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Re: REFLECTOR: Fuel system safety
Nice summary and my answers are interspersed.
Al Gietzen wrote:
> The thoughtful responses to the query about fuel system safety are
> appreciated. I'll risk giving my summary, conclusions and philosophy; and
> additional questions.
> Obviously the best way to avoid a crash and burn is to not crash. It is
> expected that we will strive for a high level of flight worthiness; both for
> ourselves and our machines. We would do this even if there were no
> flammable fuel onboard; there are plenty of other reasons. Also, we know
> that safety and reliability are related to simplicity; but the most simple
> is not necessarily the safest and most reliable. But we do know that
> Velocity accidents happen; that an engine (related) in-flight fire has
> happened, fuel leakage into the cabin has occurred as a result of an
> accident, and a fatal accident has occurred in which the plane was destroyed
> by fire. These are very low probability events. Sure we accept risks, but
> the key is to know what they are and control them to whatever degree we
> reasonably can.
> There is no doubt that the "answers" to safety related aspects of the fuel
> system in an accident are difficult or impossible. In the cabin area I
> think that the bottom line is that we rely on the considerable structural
> strength of the spar, firewall, gear bulkhead and strake area to protect our
> fuel system in the event of a mishap. What's left is to look at the routing
> and the materials of the fuel lines to minimize the likelihood of disruption
> if things were to shift around a bit. We should probably avoid having the
> fuel lines going through any of the bulkhead structures; i.e., following the
> standard design. It may also be that using braided steel sheathed line with
> threaded fittings, with some controlled slack in the line, provides the best
> bet on things hanging together.
> Question: Do we know the source of the fuel in the cabin in Hugh Hydes
> Aft of the firewall things are different. Protection of fuel lines from
> heat (and fire) is standard operating procedure. Here there are sources of
> ignition for flammable vapors. And here, in my opinion, there is also a
> greater likelihood of a ruptured fuel line in the event of an accident. If
> one could keep ones head while about to do a forced landing, closing an
> emergence shutoff valve before impact could reduce the chances of a
> post-mishap fire.
> Yes, installation of a valve introduces the tradeoff of the likelihood of
> valve causing an accident by being inadvertently in the wrong position vs.
> protecting from a fire. I don't think that valves are so complex, or that
> the installation can't be well enough thought out that the installation
> doesn't win out. Plus there is some added convenience and safety for filter
> maintenance, etc. Valves are simple and reliable, need to be periodically
> inspected and can be replaced. And, absolutely, manual or electrical; either
> installation needs careful consideration to be SURE that it cannot
> inadvertently be in the wrong position.
> Question: Jim Agnew- what kind and where is your fuel shutoff switch?
The switch is on top of the keel forward of the stick. The switch is a heavy
duty toggle type covered with a safety flip up shield (commonly referred to as a
toilet seat). There is also a red flashing LED that indicates the closed
position of the VALVE not the switch. The valve has an internal switch that
indicates the actual position. It is impossible to operate the switch without
lifting the cover yet you can quickly operate the switch with your left or right
hand. This is the same arrangement used for years for munitions arming switches
and other critical switches. The valve is a screw drive operated shuttle valve
so it is almost impossible to displace it accidentally.
> Question: Those using a manual valve with cable; what valve are you using?
> In-flight engine fires? If there is going to be one, it's going to be
> fueled by an oil or fuel leak somewhere. And if it starts, any line that is
> susceptible to fire damage is going to make the fire worse. So fireproof
> lines and a shutoff valve could reduce fire risk.
> Question: Should we be using only solid steel lines aft of the firewall? Or
> is fire sheathed, stainless steel braid sheathed line just as good?
Chances of cracking solid steel lines is very high in this high vibration area.
Race cars, boats, and aircraft have proven the dependability of the SS braid
> The addition of fire detection would seem to make sense. I'm not sure what
> can be accomplished by looking in a rearview mirror AFTER your engine has
> quit (why else would you look?) to see that you also have smoke and flames.
> A little more timely warning may be helpful.
> Question: What are the reliable options for fire detection?
This is a difficult question since you are already in a high heat area and I
feel that you are better off thinking in terms of a "possible" fire warning
system. You could get very exotic in this area, however, a simple system should
prove the best compromise. I will be using thermal fuses that are very commonly
used in everything from coffee pots to attic vent fans. The attic vent fans are
a good test since they live (in Tampa anyway) in high temperatures with a lot of
vibration. The fuses are simple small auto fuse like devices with a wire
attached to each end. These devices will be attached to the inside of the
cowling on the upper surface and around the rear outlet. I'm trying 141 C
(about 300 F) ones and they will be wired in series since the fuses open when
overheated. This gives you a foolproof system that will detect the opening of
the thermal fuse or a broken wire.
> Last thoughts. If we are going to call it a firewall, it should be
> fireproof. A nonflammable skin on the engine side (and a thin layer
> fireproof insulation?) makes a lot of sense. Perhaps the valve options are:
> full simplicty; i.e., no valve - no fire detection; or, emergency shutoff
> with fire detection. Simplicity vs. knowledge and control. I have no proof
> of which is better. You get to choose.
> Further comments welcomed.
> Al Gietzen
// James F. Agnew
// Tampa, FL
// Velocity 173 FG Elite ( http://www.VelocityAircraft.com/ ) under construction