Modern aircraft span a broad spectrum of flight control philosophy, from purely mechanical systems where cables and pushrods move surfaces directly, to fully digital fly-by-wire (FBW) architectures where pilot inputs are interpreted by computers before being sent as electrical commands to actuators. Between those extremes sit mechanical mixing units — gearboxes, bellcrank assemblies, or torque-tube arrangements that blend inputs from multiple controls before transmitting them to the flight surfaces. Regardless of the technology involved, the goal of a rigging check remains the same: confirm that every surface moves the correct amount, in the correct direction, with the correct feel, and within the limits the type certificate requires. For an Airframe AMT, understanding the distinctions between rigging a conventional system and rigging a FBW or mixing-unit system is essential both for shop practice and for the FAA knowledge test.
This article walks through the principles behind each system type, the specific steps and tools used during rigging checks, the tolerances and limits you must verify, and the common mistakes that create failed tests or, worse, unsafe aircraft.
Mechanical Mixing Units: What They Are and How They Work
A mechanical mixing unit is a device — often found in helicopters and some multi-engine aircraft — that combines pilot inputs from two or more controls and outputs a blended mechanical signal. The classic example is the helicopter's mixing unit, which accepts collective and cyclic inputs and sums them so that changes in one axis do not inadvertently disturb the other. In fixed-wing aircraft, mixing units appear in differential aileron systems and in aircraft where flap deployment automatically adjusts stabilizer trim through a mechanical link.
Inside a mixing unit you typically find a series of levers, bellcranks, and sliding spools connected by pivot pins and bearings. The geometry of these connections determines the mix ratio — how much input from one channel produces how much output on a given surface. Because that ratio is set by physical dimensions (arm lengths, pivot positions), it can change if any component wears, bends, or is reassembled incorrectly. A rigging check of a mechanical mixing unit must therefore verify not just travel limits but the linearity and ratio accuracy across the full range of motion.
Fly-By-Wire Systems: Architecture and Rigging Challenges
In a fly-by-wire system, the pilot's inceptor (sidestick or yoke) generates an electrical signal — analog voltage or digital data — rather than a direct mechanical force. Flight control computers (FCCs) receive that signal, apply control laws (which may include envelope protection, load-factor limiting, and gust damping), and then send commands to electrohydraulic or electromechanical actuators (EHAs or EMAs) at each surface. Some FBW designs retain a mechanical backup reversion mode; others are full-authority with no mechanical path.
Rigging a FBW system differs fundamentally from rigging a cable system because many of the adjustments are software parameters rather than physical hardware positions. However, hardware rigging is still critical for the following elements:
- Inceptor neutral position: The sidestick or column must be physically centered and that position must correspond to the electrical null (zero-volt or zero-digital-count output) of the position transducer (LVDT or RVDT). An off-center null causes a constant trim offset that the FCC must fight.
- Actuator mechanical stops: Even in a FBW aircraft, each actuator has hard mechanical end stops that define the absolute travel limits. These are set and verified during rigging and must match the aircraft maintenance manual (AMM) values.
- Feedback transducer rigging: The surface-position LVDTs (linear variable differential transformers) must be zeroed with the surface at the aerodynamic neutral position. If they are rigged incorrectly, the FCC's closed-loop control will hold the surface at the wrong angle.
- Autopilot and feel-force calibration: Artificial feel units (springs, Q-feel simulators) must be set so that control forces at defined airspeeds fall within certificated limits.
Performing the Rigging Check: Step-by-Step Approach
1. Documentation and Preparation
Always begin with the current, approved AMM for the specific aircraft model and serial number. Rigging tolerances are not generic — they are model-specific and sometimes serial-number-specific due to service bulletin incorporations. Gather the required special tools: rigging pins (which lock controls in the neutral or full-travel position for measurement), inclinometers or digital protractors, tension meters for cable systems that feed into mixing units, and — for FBW systems — the aircraft's maintenance computer or BITE (built-in test equipment) terminal for reading transducer outputs and actuator positions in engineering units.
2. Setting the Neutral Reference
Install rigging pins or fixtures at all designated neutral-lock positions simultaneously. On a helicopter mixing unit, this means locking the collective at a defined mid-travel point and the cyclic at geometric center before measuring any output linkage. On a FBW aircraft, set the inceptor to the mechanical neutral detent, then confirm the cockpit display or maintenance terminal shows the transducer output at the specified null value (often 0 ± a small tolerance in millivolts or digital counts stated in the AMM).
3. Measuring Surface Travel
With the neutral reference established, move each control to its full-up (or full-forward) and full-down (or full-aft) stops, one axis at a time. Measure the surface deflection with an inclinometer placed on the surface skin at the designated measurement station. Compare the measured angle against the AMM table. Typical tolerances are ±1° to ±2° from the specified travel value, but always use the AMM figure — never assume a generic tolerance.
For a mechanical mixing unit, also check that inputs from one axis do not cross-couple into an unintended surface. For example, moving the collective fully up in a helicopter should not produce a measurable cyclic pitch change at the rotor head if the mixing unit is rigged correctly. Any cross-coupling beyond AMM limits indicates a worn pivot or incorrect assembly.
4. Verifying Direction of Movement
Confirm that each surface moves in the correct direction for each input — this is a fundamental airworthiness check. On a FBW system where software can invert a signal, the physical check is just as important as the electronic check. Use the AMM control-movement verification table and mark off each item. A reversed aileron, for instance, would be immediately catastrophic.
5. Checking Breakout Force and Friction
Specifications exist for the maximum friction and minimum breakout force at the pilot's inceptor. On FBW aircraft, this is primarily a mechanical feel-unit check. On mixing-unit aircraft, excessive friction often points to a binding pivot pin or mis-rigged spring cartridge. Use a calibrated force gauge at the grip point and record results against AMM limits.
Key Numbers and Rules to Know
- All rigging must be performed in accordance with the approved AMM and applicable service bulletins — not generic references (14 CFR Part 43 requirement).
- Surface travel measurements are taken at the AMM-specified measurement station on the surface, not at the trailing edge tip (which may differ due to flex).
- LVDTs and RVDTs in FBW systems are typically considered serviceable when their output linearity error is within ±1% of full-scale range, but the AMM governs.
- Rigging pins must be removed and accounted for before returning the aircraft to service — a pinned control is an inoperable aircraft.
- Any rigging adjustment that changes control travel or feel forces requires an operational ground test and, per the AMM, possibly a functional flight test before return to service.
- Cable tensions in systems that feed mixing units are temperature-dependent; the AMM provides a tension-vs.-temperature correction chart that must be applied.
Why Rigging Checks Matter
An incorrectly rigged control system can manifest as a subtle problem — slightly heavy controls, a minor tendency to yaw — or as an immediate emergency, such as a reversed surface or a jammed actuator at the mechanical stop. For FBW aircraft, an improperly zeroed feedback transducer means the flight control computer's model of the aircraft doesn't match reality, which can degrade envelope protection and even cause the computers to command opposing surface deflections. All maintenance performed on primary flight controls must be properly recorded, and 14 CFR Part 43, Appendix D identifies the items — including flight control systems — that require inspection under the scope of an annual or 100-hour inspection. For air carrier operations, a duplicate inspection (a second, independent sign-off by another certificated person) on critical flight control work is typically required by the operator's approved maintenance program under 14 CFR 121.371 or 135.427, or by manufacturer/operator maintenance manual procedures, not by Appendix D itself.
Common Test Traps
- Confusing travel measurement location: The FAA expects you to know that measurements are taken at the AMM-specified station, not arbitrarily at the surface tip. Surface flex can make tip measurements misleading.
- Assuming software replaces hardware rigging on FBW aircraft: Mechanical stops, transducer nulls, and actuator end-play still require physical adjustment and verification — software cannot compensate for a hardware rigging error beyond narrow limits.
- Forgetting the duplicate inspection requirement where applicable: For air carrier operations, critical flight control work typically requires a second, independent inspection under the operator's maintenance program (e.g., 14 CFR 121.371, 135.427) rather than under Part 43, Appendix D, which instead defines the inspection scope for annual/100-hour inspections. Missing a required duplicate inspection is an airworthiness violation.
- Ignoring cross-coupling checks on mixing units: Exam questions may describe a symptom (e.g., collective input causing heading change) and ask you to identify the likely cause — improperly rigged mixing unit is the answer.
- Not applying the temperature correction to cable tension: A cable tensioned correctly at 70 °F will be over-tensioned when cold and under-tensioned when hot. The AMM chart corrects for this; ignoring it is a common rigging error that the knowledge test targets.