Flight control cables transmit pilot inputs to control surfaces — ailerons, elevators, rudders, and trim tabs — with precise, reliable force. Because a cable failure or an improperly secured turnbuckle can directly cause loss of aircraft control, the FAA places these components among the most scrutinized items during airframe inspection and maintenance. Every Aviation Maintenance Technician (AMT) working on airframes must understand not only what to look for, but why each defect matters and exactly how to correct it in accordance with FAA-accepted standards.
This article walks through cable construction and types, inspection criteria, turnbuckle function and adjustment, cable tension measurement, and the locking and safety requirements that must be satisfied before returning a flight control system to service. All guidance is grounded in the FAA Aviation Maintenance Handbook — Airframe (FAA-H-8083-31) and relevant 14 CFR Part 43 requirements.
Cable Construction and Types
Aircraft control cables are manufactured from carbon steel or corrosion-resistant steel (CRES) wire strands twisted together in a specific pattern. The two most common constructions found in general aviation are 7×7 and 7×19. A 7×7 cable contains seven strands, each made of seven individual wires, for a total of 49 wires. It is relatively stiff and is typically used for trim controls and in situations where little bending over pulleys is required. A 7×19 cable has seven strands of nineteen wires each (133 wires total), making it significantly more flexible and better suited for primary flight controls routed over pulleys and through fairleads.
Cable diameter is specified by the aircraft manufacturer and must be matched exactly during replacement. Common sizes range from 1/16 inch to 3/8 inch. Using an undersized cable — even by one nominal size — can result in a cable that stretches excessively under load, fails at a lower breaking strength, or jams in a pulley groove designed for a larger diameter.
Inspection Criteria and Defects
Thorough cable inspection requires good lighting, clean cables (free of accumulated grease or dirt that hides damage), and systematic coverage of the entire cable run from end fitting to end fitting. FAA-H-8083-31 identifies the following defects as cause for cable rejection or replacement:
- Broken wires: Any broken wire in a 7×7 cable is cause for rejection. In a 7×19 cable, broken-wire rejection criteria are established by the aircraft manufacturer's maintenance or structural repair manual — there is no single universal FAA numeric limit; always consult the applicable manual for the specific broken-wire threshold. Broken wires are often found at pulleys, fairleads, and swaged terminal fittings — points of highest flex fatigue.
- Corrosion: Surface rust on carbon steel cables can be cleaned with a dry cloth or fine steel wool in some cases, but pitting corrosion — surface corrosion that has eaten into the wire — is cause for immediate rejection. Internal corrosion discovered by untwisting or separating strands is also rejectable and may not be visible from the outside.
- Kinking and bird-caging: A kinked cable has a permanent deformation caused by being bent sharply or having a loop tighten under load. Bird-caging refers to a bulging or unraveling of outer strands caused by a sudden shock load or reverse bend. Both conditions permanently weaken the cable and require replacement.
- Wear and abrasion: Cables that contact structure due to misaligned fairleads or worn pulley grooves develop flat wear spots. If the wear reduces a wire's diameter by more than a small fraction, the cable must be replaced.
- Swaged terminal integrity: Swaged (mechanically compressed) end fittings must be inspected for cracks in the swage sleeve, for evidence that the cable is pulling out of the fitting, and for corrosion at the cable-to-fitting junction. A pull test or proof load check may be required after new swaging.
Pulley, Fairlead, and Hardware Inspection
Cables do not exist in isolation. Each pulley must be inspected for bearing roughness (which causes uneven cable wear), groove depth and shape (a worn groove allows the cable to ride up and chafe against pulley flanges), and proper alignment with the cable path. Pulleys should turn freely but without excessive side play. Cracked pulley flanges are an immediate rejection item.
Fairleads — usually made of phenolic plastic or aluminum — guide cables through structure openings. They must contact only the side of the cable (not wrap around more than a few degrees of arc) and must not cause the cable to change direction. FAA guidance specifies that a fairlead should not deflect a cable more than approximately 3 degrees from its straight-line path. Greater angularity causes side-loading that wears both the cable and the fairlead itself.
Turnbuckle Function and Components
A turnbuckle is an adjustable mechanical device inserted into a cable run to allow precise tensioning of the cable and to take up slack caused by temperature-induced expansion or contraction, structural settling, or replacement components. A standard turnbuckle assembly consists of three parts: two terminal ends (one with right-hand threads, one with left-hand threads) and a barrel with corresponding internal threads at each end. Rotating the barrel draws the two terminal ends toward each other, increasing cable tension; rotating it the opposite direction releases tension.
Terminal ends come in several styles: fork end, eye end, pin end, and swaged ball end — each chosen based on the attachment point geometry. The barrel is typically made from brass or aluminum alloy with a distinctive oblong cross-section so a wrench or turnbuckle tool can grip it without damaging threads.
Cable Tensioning Procedure
Before adjusting tension, the aircraft must be in a standard condition: control surfaces at neutral, aircraft leveled per the manufacturer's specifications, and temperature noted. Tension changes with temperature because the aluminum airframe expands and contracts at a different rate than steel cables; manufacturers publish tension values at specific temperature ranges or provide correction charts.
Tension is measured with a cable tensiometer — a tool that applies a known perpendicular load at the center of a short cable span through a riser or plunger. The cable's deflection under that load indicates tension on a calibrated scale. Different riser sizes are used for different cable diameters; using the wrong riser gives a false reading. Always zero the tensiometer before use and verify its calibration with a known standard if available.
Tension is adjusted by rotating the turnbuckle barrel. When the target tension is reached, check that at least three threads of each terminal end are visible outside the barrel (i.e., the terminals are not screwed in too deeply) but that no more than three threads are exposed inside the barrel (meaning threads are still fully engaged). Manufacturer manuals specify exact thread engagement minimums — always defer to them.
Locking the Turnbuckle
This is one of the most tested and most critical steps. An unlocked turnbuckle can rotate in service, changing cable tension or even unscrewing entirely. FAA-H-8083-31 describes two accepted locking methods:
- Safety wire (clip locking method): Two lengths of 0.040-inch (or manufacturer-specified) diameter safety wire are threaded through the hole in the turnbuckle barrel and wrapped around each terminal end in opposite directions, with each wire making at least four wraps around the terminal shank before being bent back through the barrel hole and twisted off. The wraps must be tight and the wire ends bent so they cannot snag on adjacent components.
- Clip-type locking devices: Pre-formed locking clips (sometimes called MS21256 or similar standard hardware clips) are inserted through the holes in the barrel and around the terminal ends without any wrapping required. These are single-use devices — they must never be reused once removed.
In either case, the lock must prevent rotation of both the barrel and the terminal ends. After locking, attempt to rotate the barrel by hand to verify the lock is effective before returning the system to service.
Key Numbers and Rules
- 7×19 cable rejection: broken-wire limits are specified by the applicable aircraft manufacturer's manual — no single universal FAA numeric standard applies.
- 7×7 cable rejection: any broken wire.
- Fairlead maximum cable deflection: approximately 3 degrees.
- Turnbuckle thread exposure: at least 3 threads visible outside each end of the barrel after tensioning (confirms terminal is not over-inserted).
- Safety wire gauge for turnbuckles: typically 0.040-inch diameter stainless steel safety wire.
- Clip-type locking devices are single-use only; never reuse.
- All flight control rigging and inspection must be documented per 14 CFR Part 43.9 maintenance record requirements.
Common Test Traps
- Confusing cable types: Exam questions often swap 7×7 and 7×19 applications. Remember: 7×19 is for primary flight controls over pulleys (most flexible); 7×7 is for trim and low-flex runs.
- Broken wire limits are manual-specific: There is no single universal FAA numeric broken-wire limit for 7×19 cable — always consult the applicable manufacturer's maintenance or structural repair manual for the exact rejection criteria.
- Thread exposure means threads outside the barrel: It is easy to confuse this with threads inside the barrel. The rule ensures enough thread engagement remains while confirming the terminal has not been threaded past the barrel's center.
- Locking clips cannot be reused: A question may imply that a clip removed during inspection can be reinstalled. It cannot — install a new clip every time.
- Tensiometer riser size matters: Using the wrong riser for the cable diameter gives an incorrect tension reading. Always match riser to cable diameter per the tensiometer manufacturer's instructions.