingly high, even these may represent only the tip of the iceberg, Almost every brigade contacted could recount a number of other "near misses". The Bridgewater Brigade, for example, had their tanker destroyed by fire on Ash Wednesday I in 1980, and the replacement seriously scorched on Ash Wednesday II in 1983, and again in 1990.

The Australian tests of 1972

In the early 1970’s, a series of experiments subjected standard passenger cars to external heat simulating the effects of a bushfire (Cheney 1972). Even in the most severe tests, simulating a forest fire, the cars proved remarkably resistant to heat , it taking around 4 minutes for ignition of the door seals and roof lining, 8 minutes for the ignition of the engine bay, and over 30 minutes before the fuel tanks ignited.

Interior temperatures remained relatively low, reaching 104 degrees F after 4 minutes, but rising rapidly once the interiors caught fire.

The 1996 US Burnover Tests

In 1996 the Missoula Technology and Development Centre conducted 3 separate trials which recorded the conditions inside fire appliances and personal fire shelters during an experimental burnover (Mangan 1997). The first, the Florida Burn, involved a standard ‘Wildland Engine’ and a standard pickup with a slipon unit. The resulting low intensity grassfire, with low flame heights and an estimated intensity of less than 1500kW/m, passed over in less than a minute and did not seriously test the survivablilty of either the trucks or the fire shelters.

The First Los Angeles Burn subjected a standard urban appliance and a pickup to a more severe grass and brush fire with flame heights averaging 10-20 feet and an estimated intensity of 2000-3000 kW/m (Figure 1). The peak intensity of the fire passed within a minute. The urban appliance suffered severe damage in the fire, with cracked windows in the cabin, and ignition of paintwork, hoses, and mudflaps. The pickup was similarly affected, with scorched paint and cracked windows, and, disturbingly, filled with heavy smoke even though the interior did not ignite. The interior temperatures however, remained low, ranging from 60 degrees C in the pickup to 85 degrees C in the urban appliance, and were significantly lower on the floor of the cabins. The temperatures inside the fire shelter adjacent the urban appliance were higher than those inside the engine, reaching 220 degrees C at 12 inches above ground, and 160 degrees C at 1 inch above ground.

Conclusion :
Temperatures inside the vehicles remained survivable in this moderately high intensity burnover. Dense smoke however, possibly from near ignition of the door trim, may have forced the crew from the pickups’ otherwise survivable interior during the burnover. Temperatures inside the fire shelters were higher than in the vehicles.

The Second Los Angeles Burn was of lesser intensity than the first, with an estimated intensity of 2000kW/m , and with the peak of the fire passing within 30 seconds. The urban appliance again appeared to provide a survivable environment, with maximum interior temperatures of 75 degrees celsius (and only 40 degrees C on the floor). The pickup interior, however, became uninhabitable within 60 seconds when the interior trim of the door adjacent the fire ignited due to the radiant heat load on the exterior (Figure 2).

This rapidly filled the cabin with smoke, and elevated the interior temperature to 280 degrees C. The temperature in the fire shelter adjacent the urban engine was below 80 degrees C. The shelter adjacent the pickup recorded a maximum interior temperature of 180 degrees C.

Conclusion :
Interior temperatures inside the fire shelters and the urban appliance remained survivable, (and again lower inside the vehicle) in this moderate intensity fire. Ignition of the interior door trim due to external radiant heat would however have forced the crew from the cabin of the pickup during the burnover.

The final burn, the Montana Burn subjected a Wildland engine and a pickup to radiant heat from an adjacent pine bonfire. This produced a more prolonged and severe exposure to heat than the earlier tests. Thick smoke and high temperatures made the Wildland Engine cabin uninhabitable after several minutes, again following ignition of the interior from the radiant heat. Conditions in the adjacent personal fire shelter however, remained survivable, with maximum temperatures of only 75 degrees C recorded after 12 minutes. The pickup , similarly, became uninhabitable after several minutes, when ignition of the interior caused the cab to fill with smoke and heat . The fire shelter adjacent the pickup recorded survivable interior temperatures of 105 degrees C.

Conclusion :
The drivers cabins of both vehicles became uninhabitable within 3 minutes when radiant heat ignited the interiors in this high intensity, long duration fire. The interior of the personal fire shelter appeared to remain a survivable environment.

Overall, this series of tests appeared to show that interior temperatures inside vehicles were generally lower than inside fire shelters, provided the interior door trims were not ignited by radiant heat loads on the door exteriors.