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Wiring Practices

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FAA Requirements

There are many misperceptions about requirements for replacing wire.  The FAA has made several of their resources available on line.  AC43.13-1B Chapter 11 makes recommendations regarding wire  inspection, substitution and replacement.  Paragraph 11-97 specifically  addresses wire replacement.  11-98 covers terminal replacement.  Manufacturer certification for aircraft wiring is available on PDF upon request.

FAA/Industry Resources

By the Book - CPA article by Mike Busch of Savvy Aircraft Maintenance 

FAR   43.13
FAR  43.16


FAA Airframe Handbook (See CHPT 9)

AC 23-27 - Obsolete Parts Guidance 

Crimp Guide

Solderless Terminals 

8130-3 required?  See paragraph 14.       

Electrical Grounding

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Some time ago I was called to a local airpark for a Piper Cherokee with alternator whine reported by ATC.  A leaking diode prompted an alternator overhaul.  The owner mentioned two recent starter replacements.  As I'm generally inspecting the engine bay I notice the absence of a ground cable between the engine and airframe.  Both the starter and alternator need a large ground path to the battery thru the airframe to function properly.  Older model Cessna and Piper wiring diagrams show component grounds, but are vague on specific locations and wire size.  If you don't have a dedicated ground cable equal size of the starter cable between the engine and airframe, you will have alternator and starter performance problems.  


172 Six Cylinder     172 Four Cylinder/182  


Engine mounts have large rubber isolators that make poor electrical conductors.  Cable housings may complete the circuit, but performance will be diminished and may pose a fire hazard if your entire electrical system attemps to ground thru one or two small cable housings.  We installed a heavy 4 AWG equivalent braid on the Piper between the engine and airframe side of the motor mount isolators.  The owner reported an obvious increase in starting power.


AC43.13-1B Chapter 11, Section 15 is dedicated to grounding and bonding.  Scroll down thru the gallery for cleaning and corrosion control.  

A common industry practice is to clean grounding surfaces with a stainless bonding brush and treat with an alodine stick.

Firewall Shields

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Cessna Type

Cessna used a number of different sized two piece shields for firewall feed thru.  Note the 172M IPC allows 3 different hole diameters to accommodate wire bundle sizes.  NAS557 split grommets are also inside the hole.  


Contact me if you damage this nylon grommet, I can send a replacement.


Aircraft Spruce sells shield kits and several types of firewall sealant.  Gallery displays customer supplied photos.

Wiring Security

Lacing Tape

Techflex TechLace nylon wax finished LT1-S3-FB-BK is my all purpose lacing tape.  I've tried synthetic rubber and it just doesn't lock knots like synthetic wax.  Size 3 isn't too bulky and doesn't cut into your hands like smaller tape.  Fiberglass LT4-S3-FD-NT works best in high temperature areas, but care must be taken during tying  because it breaks much easier than nylon.  

AC 43.13-1B Chapter 11, FIGURE 11-17

PLATO Model 170 diagonal cutters are excellent for cutting lacing tape (tie string) and zip ties from old harnesses.

RF Shields

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PPH voltage regulator harness shields are grounded by attaching the 1/4" ring terminal to a voltage regulator mount  bolt.  The FAA AC 43.13-1B Chapter 11 recommends the shields be  grounded on both ends.  Most avionics shops will recommend terminating all  shields at one end only.  Single point shield ground is required by all  audio panel manufacturers for shielded wires used in audio systems.  These  would be aural warning, headphone, music and entertainment wiring.   

CONNECTING SHIELD TO POWER THRU ANY MEANS WILL RESULT IN IRREPABLE DAMAGE.

I use Ideal 45-162 to strip tefzel insulation from shielded conductors.  The blade can be adjusted for depth to cut into the insulation without going  thru.  This prevents shield damage by the razor.  Then just bend  the conductor around to break the insulation.  A piece of rubber or emery cloth helps to pull the insulation free.  Any razor will work, just be careful not to damage the shield while rolling the blade around the  insulation.  Should you choose to ground the other end of my harness, read the shield pigtail link provided above.  I fold the shield over the outer insulation as an extra measure of protection.  Then solder a wire to a piece of exposed shield and protect the joint with heat shrink.  Ground the other end of that wire with a quality ring terminal.  TYCO solder sleeves require a heat gun capable of melting solder.

AC 43.13-1B Chapter 11

11-89. SHIELDED WIRE  -  Shielded wire or cable is  typically connected to the aircraft ground at both ends of the wire, or at connectors in the cable.

11-98. TERMINALS  AND TERMINAL BLOCKS -  g. Shielding should be dead-ended with suitable insulated  terminals.


11-106.  ELECTROMAGNETIC INTERFERENCE (EMI) -  Use of shielding with 85 percent coverage or greater is recommended. Coaxial,  triaxial, twinaxial, or quadraxial cables should be used, wherever  appropriate, with their shields connected to ground at a single point or  multiple points, depending upon the purpose of the shielding.

Avidyne AMX 240 Installation Manual


2.4.1  Electrical Noise - Ground loop noise occurs when there are two or more ground paths for the same signal (i.e., airframe and ground return wire).   Large cyclic loads such as strobes, inverters, etc., can inject noise signals onto the airframe that are detected by the audio system.  The shields can be daisy-chained together, and then connected to the ground lugs mounted on the back plate shown in Appendix B.

AMX240 Connector, J1 (Sub-D  44-pin, male on tray) - All shields should be grounded as shown, at signal source only.  Other end remains floating(refers to audio wire  only).

Noise Tips

Alternator manufacturers have FAQ pages to assist customers with charging and noise issues.  They're worth a read.  The following are known sources of noise.


  • Alternator Whine - Rectifier diodes should not leak more than .2 VAC.
  • Magneto Static - Ungrounded P lead shield; bad capacitor, filter and/or spark plug noise suppression resistor.  Switch between mags.
  • Strobes, blower motors and low grade phone chargers are noise generators.
  • GROUNDS - noise suppressors such as filters and shields need good contact with ground!
  • Please feel free to share your own experiences.

Stripping/crimping

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IDEAL 45-635 Custom Stripmaster Lite pictured below is a prefered wire stripper for aviation.  Die type blades have precisely machined counterbores with an inner ridge for stripping the insulation.  Lower cost knife type blades can contact and damage the conductor.  45-635 is for thick wall Type EE insulation.  Note the small gap around the standard wall Type E M22759/16 wire in the die.  This combination strips well and puts less pressure on the expensive blades compared with Type E stripper 45-2133.  Custom Lite frames are smaller and require less hand pressure than full size Custom frames.


Pre-insulated(PIDG) and uninsulated terminals are best crimped with a ratcheting type crimper as  recommended in AC 43.13-1B Chapter 11, Para. 178.  The 3 chamber crimper below is TYCO 58433-3, an affordable field grade crimper for AMP PIDG splices and terminals.  This crimper belongs in every A&P tool box and crimps terminals for 10- 22 awg wire.  The "T" head dual chamber 59250 is a high cycle manufacturing grade crimper. 

Solenoids

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Hartzell Engine Technologies, parent of Sky-Tec, informed me of an inherited problem within Sky-Tec master solenoids shortly after acquisition.  The contactor alloy corrodes and will not carry a load.  Voltmeter will indicate correct voltage across with no current output.  All solenoid production was permanently ceased.  By SEP of 2018 Lamar purchased the Sky-Tec solenoid PMA's and product.  According Jim Errington at LAMAR, Sky-Tec starter solenoids operate as designed and will continue being produced until original parts are depleted, then replaced with LAMAR models.  The easiest way to tell the difference between Sky-Tec and LAMAR is the numbercoil posts with 10-32 threads.  Sky-Tec solenoids have a single coil post and LAMAR two coil posts.  


Read Service Bulletin 72-A.

  

Post hardware can limit available real estate when multiple cable lugs are installed.  Sky-Tec informed me replacing the thicker anchor nut with a thinner AN316-5 nut and lock washer MS35333-41 is acceptable, be careful not to allow the post to slip inside the solenoid case.  Silicon boots can be tough to install sanitarily.  Below is a battery solenoid using boots with 2-3 conductors per post.  Running every wire for a single post thru the boot allows complete encapsulation of the post and terminals.  


Jim, at LAMAR, also offered a few tips for solenoid installation.  Mounting orientation can be any direction.  Coil energizing from either direction.  OVER torqueing will damage the internal nylon post insulator.  Use a wrench to back up the inside nut to prevent internal damage from post rotation during removal and installation of lugs.  Seal all seams and posts with an aviation grade RTV to prevent moisture contamination.