Fire suppression -- worth the weight?

I came across an interesting product called Blazecut that would be a lightweight solution to fight an engine compartment fire. Check out the detailed spec sheet and the video demonstration on the site. No weight is given, but it holds 0.5 kg of suppressant so I'd guess a 1-2 kg total weight.

It seems to me that the most likely scenario for fire in a homebuilt is not an in-flight fire, but a post-crash fire because fuel from a ruptured tank or lines contacts hot exhaust, electrical sparks, or sparks from metal scraping the pavement. In that case, especially in aircraft that have the fuel tank in the fuselage near the crew for CG reasons, something like one of these manual/automatic systems would provide awfully cheap peace of mind. Installed weight is about 4-5 kg.

What do you think? I don't think it's a money question -- if you can afford to build and fly a plane, you can afford to spend a few hundred bucks more to prevent a fire -- but are you willing to add 2-7 kg to the empty weight of your aircraft for fire safety?

Cheers,

Matthew

And if you can afford to build and fly a plane, you can afford....
1. Air Bag Shoulder Harnesses.
2. BRS Chute.
3. ADSB everything.
4. Hi Tech everything panels.
5. lots of other government mandated ballast equipment.
6. The list goes on and on.

All of us draw the line at some point to reach a balance of weight, safety and cost. Personally, I haven't installed one of these in any of the planes I have built. However, one of the EAB planes I maintain for a customer does have a fire suppression system. That's OK too as it's his choice for his safety. Depending on what is leaking and where, I think you'll find an under cowl fire suppression system to be pretty useless in the case of a post impact fire. These are mostly only good for an in flight fire under the cowl and only if you shut off the fuel source before firing the extinguisher. No doubt there are exceptions to that.

One other thing I would suggest is to try fighting a small gasoline fed fire with a fire extinguisher of similar size. You'll be amazed at how quickly the extinguisher expires and how quickly that tiny amount of fuel can become a roaring inferno once you are out of suppressant.

I'm not knocking these units, but know what you are getting and what it can and can not do for you. Then evaluate whether it's worth the cost and weight going into your project. There is no absolute yes or no answer. Many builders will come up with no for an answer. Many others decide they want it.

-CubBuilder

In my opininion it has merit. I checked their web site for an available DETECTION system. A must have. May have missed it. Many aircraft have fire warning systems, a lesser number have the means to fight it once detected. I am familiar with three types of detection.

The first type has been around since the stone age. an un insulated conductor is strung around the inside of the engine cowling. It has a suitably low melting point. When continuity is broken, a light & horn on the panel activate. It proves to be almost fool proof.

A more recent (almost half century) system uses "flame detectors" mounted inside the cowling. Just another way to say photo cell. Its more expensive than the other one and has a quirk. When the flame detectors get old and deteriate they get more sensative. Like some people. Early morning or late afternoon when the sun is low, it shines into the cowling and the alarm goes off. Those of us that have experienced this have learned to turn away from the sun briefly. If the light and horn stop,its a false alarm.

Some baggage compartments have kitchen type smoke alarms but rarely means to fight the fire. This type certainly would not work in engine compartments. Some types of newer (10 years) baggage smoke alarms use a different principle and are set off by unauthorized cell phones.

The Blazecut system is somewhat like the systems required on some certified aircraft. Its much less expensive. It has about the same amount of agent that i've seen on 1,000 hp engines. The agent in certified systems makes a poisonous gas when exposed to flame. The certified systems work like this:
When the alarm goes off, pull the "T" handle to the rear to arm the system. Then hit the switch to discharge the agent. When the "T" handle is pulled aft, system is armed, generator is cut, fuel valve is closed and throttle closed.

My very first emergency was when the cabin filled with smoke. All gauges in the green and no alarms. Landed ok next to the fire trucks, killed the engine and master switch and ran about 50 yards away. Could barely see the bird for the smoke cloud. A two by three foot hole was burned in the side and the battery was gutted.

Years latter we departed a ship for a 1 hr 20 min over water trip home. Left engine would not make 85% on climb out. After 1 hr it would only make 30%. Landed OK and maintenance tech found a loose "B" nut (prime port) on a SS fuel line on top of the engine. It had been spraying an increasing mist of Jet A over the hot section & tailpipe for more than an hour. This a/c had fuel pressure gauges and they read normal. Most of this type have fuel flow gauges and the high fuel flow would have tipped us off to shut it down. No sign of fire. I doubt that our fire system could have coped.

Bob

Mr Dingley makes a very good point. In an airplane, unless you detect an in-flight fire early, you will not likely have enough extinguisher on board to put it out. After a crash, IF you are conscious, then an extinguisher system might buy time to get out if you can extricate yourself. The unfortunate reality is that if you are trapped in some manner or you are lights out, an on-board extinguisher system is unlikely to buy as much time as you really need.

So my personal value judgement is that on-board fire suppression in a light aircraft is a mirage. The promise is unlikely to be realized in the real world and they add weight and complexity. All equipment in the passenger compartment must be secured by a structure that can take 9G of stress in the forward direction. You don't just strap a bottle to a longeron using a couple of hose clamps. Plus you need to remove and inspect and test at regular intervals. Nothing like being on fire and discovering that the contents of your extinguisher leaked away when you weren't looking. Some multi-engine Cessna's had that problem.

It is relatively easy to make fire an extremely unlikely event. When you build, mount your hoses and plumbing securely and so that they are not subject to chafing. Take care of your exhaust system and spot cracks early. Look for signs of leaks in the fuel system when you preflight. Cessna 182's are famous for having blue stains at the wing roots when the bladders get old. Decathlons get blue stains and drips at the trailing edges of the wings when fuel tanks crack. Don't see these signs and hope they will get better.

Fuel fires obviously require the most decisive and rapid response. Get on the ground or use your parachute. Electrical fires seem progress more slowly and seem to be easier to handle. When I got smoke in the cockpit I shut off the master and the smoke started to clear right away. I still landed immediately. Electrical fires seem only seem to start to move if they make contact with something else flammable. If your airplane has been constructed using modern wire and cockpit materials that are flame resistant (FAR 25 anyone?) then an electrical fire has a lower likelihood of accelerating.

Fires are so infrequent that the stories get our attention, and of course the potential consequences are huge, but the amount of consideration given to fire in the design and building of our aircraft should be proportional to the likelihood of its occurrence and the options we have to respond to it. I will hazard a guess that only a small minority of the readers here have ever seen smoke in flight, or had to crawl out of an airplane that was not upright in its expected parking spot.

All of that said, if a fire detection system is light and reliable, you can make a good case for building it into the engine compartment. Even then, if you make a habit of starting the engine by flooding it and then blowing flames out of the exhaust, you might get enough false indications to motivate you to remove it. There is no free lunch.

Experimental Amateur Built is great. You get to decide what trade-offs of performance vs utility vs weight and balance you want.

Best of luck,

Wes
N78PS

Thanks, all, for the comments, this is exactly the kind of conversation I was hoping to spark. I don't have strong feelings either way, I can see arguments for and against. I agree that if we carried every safety device available we'd be very safe because we could no longer fly. At the same time, that Blazecut device intrigued me because it was relatively small, light and easy to install. That said, I am not sure that something like that would do much good in that worst-case scenario of a post-crash fuel fire as I think the effectiveness of the demonstration was due to the enclosed nature of an engine compartment. But it is all food for thought...