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Electrical Wiring Interconnect Systems
There are many misperceptions about requirements for wire repair and replacement. 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. Look thru the EWIS training aid for excellent wiring practice recommendations.
PPH can provide manufacturer certification for aircraft wiring upon request.
FAA/Industry Resources
FAA Airframe Handbook (See CHPT 9)
AC 23-27 - Obsolete Parts Guidance
8130-3 required? See paragraph 14.
Electrical Grounding
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. While 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 bonding brush and treat with an alodine stick.
Cessna/Whelen White Connectors
Mate-N-Lok Commercial Series
White connectors used by Whelen and early Cessna models are TYCO Mate-N-Lok Commercial series with single locking lance contacts. Many Cessna models have two 15 position connectors behind the pilots side instrument panel. These exist to aid mass production on an assembly line. When I build harnesses that replace any of the wiring thru these connectors, such as "P" leads or voltage regulator power, I bypass the connector unless instructed otherwise by the customer. If you pay for the benefit of modern shielded wire, why break the shield to comply with 40-50 year old mass production techniques? Scroll to the bottom of my Services page for an example of a 15 position connector damaged from a short circuit.
15 Position Plug - 1-480323-0
Socket 14-20 AWG - 60619-1
15 Position Receptacle - 1-480324-0
Pin 14-20 AWG - 60620-1
Crimper I recommend is 90575-1.
Extractors are 1-305183-1-W for pins, 1-305183-2 for sockets or 465644-1 for both.
Commercial series are found throughout General Aviation and routinely mistaken for Molex brand.
Firewall Shields
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.
RF Shields
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 1 VAC. Jeff at Aircraft Accessories in Tulsa regarding acceptable limit of AC allowed from properly functioning alternator diodes, "The battery can absorb a lot of Alternating current, to answer how much is allowed, one half volt (.5) would be the most I would LIKE to see on the bus, however one volt maximum could be present with all alternator diodes being “good”. This reading would be taken with alternator running at full rated amperage. The different stator windings of different alternators ,specifically “WYE” (a WYE winding can run 1.5 volts ac leakage) or “DELTA” will effect your reading as well as the number of diode pairs. In short 1 Volt AC."
- 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!
Stripping/crimping
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
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.
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 internal moisture contamination.
PPH customers receive Aircraft Spruce dealer pricing all solenoids!
