12.4 High-Tension Ignition Cables & Coaxial Transmission Lines
Key Takeaways
- A magneto delivers high voltage at low energy, while a turbine capacitor-discharge exciter delivers high voltage at lethal energy, so the handling precautions differ.
- The ignition harness metallic screen suppresses radio interference and contains corona and altitude flashover; it is not there for mechanical protection.
- Turbine ignition work requires the manual's wait period after power-down, disconnection of the exciter input lead first, then grounding the igniter lead centre electrode to the engine.
- Aircraft radio coaxial cable is 50 ohms; 75 ohms is video. Crushing, kinking or bending tighter than 6 times the outside diameter permanently changes impedance and raises VSWR.
- Low-frequency screened signal cables are earthed at one end only to avoid ground loops, and the braid pigtail is kept as short as possible.
The Missing Half of 6.11
Sub-module 6.11 Electrical cables and connectors (level 1 for A, level 2 for B1, B3, B2 and B2L) lists four bullets, and the second is:
"High-tension and coaxial cables."
Sections 12.1 to 12.3 covered general cable types, construction and characteristics, bundling and routing, crimping and connectors — and coaxial cable appeared there as a routing and bend-radius case. This section deals with the two special cable families in their own right: the high-tension (HT) ignition cable, which is the highest-voltage conductor on the aircraft, and the coaxial transmission line, which is the most impedance-sensitive.
They are grouped in one syllabus bullet for a reason: both are screened (shielded) cables whose screen is functional, not incidental. On an HT lead the braid contains radiated interference; on a coaxial line the braid is the return conductor and defines the characteristic impedance.
High-Tension Ignition Cable
Where the voltage comes from
| System | Source | Typical secondary output |
|---|---|---|
| Piston engine magneto | Rotating magnet and step-up coil, triggered by contact breaker or electronic module | Rises to whatever voltage is needed to jump the plug gap — commonly 15,000–20,000 V, higher with a fouled or worn plug |
| Turbine engine ignition | Capacitor-discharge exciter box fed with 28 V DC or 115 V AC | Very high voltage at very high energy — the exciter dumps a stored capacitor charge into the igniter in a single pulse |
The distinction matters: a magneto delivers high voltage at low energy; a turbine exciter delivers high voltage at lethal energy. The maintenance precautions differ accordingly.
Construction
An HT lead is not simply a thick wire:
| Layer | Purpose |
|---|---|
| Conductor | Stranded copper, or a resistive/ferrite core in suppressed leads. The core is often deliberately resistive to damp the discharge and reduce radiated interference |
| Primary insulation | Silicone rubber or PTFE, chosen for very high dielectric strength and for continuous service at engine-compartment temperature |
| Metallic screen (braid) | Tinned copper or stainless braid that surrounds the whole lead |
| Outer jacket / glass braid | Mechanical, thermal and abrasion protection |
| End hardware | Threaded terminal nuts (the classic all-weather spark-plug lead uses a 5/8-24 threaded coupling), ferrules, insulating sleeves and elbows |
Why the screen exists
- Radio interference. An unscreened ignition discharge is a broadband radio transmitter. Without a screened harness, VHF communications, ADF and navigation receivers are swamped by ignition noise every time a plug fires. The braid conducts that interference to airframe earth instead of radiating it.
- Flashover control. Air is a poorer insulator at low density. As the aircraft climbs, the same voltage becomes far more likely to arc from the lead to the nearest earth — altitude flashover. HT insulation is specified for the aircraft's maximum operating altitude, and the screen keeps the field contained.
- Corona. Sustained partial discharge inside a lead progressively erodes the insulation and produces ozone and carbon; carbon tracking then provides a conductive path and the lead fails completely.
Maintenance and inspection
- Cleanliness is airworthiness. Oil, dirt and moisture on a terminal or an insulator create a leakage path. Terminals and ceramic insulators are cleaned and dried before reassembly.
- Carbon tracking. Look for thin black lines on insulators and inside terminal sleeves. Any tracking condemns the component — it cannot be cleaned off, because the carbon has been driven into the surface.
- Bend radius and routing. HT leads are supported clear of exhaust stacks and moving parts, and are never bent tighter than the manufacturer's minimum radius; a sharp bend cracks the insulation internally where nothing can be seen.
- Testing. Continuity and, critically, high-voltage insulation resistance testing with a purpose-built harness tester that applies a voltage of the same order as the system's working voltage. A standard 500 V insulation tester will pass a lead that flashes over at 15 kV.
- Torque the coupling nuts by hand tools only, and never use the lead as a handle to pull the plug elbow.
Turbine ignition safety — the highest-risk task in this module
A turbine exciter stores enough energy in its capacitor to be lethal even after the aircraft is powered down. The procedure is fixed:
- Switch off and isolate the ignition system, and placard it.
- Wait the period specified by the engine maintenance manual before touching any ignition component. Exciters normally contain a bleed-down resistor, but the published wait is typically a minimum of about three minutes, and some manufacturers (Pratt & Whitney Canada, for example) require the system to have been inoperative for at least six minutes.
- Disconnect the exciter input lead first.
- Only then disconnect the igniter lead, and ground the igniter lead centre electrode to the engine case to bleed off any residual charge.
- Use insulated tools on the cable coupling nuts, and handle the lead by its hardware.
Reversing steps 3 and 4, or skipping the wait, is a recognised fatal-accident mechanism.
Coaxial and Screened Signal Cables
Construction and why impedance matters
A coaxial cable is a concentric transmission line: centre conductor → dielectric → braided (and sometimes foil) screen → outer jacket. The ratio of the screen's inner diameter to the conductor's outer diameter, together with the dielectric constant, fixes the cable's characteristic impedance — the value the cable presents to a signal regardless of its length.
| Impedance | Use |
|---|---|
| 50 Ω | Virtually all aircraft radio: VHF/HF comms, VOR/ILS, DME, transponder, TCAS, GPS, radio altimeter |
| 75 Ω | Video and some data distribution |
Because the impedance is set by geometry, anything that deforms the cable changes its impedance: a crushed section, a kink, a bend tighter than the minimum radius, or a badly assembled connector. The result is a reflected wave and a rise in voltage standing wave ratio (VSWR). A high VSWR means transmitted power is returning to the transceiver instead of reaching the antenna — the symptom is weak transmission, and in severe cases a damaged output stage.
Other characteristics you should be able to state:
- Attenuation rises with frequency and with cable length. A run acceptable for a VHF comm may be unusable at transponder or GPS frequencies, which is why installations specify a cable type and a maximum run length.
- Velocity factor — the signal travels slower than in free space, which matters when cable length is used for timing or phasing.
- Screen effectiveness — a single braid may give around 90 % coverage; double-braid and braid-plus-foil constructions are specified where interference is critical. Common aerospace types include RG-142 and RG-393 (PTFE, double-screened, high temperature) in place of the older PVC-jacketed RG-58.
Assembly and installation
- Strip dimensions are dimensional, not approximate. Each connector has a published strip length for the jacket, braid and dielectric. Nick the centre conductor and you create a stress riser and a resistance change; leave a stray braid whisker across the dielectric and you short the line.
- Use the correct crimp tool and die set for the connector's contact and ferrule, with the same go/no-go calibration discipline described in Section 12.3.
- Minimum bend radius for coaxial cable is 6 times the outside diameter — tighter bends permanently deform the dielectric and shift the impedance.
- Moisture is the enemy. Water wicking into a braid raises attenuation and corrodes the screen. Connectors in unpressurised or exposed areas are weather-sealed, and drip loops are provided as described in Section 12.2.
- Connector families: BNC (bayonet, low frequency, general avionics), TNC (threaded BNC — preferred where vibration is present), N-type (larger, higher power and frequency), SMA (miniature, high frequency).
Screening low-tension signal cables
The same principle protects sensitive low-level signals:
- Twisted pair cancels magnetically induced noise; twisted and screened pair adds electrostatic protection.
- Triaxial cable adds a second, isolated outer screen so that screen currents do not flow in the signal return.
- Screen termination discipline is what makes or breaks the installation: screens are normally bonded to earth at one end only for low-frequency signals, to avoid a ground loop in which a circulating current between two different earth potentials injects noise into the very circuit the screen was meant to protect. The pigtail connecting the braid to the earth point is kept as short as possible, because a long pigtail is an inductor that defeats the screen at high frequency.
Exam Traps
- High tension = high voltage; high energy = the turbine exciter. A magneto can hurt you; an exciter can kill you. Questions on ignition safety are always about the capacitor-discharge system.
- The wait period before handling turbine ignition components is specified by the maintenance manual — quoting a universal figure is the wrong answer; the correct behaviour is to apply the manual's figure, disconnect the exciter input first, then ground the igniter lead.
- The ignition harness screen is there for radio interference suppression and flashover containment, not for mechanical protection — the outer braid or jacket does that job.
- 50 Ω is the aircraft radio standard, 75 Ω is video. Fitting a 75 Ω cable to a comm antenna produces a mismatch and a high VSWR.
- Coaxial minimum bend radius is 6 × OD; deformation changes impedance permanently, so a kinked coax is replaced, never straightened and reused.
- Screens are earthed at one end for low-frequency signal cables to avoid ground loops; "earth both ends for a better screen" is the distractor.
Why is the ignition harness of a piston engine fitted with a metallic braided screen over each lead?
Which sequence is correct before removing an igniter plug from a turbine engine?
What is the characteristic impedance of the coaxial cable used for aircraft VHF communication, VOR/ILS, DME and transponder installations?
Why is the screen of a low-frequency screened signal cable normally bonded to earth at one end only?
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