I built a flow path app, so I should say where I stand on flow path.
It matters. The research out of UL's Fire Safety Research Institute over the last fifteen years changed how a lot of us think about ventilation, and it should have. Where the air comes in and where the heat goes out decides what the fire does next, and it decides it faster than most of us were taught. If your department hasn't looked at that work, look at it.
But I want to be careful about the next step, because I see a lot of people take it: turning the research into a set of hard rules and teaching the rules as if they were the science.
The building doesn't know the rules
The research burns are controlled. Same house, same furniture, same openings, run again and again so the variables can be isolated. That's what makes them science. The building you pull up to is the opposite of that. It's a 1920s two-and-a-half-story that got a rear addition in 1975, a finished attic in 1998, a wall taken out for an open floor plan in 2015, and a dropped ceiling nobody can explain. The occupant left a door open, or didn't. There's a window unit in the second-floor bedroom. The basement stairs were enclosed by somebody who owned a nail gun.
Every one of those things changes the flow path, and you can't see most of them from the street. So the honest version of what we know is: the physics is real, and the inputs are unknown. That's not a reason to ignore it. It's a reason to stop pretending there's a card that tells you what to do.
What actually matters on arrival
The most useful thing I can say about flow path is small. Look at the building. Look at where the smoke is going and how hard it's moving, where the air is going in, what's open, what's about to be opened by your own people. Then say it out loud, on the radio, early, so the rest of the fire department shows up knowing what you saw. "Heavy smoke pushing from the B/C corner second floor, front door's open, we're going to control it." That sentence does more for the crew coming in behind you than any rule does.
And be willing to call an audible. If what you're seeing doesn't match what you planned, change the plan and say so. The dynamic thing needs a dynamic response, not a memorized one.
Why this is hard to teach
Two reasons, and they're connected.
The first is the firehouse itself. A company is a small group of people who eat together, sleep in the same building, and go to the same calls for years. That's a great thing on a fireground. It's also the textbook setup for groupthink: the group agrees with itself, the senior guy's tactics become the company's tactics, and anybody who says "that's not what the research shows" is arguing with the family, not with an idea. Getting a fire department to actually look at its own tactics is harder than teaching it new ones, and most training programs skip that part.
The second is on the other side. Every few years something new comes along, somebody builds a class around it, and it gets pushed hard before it's finished cooking. The technique gets taught as the answer before anybody's worked out the edges. Then the first time it doesn't fit the building, the guys who were skeptical say "see," and now they don't want to hear the next new thing either. The overselling causes the resistance. I've watched that cycle more than once and I think it costs the fire service more than any single bad tactic does.
So, the app
The Flow Path Analyzer is a beta and it says so on the screen. It's a building you can change, doors and windows you can open, and a picture of where the flow goes. It's not calibrated to your building, because nothing can be. What it's for is the argument: put it on the TV, open the front door, watch what happens, and let your crew fight about whether that's what they'd expect. If it gets a company to look at its own habits for twenty minutes, it did its job. If it ever gets taught as fact, somebody's misusing it, and that somebody might be me.