Captain Kenneth R. Adams
Air Line Pilots Association
Chairman Central Air Safety Committee
Ticking faults have been identified for over 30 years as having caused or as a causal factor in aircraft accidents. For over 30 years the industry has ignored this type of fault. As the average aircraft has aged, aging wire has increased the potential for this type of activity to occur. This paper defines ticking faults and identifies conditions that enable ticking faults. It also identifies how aging wire contributes to these conditions
I have been a safety volunteer and “tin kicker” with the Air Line Pilots Association for over 16 years. My background of scientific and engineering training has helped me in these endeavors but my real job is flying a large aluminum tube filled with wires, electricity, flammable liquids, a lot of flammable material and people. This presentation will not be electricity 101 but a synopsis of the real world. The one of flying aircraft with passengers in the seats of those aircraft.
I am here as a representative of the US Air Line Pilots Association to identify to you the problem of ticking faults and its new ally aging wire. For over thirty years we have lived with ticking faults and the accidents that have resulted from them. We think it is time to finally address this problem as well as its new ally. My purpose here today is to challenge you and this industry to finally step up to the plate and work with us.
The 1943 Aeronca Chief pictured below had a very simple need for wiring. Its cockpit is composed mainly of mechanical instrumentation and what wiring that was installed was used to provide electrical power to the running lights. Common household lamp wire or “zip cord” was used originally in this aircraft. The original wire lasted fifty years but the aircraft had been stored for much of that time. The restorer’s wisely replaced it but used lamp wire to keep the aircraft original. The most interesting point to me is that the thickness of the insulation is as great or greater than the wire thickness.
This is the cockpit of the B-777, one of the newest aircraft entering the fleet. All the instrumentation is electrically powered as well as many aircraft systems.
This is the wiring of a MD-11. The bundles are composed of up to 50 or 60 wires each, which are then crammed into a small area with many other bundles. The wires are mostly 20, 22 and 24 gage wires and the insulation thickness is 5 to 6 mils in thickness, roughly about the thickness of 3 human hairs. The reason for this is very simple. Weight. Modern wide body aircraft have 187 miles or more of wiring.
The FAR’s regulating electrical equipment and installations go back to 1964 with the latest revision in 1978.
FAR part 25.1353 paragraph (a) “Electrical equipment, controls, and wiring must be installed so that operation of any one unit or system will not adversely affect the simultaneous operation of any other electrical unit essential to the safe operation.” defines the requirement to prevent a single point failure in electrical equipment, controls and wiring. Paragraph (b) “Cables must be grouped, routed, and spaced so that damage to essential circuits will be minimized if there are faults in heavy current carrying cables.” deals with the wire grouping, routing and spacing. Paragraph (c) deals with batteries and is not of our concern at this moment.
Electrical equipment and wiring have changed a great deal since then. The number and type of “black boxes”, computers, entertainment systems have grown exponentially.
We need a more defined part 25.1353. The bundles pictured below contain wire with different insulation type. Because of these differences, we have seen wire bundles where one type of wire was sawing into and cutting the insulation of the wire with a different insulation type.
We presently have wire bundles that are composed of AC power, DC power, ground wires and signal wires or otherwise known as circuit controlling wires. One can easily use their imagination to see what difficulties could arise from shorting, arcing or some other type of damage to a bundle of this type.
We presently have bundles that are not only comprised of the conditions listed above but bundles that have different bus power sources contained in them. It makes for a difficult chore to determine how to protect a circuit in which it could have possibly three or four different power sources to feed it.
Think of these bundles as I have described them and now imagine a “ticking fault”. A ticking fault is electrically much different than a short to ground type of fault. The “ticking fault” is an arcing event that has time duration of just a few milliseconds. Typically the voltage will drop to some mid level point while the amperage will multiple upwards of by a factor of 10 or more. This is a discharge of a great amount of localized heat and energy yet because of the short duration of the arcing event, current circuit breaker design will not protect against this type of failure. While the pilot is counting on the circuit breaker to prevent a possible electrical fire, he is generally not aware that the circuit breaker will not protect him from a “ticking fault”.
A ticking fault or arcing has been implicated in many accidents: Apollo 1, Philippine Air Lines 737, U.S. Navy Aircraft, TWA 800 and SWR 111.
Mr. Jerome Leder, founder of the Flight Safety Foundation, encountered wire faults over 30 years ago and determined that these “wet wire “ fires as he called them, caused more than a few accidents. The phenomenon appears to have been a form of the “ticking Fault” or wet arc tracking. This phenomenon has been shown in laboratory test to explosively track along the wire. Visually it would resemble an old fashioned black powder fuse.
The below picture is an example of a “wet wire” fire or more accurately, ticking fault enabled and supported by moisture damaged insulation. The moisture in this case was “blue water” and due to the absence of a suitable fuel this fire was contained to a small area. Those of you that have ever been “rained upon” in a commercial aircraft can attest to the fact that there is suitable moisture present in our modern aircraft that can cause moisture damage to wire insulation.
The fact that the wet insulation will dry out over time does not make the situation any better. The drying forms what are known as “dry spots” in the insulation. The dry spots have changed their chemical composition, getting nearer to the carbon molecule, which increases the conductivity of the insulation. This creates the phenomena knows as dry arc tracking.
Another form of ticking fault or arcing occurs when microscopic cracking occurs in the insulation. This cracking occurs over time and aging of the insulation. The ally of ticking faults, aging increases the probability of ticking faults occurring in our aircraft.
Aging in some cases has been defined by a certain number of years and in the case of aircraft structure we are looking somewhere in the 20-year time frame. I think we need to realize that wire is not the same as structure and that it ages at a different rate.
Vibration is one of the factors affecting wire aging. Vibration in not constant throughout the frame of the aircraft. It varies greatly and as such it is affecting the wiring running through those areas differently as well. Wheel wells, engine compartments, areas near the air-conditioning packs all have different vibration cycles and yet the current approach to wiring does not take those differences into account.
Moisture is another major contributor to premature wire aging. Most insulation material is a very complex long chain polymer and moisture accelerates changes to this complex polymer which decreases the insulation qualities over a very short period of time.
Temperature is another of those major contributors. If you ever overheated your saw motor or had an electrical device short and seen the damage to the cord from the internal heat you can easily imagine what is happening in our aircraft. We not only have to contend with internal overheating but external heat from just about any device we have on an aircraft. We use a great deal of energy and energy is heat.
Installation of the wire also has a large effect on wire aging. Most of the current insulation types are unable to withstand tight radius bends, yet in our aircraft, we have thousands of examples of this type bending. We have yet to be able to translate two-dimensional design into three dimensions. Each aircraft is somewhat different; each wire run is installed somewhat differently. The clamping devices and bundling devices also add to stress and strain on the insulation. All this adds to premature aging.
Location of wiring in the aircraft structure is really a combination of the previous items, as different locations will have some or all of these items. Depending on the combinations the aging process will be quicker or slower.
The wire type as differentiated by construction method, by insulation type and design purpose affects the aging process as would normally be expected
The challenges I would like to present are really quite simple.
1. Enhance FAR 25.1353. It is inadequate considering the expansion of electrical devices and amount of wire in our modern aircraft. . Electricity is different than hydraulics. The current regulations treat the electrical system as if it were a hydraulic system. New aircraft are being planned that will make greater use of electricity and the regulations must be changed to recognize and deal with the complexities of electrical systems.
2. Installation practices: What power sources, what insulation types are contained in a single wire bundle, fastener design, bundling methods.
3. Modular design: Wire is not for ever, aircraft go through major overhauls at set times, modular wiring would allow for ease of replacement on a scheduled basis.
4. Fiber Optics: Not all the systems in our aircraft need wires. My pool light is a fiber optic light. Aircraft interior lighting, entertainment systems seem to be a natural area for this type of technology
5. IR/FM Technology: This type of technology is used in other industries, maybe there is a way for us to use it as well.
From the recent accidents we have been involved in, I would like to present you with some goals we at US ALPA have developed.
A. Checklist and design of electrical system coordinated to remove as much electrical power as possible from the aircraft as soon as possible, reducing power down to a level of “essential power.”
B. Some type of cockpit fire detection/extinguishing system available to the crew while in their seat attached to the oxygen system.
C. Integral power supply for standby horizon.
D. Separate by location wiring system for Captains flight instruments, communication and navigation systems.
E. Checklist design as it pertains to font size and color.
F. Smoke/heat detection system in avionics compartments.
This problem was first identified over 30 years ago. The consequences have been documented. They continue to be possible every time we operate an aircraft. I believe that this situation is totally unsatisfactory.
Think about it. It could happen to you.
The author would like to take this opportunity to thank Mr. Dick Healing, Director, US Navy Safety and Survivability, Mr. Joel Walker, Navy Safety and Survivability, for their help in developing this presentation.
Picture of the Aeronca Chief courtesy of and permission from EAA and Vintage Airplane, picture by Jim Koepnick. Picture of B-777 cockpit courtesy of and permission from LB Adams. Pictures of MD-11 wiring by Jim Shaw, ALPA.