Fire Dynamics, Firefighting

USFA: Backdraft and Reading Smoke

Fire erupts from flashover can
Image courtesy the U.S. Fire Administration

The U.S. Fire Administration (USFA) released a training bulletin dealing with recognizing the dynamic fire phenomenon known as backdraft.

Recognizing backdraft conditions can save firefighters’ lives. The training bulletin seeks to underscore why situational awareness is critical for recognizing when potential backdraft conditions exist.

RELATED FIREFIGHTER TRAINING

USFA: Recognizing Flashover

Phil Jose: Reading Smoke and the Transfer of Command Process

David Dodson: The Art of Reading Smoke

FLASHOVER AND BACKDRAFT: A Primer

Differentiating the “Fireground Phenomena”

A backdraft is an air-driven event, unlike a flashover, which is thermally driven. Backdraft is usually defined as a deflagration resulting from the sudden introduction of oxygen into a ventilation-limited space containing unburned fuel and gases. When the air combines with the unburned fuel, rapid ignition can occur with devastating force. The normal oxygen level in air is approximately 21 percent. Below 14 percent, visible flame is reduced. When these fuels mix with air they ignite and burn quickly resulting in overpressure.

Backdraft Indicators

Conditions for backdraft might include:

  • Black smoke becoming dense, greyish yellow without visible flames. The smoke color is indicating incomplete combustion. Usually the darker the smoke the more incomplete the combustion.
  • A well-sealed building might indicate air confinement and excessive heat buildup.
  • High concentrations of flammable carbon monoxide could be present as a result of incomplete combustion.
  • Little or no visible flame. If flames are present, they may be blue in color. Another indicator, might be flames in smoke exiting the structure, especially in eaves of the structure.
  • Smoke leaving the building in puffs and being drawn back in. Fire is trying to find oxygen, and this is the appearance of smoke pulling in under doors or through cracks.
  • Smoke stained windows, brown in color, with visible cracking and/or rattling.
  • Sudden, rapid movement of air and smoke inward when an opening is made.
Dale Pekel/YouTube

Situational awareness and defensive steps to protect yourself and your crew:

  • Perform a 360° size-up prior to opening-up wearing full personal protective equipment.
  • Create vertical ventilation prior to making entry.
  • Operate from a position of safety. Always plan an escape route.
  • Pencil the ceiling with a straight stream, without disrupting the thermal layer, to cool gases below their ignition temperature.