Cessna 172: Electrical Fires and Cabin Ventilation Connection

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DCR

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rudy
Hello,

I have been wondering for a while - do the air vents which feed the passenger compartment on a 172 (N model if it matters) pass through some area shared with electrical connections / wiring?

I have always reasoned "close air vents" on the "Electrical Fire in Flight" procedure as an attempt to stop fanning the flames back there, but if the ducting is isolated from the electrical equipment / wiring then this logic is nonsense...
 
The air vents being fresh air into the cabin which also causes air motion which “fans flames” to any incipient phase fire in the cabin. Similar to the Santa Ana winds.
 
455 Bravo Uniform said:
The air vents being fresh air into the cabin which also causes air motion which “fans flames” to any incipient phase fire in the cabin. Similar to the Santa Ana winds.
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Should have been more clear - I was referring to a fire behind the panel.
 
DCR said:
Should have been more clear - I was referring to a fire behind the panel.
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The air coming in through the wing vents has to come out somehow. That exit path could help fan a fire in an unexpected part of the cockpit.
 
AV8R_87 said:
The air coming in through the wing vents has to come out somehow. That exit path could help fan a fire in an unexpected part of the cockpit.
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Gotchya - apparently I am just totally clueless on where the ventilation comes from in the 172.

I thought it comes from behind the panel similar to ventilation in a car.
 
texasclouds said:
Are you flying this model? Did you check the AFM/POH for diagrams/procedures?
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In the diagram I have it looks like it comes from behind the panel. That doesn't answer my original question though.
 
There is no electrical wiring inside the air ducts. However the outside ram air will “fan the flames”, so Cessna wants the pilots to close the vents to reduce airflow.

Some air comes from the wing leading edge and puts air into the upper cabin. Some comes from the cowl area and enters the cabin from behind the control panel (poke your head under there and see). Some is available to discharge onto the windshield. Diagram below.

What three things do you need for fire to occur? Fuel, heat, oxygen.

Closing the vents reduces the oxygen.

Turning off the electrical will reduce heat (from shorted wires being energized).

I would argue this to be sound advice, not nonsense at all.



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The inlet ducts on the side of my fuselage I have taped off year round since 2018. I have a panel of all garmin that has internal fans to cool it's self. Mainly have them taped off to stay warmer during the winter.
 
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texasclouds said:
There is no electrical wiring inside the air ducts. However the outside ram air will “fan the flames”, so Cessna wants the pilots to close the vents to reduce airflow.

Some air comes from the wing leading edge and puts air into the upper cabin. Some comes from the cowl area and enters the cabin from behind the control panel (poke your head under there and see). Some is available to discharge onto the windshield. Diagram below.

What three things do you need for fire to occur? Fuel, heat, oxygen.

Closing the vents reduces the oxygen.

Turning off the electrical will reduce heat (from shorted wires being energized).

I would argue this to be sound advice, not nonsense at all.
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I'm still not following...

Opening the ducts increases airflow into the passenger compartment. But if the fire is behind the panel how does opening the ducts increase airflow back there? From the diagram it is effectively a closed compartent with no interaction with the air inside the ducting, nor the air in the cabin.
 
Stick your head under the panel near the rudder pedals and look up. All your electrical items (intercom, fuses/CBs, radios, transponders, GPS, electrically operated indicators/gauges, and other items containing magic smoke) are open air to the cabin. More air in cabin means more oxygen for your electrical fire.
 
A problem is you still have to breathe.

A bud had the tailpipe break off on his Glastar.

Cabin immediately filled with heavy smoke.

His strategy was to open a side window while slipping.

This restored breathing and visibility to enable a safe landing.

It then was destroyed by the fire. It would be nice if there were smoke

masks or an emergency pilot breathing vent.

I thought I posted the pix here.

This happened April 2024.
 
Fire behind panel is really bad. Master off and get on the ground.
Vents open or close. Root vents open or close. Window open or close.
Whatever it takes. Doubt Cessna started a cabin fire to test their theory.

Fire is main reason pilots wore parachutes back in the day.
 
I wonder if the procedure was developed through testing, or a combination of experience in other airframes and theory. It’d be interesting, albeit a bit expensive and time-consuming to take a 172 that’s bound for the salvage yard and test various configurations with cabin electrical fires.
 
ateamer said:
I wonder if the procedure was developed through testing, or a combination of experience in other airframes and theory. It’d be interesting, albeit a bit expensive and time-consuming to take a 172 that’s bound for the salvage yard and test various configurations with cabin electrical fires.
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I agree it would be very interesting. I wasn't trying to say their procedure was wrong or pointless - just that my own personal justification may have been wrong!
 
DCR said:
I agree it would be very interesting. I wasn't trying to say their procedure was wrong or pointless - just that my own personal justification may have been wrong!
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I’m not trying to say if any comments in here are right or wrong. With wondering about the why of the factory-recommended procedure, it seems to me that closing the vents won’t really reduce the amount of oxygen. It could reduce the draft, maybe.
 
They say for a engine fire you shut off fuel and dive the plane to snuff out the fire. Don't know about that one either.
 
bonefishpete said:
They say for a engine fire you shut off fuel and dive the plane to snuff out the fire. Don't know about that one either.
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I'd pay to see that one get tested! Where's mythbusters when you need them. Someone needs to restart that show with a focus on aviation.
 
DCR said:
I'm still not following...

Opening the ducts increases airflow into the passenger compartment. But if the fire is behind the panel how does opening the ducts increase airflow back there? From the diagram it is effectively a closed compartent with no interaction with the air inside the ducting, nor the air in the cabin.
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The space behind the panel is not hermetically sealed. Stick your head in the footwell and look up: It's wide-open under there.

When the vents are open, air is blowing all around, and some of it gets into that open space behind the panel.

Make the air in the cabin more still by closing the vents, and you reduce the amount of new oxygen finding its way behind the panel to feed the fire. Whether the effect is "a lot" or "a little", it's not nuthin', and it's an easy thing to do to make a positive impact on a bad situation. It's not pointless.
 
Jim_R said:
The space behind the panel is not hermetically sealed. Stick your head in the footwell and look up: It's wide-open under there.

When the vents are open, air is blowing all around, and some of it gets into that open space behind the panel.

Make the air in the cabin more still by closing the vents, and you reduce the amount of new oxygen finding its way behind the panel to feed the fire. Whether the effect is "a lot" or "a little", it's not nuthin', and it's an easy thing to do to make a positive impact on a bad situation. It's not pointless.
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That makes sense. I thought it was a little more closed of than that. I'll stick my head under there before my next flight.

Sorry Texas that's probably what you were trying to say the whole time it just wasn't getting through my thick skull
 
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