Aerodynamics 2.3 p1-31: Mechanical Flight Control System

Mechanical Flight Control System Sections

• A flight control system contains three main sections.
• These sections are the input/handling device, the transmitting system/device, and the control surface.

Input/Handling Device

• Control columns or wheels serve the purpose of moving the ailerons and elevators.
• They operate within the lateral and longitudinal axes and are common in light aircraft.
• Pedals control the yaw axis and have two or three functions.
• These functions include rudder control, landing gear brake actuation, and nose wheel steering.

Additional Input Devices

• Main control input assemblies have additional components.
• These components include trim wheels for all three control directions.
• Flap levers are also included.
• Electrical switches are present for alternate control of trim motors and automatic controls.
• Secondary control inputs on large aircraft are located at the center pedestal and the trim and autopilot switches at the control wheel.
• The flap and spoiler levers are a part of the additional components on large aircraft.

Transmitting Systems/Devices

• Transmitting systems consist of control cables, cable drums, pulleys, turnbuckles, tension regulators, and cable quadrants.
• Control rods, levers, and cranks also constitute the system.
• Complete control servo systems with hydraulic power supply and valves are included.

Control Surface

• Early aircraft generations used conventional mechanical controls to move control surfaces.
• They consist of cables, chains, and torque tubes.
• The control surface is operated by a conventional control column, control wheel, pedal, and lever.
• Operation is instinctive to the pilot.

Control System Components - Cables

• Aircraft control cables are available in both Corrosion Resistant Steel (CRES) and Carbon Steel.
• CRES cables are more expensive and have lower strength compared to carbon steel, but are more appropriate when corrosion is a factor.

Cable Types

• There are three main types of cable: non-flexible, flexible, and extra-flexible.
• Non-flexible cables are 1X7 or 1X19.
• Flexible cables are 7X7.
• Extra-flexible cables are 7X19.
• The numbers indicate the number of bundles or strands and the number of wires in each strand.

Cable Applications

• Non-flexible cables are used for straight runs where the cable does not run over pulleys.
• Used often as a push-pull cable for engine controls or environmental controls.
• These cables are often encased in an outer sheath, similar to a bicycle cable.
• Flexible cables are suitable for straight runs with large pulleys if extreme flexing is not required.
• Extra-flexible cables are used for primary flight control systems.
• The minimum cable size for primary control systems is 1/8 inch.

Cable Terminals

• Swage-type terminals are manufactured according to Army-Navy (AN) and Military Standards (MS).
• Swage-type terminals are suitable for civil aircraft up to maximum cable loads.
• Instructions for swaging tools, including 'go' and 'no-go' dimensions, are followed.
• Doing so exactly avoids defective swaging.
• When using the gauge, the opening must be the minimum size and the other the maximum.

Cable End and Splice

• The Nicopress cable termination using copper sleeves can be used to the full rated strength of the cable when looped around a thimble.
• Tools, instructions, and data supplied by the manufacturer must be followed when using the Nicopress process for cable splicing.
• Doing so ensures the desired cable function and strength.
• Nicopress and cable-cutting instructions are within the AS-70-3 Training Board.
• The cable used with training aids measures 1/16" in diameter and requires a single swage.
• The order of the 3 swaged nicopress points is standard for all other sizes.

Cable Inspection and Servicing

• Always refer to the Aircraft Maintenance Manual (AMM) for specific criteria.
• Inspections occur during annual inspections or specific intervals set by the aircraft manufacturer.
• Inspection intervals may need to increase due to operating environments (saltwater).
• Critical fatigue areas are where the cable runs over, under, or around a pulley, sleeve, or fairlead.
• Critical fatigue areas include sections where the cable is flexed, rubbed, or worked in any manner, or within 1 foot of a swaged fitting.

Cable Inspection Continued

• Check control cables for condition, attachment, alignment, clearance, and proper operation.
• Replace cables that have three or more broken strands along any three-foot segment, or evidence of corrosion anywhere on the cable.
• Any cable that has one broken wire strand in a critical fatigue area is replaced per AC43.13-1B.
• Inspect swage fittings for slippage e.g., paint marks, and broken wires at the swage from the swaging process per AC43.13-1B.
• Replace flexible/non-flexible cables when individual wires in each strand appear to blend, or if outer wires are worn 40-50% per AC43.13-1B.
• Avoid kinking cables, because a kinked cable cannot be fixed.
• Replace any cable that has a popped core or loose strands.
• Inspect cables in areas conducive to corrosion like battery compartments, lavatories, and wheel wells.
• Relieve tension and reverse-twist the cable to inspect inside if there is surface corrosion.
• If no internal corrosion exists, remove loose external rust/corrosion, using a fiber brush, not steel wool or solvents.
• Using steel wool will embed dissimilar metal particles in the cable, creating corrosion problems.
• Solvents will remove internal cable lubricant, which can cause cable strands to abrade.
• External surface wear indicates wear is also occurring internally, where cables pass over pulleys and bell cranks due to the relative motion between inner wire surfaces.
• After cable cleaning apply MIL-C-16173 grade 4 corrosion-preventive compound sparingly.

Cable Inspection Continued - Additional Checks

• Examine cable runs for incorrect routing, fraying, twisting or wear at fairleads, pulleys, anti-abrasion strips and guards.
• Check for interference with adjacent structure, equipment, wiring, plumbing, and other controls.
• Use a mirror to check difficult areas.
• Inspect front to back of the aircraft.
• Use another person to move controls while inspecting around pulleys.
• Use a string method when removing cables.
• Ensure all cable guides are reinstalled after cable replacement, and cables are not crossed anywhere.

Cable Tensioning

• Use the correct riser for the cable, typically risers #1, #2, and #3 are included for specific cable diameters.
• The British use different sizes of cables, so ensure CWT risers are used when necessary.
• Use a gauge or Vernier calipers if unsure of cable size.
• Do not take readings too close to an attachment point or a pulley.
• Take multiple readings and average them for long cables.

Cable Tensiometer Use

• Identify a suitable location to place the tension meter to measure cable tension.
• Raise the lever and hang the tension meter on the cable.
• Lower the lever gently until the cable is held by the tension meter.
• Take the reading displayed on the dial.
• Consult the chart provided by the manufacturer to find the tension corresponding to the cable diameter.
• If the cable diameter is unknown, use the cable diameter gauge located in the cable tension meter case.

Tensiometer Chart Use

• Tensiometers are measuring instruments that must be calibrated on a regular basis as per company approval manual.
• Because there are three different size risers for the different diameter cables, each diameter of cable will get a different reading for the same tension
• The chart reading corresponds to cable diameter (and respective riser) and the column.
• Read for actual tension based on the position relative to riser #1.
• Horizontally cross-reference the reading the tension column.
• If a value of 34 occurs using riser #1 on a 1/8" diameter cable, the tension is 35lbs.

Control System Components - Turnbuckles

• Turnbuckles are used to adjust cable tension.
• Consist of an externally threaded cable end and an internally threaded turnbuckle barrel.
• Each turnbuckle includes right and left-hand threads that are identifiable by a groove.
• A turnbuckle can be secured using wire-locking or a locking clip for safety.
• Brass is typically the metal of choice when making turnbuckles.
• Brass turnbuckles used to join two cables, allowing cables to be tensioned to the correct value.
• To reach its full strength, a turnbuckle has a maximum of three threads exposed at either end of its barrel.
• Turnbuckle barrels are available in two lengths.
• If there are too many threads showing, a longer barrel must be installed.

Control System Components - Locking Clips

• Anti-twist protection for modern turnbuckles exists in the form of clips.
• The threads on both the cable end and turnbuckle must have splines or a horizontal groove cut lengthwise.
• The design allows the parts and locking clip to interface, which allows for ultimate protection against twisting.

Control System Components - Locking Wire

• Older-style turnbuckle protection exists in the form of a lockwire.
• The system is available in both single and double-wire configurations.
• This design does not guarantee protection against twisting a little until the wire tightens.

Turnbuckle Inspection

• Inspect turnbuckles for proper thread exposure and cracks.
• Verify no safety wires or clips are missing or broken.
• For details on how to safety wire and use clips, refer to AC 43.13-1B pages 7-43 through 7-48.

Control System Components - Cable Pulleys

• Pulleys are made of Phenolic, phermica, aluminum or plastic.
• Phenolic is the most common material, with a ball or roller bearing pressed into the center.
• These are used to change the direction of a cable run or the length of a cable and to prevent cable vibration.
• They typically have guards to prevent the cable from falling out of the groove when slack.
• Pulleys have to be free to rotate throughout their full travel.

Pulley Inspection

• Inspect pulleys for roughness, sharp edges, and any foreign material embedded in the grooves.
• Examine pulley bearings for proper lubrication, smooth rotation, and freedom from flat spots, dirt, and paint spray.
• Rotate pulleys to provide a new bearing surface for the cable.

Pulley Inspection - Continued

• Check pulleys for wear, such as misalignment, excessive cable tension depressions, misalignment, and incorrect pulley size or flat spots.
• Inspect pulley brackets and guards for damage, alignment, and security.

Control System Components - Bellcranks

• A bell crank changes motion through an angle.
• The angle can be any angle between 0 and 360 degrees.
• 90 degrees 180 degrees are more common.
• Pulling or pushing one arm rotates the bar around the pivot point, pulling or pushing on the other arm.

Control System Components - Cable Quadrant

• A cable quadrant is like a flat cable drum which only allows a few degrees of rotation.
• It has a similar operation to a two-armed lever with a constant lever arm.