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Preventing inflight fires


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Fire in flight requires (amongst other things) (a) A fuel source and (b) an ignition source. Prevention of fire essentially means keeping these two apart.

 

Let's start with ignition sources:

 

One major potential ignition source is electrical- sparks, arcs, or overheated wiring or components. One good general rule is to locate potential ignition sources separate from, and above, potential fuel sources.

 

So electric circuits should NOT be bundled together with fuel lines, run inside fuel tanks, or located where leaking fuel is likely to dribble on them. FAR 23.863 refers:

 

§ 23.863 Flammable fluid fire protection.

 

(a) In each area where flammable fluids or vapors might escape by leakage of a fluid system, there must be means to minimize the probability of ignition of the fluids and vapors, and the resultant hazard if ignition does occur.

 

 

 

(b) Compliance with paragraph (a) of this section must be shown by analysis or tests, and the following factors must be considered:

 

 

 

(1) Possible sources and paths of fluid leakage, and means of detecting leakage.

 

 

 

(2) Flammability characteristics of fluids, including effects of any combustible or absorbing materials.

 

 

 

(3) Possible ignition sources, including electrical faults, overheating of equipment, and malfunctioning of protective devices.

 

 

 

(4) Means available for controlling or extinguishing a fire, such as stopping flow of fluids, shutting down equipment, fireproof containment, or use of extinguishing agents.

 

 

 

(5) Ability of airplane components that are critical to safety of flight to withstand fire and heat.

 

 

 

© If action by the flight crew is required to prevent or counteract a fluid fire (e.g. equipment shutdown or actuation of a fire extinguisher), quick acting means must be provided to alert the crew.

 

 

 

(d) Each area where flammable fluids or vapors might escape by leakage of a fluid system must be identified and defined.

 

 

The above rule is not confined to the engine compartment; it applies throughout the aircraft. Be aware of such possibilities as a DC electric motor - e.g. a flap drive motor - being located in the same part of the wing as a fuel line.

 

 

 

Incidentally, the ignition of liquid that falls on a hot surface is NOT indicated by the "flash point" of the liquid; a useful rule of thumb in regard to hot surfaces, such as parts of the exhaust system, is whether or not they are hot enough to melt solder; petrol in liquid form needs a bit over 300 C to ignite. Lubricating oil ignites at a considerably lower temperature.

 

 

 

Fuel Systems:

 

 

 

There is a considerable advantage in the use of an "updraft" carburettor, especially if it is combined by an "up exhaust" engine layout, because this places the hot bits on top and the potential fuel leaks underneath the engine. If the engine layout does not do this inherently, then it is necessary to use splash trays having drain tubes or liquid flow deflectors under carburettor bowls, etc. Also, the use of separate fire sleeving on hoses and lines carrying flammable liquids in the engine compartment has a "hidden benefit" in that it also serves to catch liquids escaping through small cracks in the tube, and thus converts a highly-flammable fuel spray into a dribble at the end of the fire sleeve.

 

 

 

Fuel lines that are under pressure - e.g. fuel injector lines - are a particular hazard, especially when the injector nozzles are located high on the engine. The traditional fuel distributor placed on top of an injected engine is an unnecessary hazard, and special care is needed to prevent the injector lines running from it from resonating to any engine vibration frequency. There have been a number of engine fires from this cause, over the years. I have seen EFI systems on recreational aircraft engines that are an accident waiting to happen, from this cause.

 

 

 

Fuel lines in engine compartments are required, nowadays, to comply with FAA Technical Standards Order (TSO) C 53 Type D. It is foolish to use less. See, for example, http://www.aircraftspruce.com/catalog/appages/ae466.php

 

 

 

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Subject like this one should be part of design recommendations. Not to be restrictive rather to be informative. WE as RAAus don't build a high percentage of the planes we have (which is a bit of a pity and something I hope will change) we should aim to build better and fly better through being better informed. Nev

 

 

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Someone on another thread commented that they would never reset a circuit breaker in flight. That is a really good idea, but I had not thought it through before and I was taught to try a reset once. I have never experienced a circuit breaker pop out, but I will leave it alone if it does in future.

 

 

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A circuit breaker is a load limiter and will pop quickly again if the load is above the limit ( IF nothing has changed) If the problem is an intermittent one it may not pop again for ages. You can have an electrical overheat without popping the circuit breaker or blowing the fuse if there is a fault not bad enough to show that way, at least initially. There is a reason why breakers pop and it should be checked out properly . Not resetting is the sure way of not having the fault bother you at the time. Don't ever put thicker wire in a fuse or you may destroy your wiring harness where all the wires are together and fuse some together and making a real problem out of a small one. Nev

 

 

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Circuit breakers work on heating a bi-metallic strip, if the current is an instantaneous high Amp load it will trip immediately. If the current is a few amps over the rating, it may take minutes to trip. Resetting a breaker in that mode can cause more problems as you tend to believe the problem is just a "trippy" breaker and not a sustained overload problem. Although, if you reset it continually the pop time will decrease as the bimetal strip stays hot between resets.

 

I would add to Nevs comment about wire bundling to use Tefzel, or similar aircraft standard wire, definitely not PVC automotive wire!

 

 

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I didn't see any comment by Nev about bundling, but I have always wonered about the wisdom of bundling wires on aircraft. Electricians have to allow for the greeating of bundled wires, but I have seen as many as a dozen wires in a bundle in an ultralight. Doesn't look good to me. When I wired my panel I kept bundling to a minimum and made sure that heavy loaded wires were separated.

 

Another fire source is the connections at fuel taps. It is not unknown for them to start leaking a long while after installation. The torqueing methods sometimes leave a lot to be desired, by just over torqueing to get pipes aligned for example.

 

 

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Circuit breakers work on heating a bi-metallic strip, if the current is an instantaneous high Amp load it will trip immediately. If the current is a few amps over the rating, it may take minutes to trip. Resetting a breaker in that mode can cause more problems as you tend to believe the problem is just a "trippy" breaker and not a sustained overload problem. Although, if you reset it continually the pop time will decrease as the bimetal strip stays hot between resets.I would add to Nevs comment about wire bundling to use Tefzel, or similar aircraft standard wire, definitely not PVC automotive wire!

Dead right; PVC has no place in an aircraft; for a start, the products of combustion are extremely toxic.

 

In regard to the entire subject of aircraft wiring, ALL the answers are in Chapter 11 of FAA AC 43.13-1 ; ALL circuits need to be protected by fuses or circuit breakers and wire ratings, including reduced ratings for bundled wires, are given in that document.

 

 

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