Bi-Directional Flow Path; Are Openings at the same height, there can be one or more bi-directional flow paths in a structure depending on doors and windows open across it.
Bi-Directional Flow paths have an over pressure area where the smoke escapes from the upper parts of the door. A clear line exists between the hot buoyancy driven air/smoke outlet and the Air train (air track) that is cooler air being the pulled into the structure; this line is referred to as the neutral plane.
Air Training, I like to say is the train track and the smoke is the train thus creating a flow path. As we create a flow path (by opening the front door) the fire takes a breath, it inhales, several things happen at this point and we can learn a lot. As cooler air draws in behind the firefighters it keeps our turnout gear cooler and we absorb less energy and thermal assault. This should not be considered a safety zone because of convection and radiation. The speed the air pulls past us tells us if this fire is ventilation limited, the faster and deeper the fire inhales, well it has to exhale just a forcefully, if you have stuff going in you bet there is stuff coming out. keep your head below the neutral plane,
(you have been warned).
Air training (air track), neutral plane and the over pressure, tell the nozzle person and company officer a lot of information. Air track will tell you help give you indicators of where the fire is located, remember the fire is breathing so follow the breath, your thermal imaging camera can help you identify this. (Andrew Starnes from Kill the Flashover is an amazing Thermal tactician I highly recommend you look at his stuff).
Neutral Plane, another indicator of not only the location of the fire, but height of the plane can tell you the stage of the fire. If the neutral exist mid door there is a good chance the fire is on the same level as you are. If the neutral plane is nonexistent and smoke is bottom to top the fire may be below you (cellar or basement). If the neutral plane is above the door jam and lazy smoke the fire is more than likely above you on the 2nd floor.
A tactical 360, door control and tactical use of TIC are just some of stuff we will cover in the near future, stay tuned.
Thank you The Departed Ones F.O.O.L.S.
(pictures courtesy of KTF, ATF, FF toolbox)