Glider electrical system primer
From WikiGlide
Wire
Contents |
General
A great deal of information on electrical systems and especially aircraft wiring standards can be found in “AC43-13”.
For many years the standard wire used in light aircraft has been MIL-W-5086, which was a tin-coated copper conductor rated at 600 volts and temperatures of 105°C. This basic wire is then coated with various insulating coatings including nylon, PVC (PolyVinylChloride) or glass or nylon braid.
Most gliders in Australia would be likely to have been at least partly rewired over their lifetime. Often this wiring has been obtained from the auto or electronics industry. While this is most likely satisfactory when new, lack of specification compliance means that material quality (particularly the insulation), temperature ratings and longevity is unknown.
There has been some controversy in the past few years about the use of wire with some insulating materials in aircraft. If there is a fire the wire releases compounds depending on the chemical make up of the insulation. PVC releases chlorine based toxic fumes including such delights as HCl, hydrochloric acid and Cl2, First World War poison gas. There is no formal restriction on the use of MIL-W-5086A wire in civil aircraft and sources of ignition (other than the battery) are rare in pure gliders.
A number of other aircraft wire specifications with tradenames such as “Kapton”, “Kynar”, “Teflon” and “Stilan” have been used in aircraft but are often difficult to get and some have also been discontinued in harsh use applications.
The newer specification MIL-W-22759 calls for standard tin-plated copper conductor and is insulated with extruded Tefzel (ethylene/tetrafluorbethylene). Tefzel has similar properties to PVC but a maximum temperature rating of 150°C. This would be a better choice than PVC but it will still release toxic fumes if burnt. The Microair M760 manual recommends 22 gauge Tefzel for auxiliary wiring and 18 gauge for the main power and ground leads.
Things to look for
- Insulation: breakdown voltage, temperature, fuel resistance (MG), availability of colours.
- Stranding: Finer stranding equals greater flexibility.
- Tin Coating: resists corrosion but lack of coating is not a problem with proper installation.
- Typical Gauge (size) selection:
Mechanical strength
Most wire sizes will carry adequate current flows for our size equipment over the small distances found in gliders. Often a slightly larger wire is better for simple mechanical robustness. Wire data is available.
Magnetic/electric fields and interference.
When a wire carries a current a magnetic field is created. Keep wires as far from compasses as possible, especially those carrying high currents. Digital devices such as loggers etc can induce electrical noise into more sensitive equipment such as varios and radios.
To reduce magnetic effects
Consider one or more of the following
- Relocate
- Twisting
- Bypass Capacitors
- Ferrite Beads/rings
- Shielded cable
Circuit breakers and fuses
Miniature circuit breakers (MCBs) work by having a heating element which carries the load current. This heating element causes a bimetallic device to operate a switch. There are very many sorts of MCB available, suitability of each type should be assessed individually. Often these MCBs are rated for 240VAC but are commonly derated for use at lower DC voltages. Any MCBs used should be “trip free” types.
Fuses consist of a simple wire element which heats up with load current and melts at a sufficiently high load. Commonly used fuses in gliders are:
- 3AG glass types. These are rated for 240VAC but are derated for automotive applications.
- SAE J1284 (spade type) automotive rated for 32VDC
| FEATURES | CB | FUSES |
| Cost | Moderate | Cheap |
| Complexity | Moderate | Simple |
| Reliability | High | Very high |
| Resettable | Yes No | |
| Speed of Operation | Moderate | Fast |
| Number of Operations | More than once* | Once only |
* On high quality devices. Some standards only require circuit breakers to operate on FAULT current once.
Things to look for
- Current rating/impedance, voltage drop (see below Over-rated equipment).
- Termination type: Screw, push-on, solder.
- Termination looseness
- Corrosion
Overated fuses/CBs/switches
Beware of high current capacity switches or relays in low current applications. The contact material may not conduct properly below a threshold current.
Connectors
Only use connectors which are polarised, ie can only be connected in one way. Where more than one connector is required at the same location, use different types. Where more than one connector is used separate each by function (eg at the instrument panel, have one 2 pin connector for power and another for all ancilliary wiring).
Where there are only two wires and a connector plug would be excessive, spade terminals (or similar) can be arranged to give a polarised connection.
Note that there is no protection against inadvertent joining of the (say) +ve and –ve leads.
If connections do not have to be made/broken the most reliable way to join them is to use a good solder joint. Some small connector types (eg computer D9 etc) also use solder due to space limitations preventing mechanical connections. A good solder joint takes some skill to do well and is best done on a bench (hence the use of connectors). When a joint is soldered, the solder can wick up the stranded cable and reduce its flexibility but the use of heatshrink tubing can give mechanical support to the joint as well as insulation. Under no circumstances should corrosive fluxes (Bakers fluid or hydrochloric acid) be used on glider wiring.
Things to look for
- Termination looseness
- Termination type: Screw, push-on, solder.
- Corrosion
Diodes
With more modern electronic equipment reverse polarity may or may not damage the device. Suitable polarity sensitive plugs can reduce the chance of reversing the voltage on equipment when assembling or disassembling items, but a diode could also be used. There are two ways to do this:
- Reverse Biased. The diode is placed between the positive and negative rails in such a way that it won’t conduct. Should the voltage be reversed, the diode will conduct as if it were a short circuit between positive and negative. This will immediately bypass the protected instrument and blow the main fuse and probably the diode too! This is an extreme protection method, but as battery polarity reversal is only likely during maintenance work, it can be useful.
- Forward biased. The diode carries all the current under normal conditions. Reversing the voltage stops current from flowing so no damage can occur. A disadvantage to this method is there will always be a voltage drop across the diode of about 0.7 to 1 Volt.
Things to look for when selecting diodes:
- Forward Current rating - surge and continuous
- Voltage rating
- Mounting considerations (might require circuit board, terminal strip or be bolted on)
Wiring diagrams
In the past, gliders have rarely had an electrical system complex enough to require a wiring diagram, but more complex gliders have more complex instrument (and other) systems. A wiring diagram is useful to help you, or others, fault find the glider later. Adherence to the latest drawing standards isn’t really required, although common symbols for batteries and switches make the diagram much easier to follow.
Schematic diagrams
- If you are going to do some significant changes to a glider electrical system, layout a basic outline of what you want to put in. If the aircraft has removable instrument panels or other parts include the disconnection points. A good part of the work can then be comfortably be done on a bench instead of awkwardly in the aircraft. It also makes future service easier, enabling devices to be unplugged for maintenance instead of removing individual wired connections.
- Make a list of the connectors and the required pins on each one.
- Draw a wiring diagram of what is there/what you will add. Include connector interfaces/terminals and data such as wire gauge, colour, connector types, breaker ratings, etc.
- Determine how much current will go through each device (switch, fuse/circuit breaker, connector, relay, diode, etc.) if it is significant. From this information, select wire sizes
- You can do this for the pneumatic system too!
