Flight controls must translate a pilot's physical input — pushing a rudder pedal, deflecting a yoke — into precise, reliable movement of a control surface. In many aircraft designs, the geometry of the airframe makes a direct, straight-line connection between the cockpit and the surface impractical or impossible. That is where the bellcrank enters the picture. A bellcrank is a pivoting lever, typically L-shaped or triangular, that changes the direction of cable or pushrod force within a control system. Understanding how bellcranks function and how to rig them correctly is an essential skill for any AMT working on airframe assembly and rigging.
This article covers the mechanical principles behind bellcranks, how they are installed and adjusted, the rigging procedures used to verify correct travel and alignment, and the specific test traps the FAA favors on the AMT Airframe knowledge exam.
What a Bellcrank Does
At its core, a bellcrank is a direction-changing lever mounted on a pivot (called a trunnion or pivot bolt) that is anchored to the aircraft structure. When a cable or pushrod applies force to one arm of the bellcrank, the lever rotates about its pivot, and the other arm moves in a different direction, pushing or pulling the next segment of the control system. By choosing the angle between the two arms and the length of each arm, a designer can accomplish three things simultaneously:
- Change the direction of force — for example, converting fore-and-aft cable movement into side-to-side pushrod movement.
- Change the mechanical advantage — a longer output arm relative to the input arm reduces the force required but increases the travel distance needed; a shorter output arm increases force but reduces travel.
- Reverse direction — a straight (180°) bellcrank causes the output to move opposite to the input, which is useful in aileron systems where one aileron must go up while the other goes down.
The classic L-shaped bellcrank has its two arms at roughly a 90-degree angle. The right-angle geometry means that when the input cable pulls one arm aft, the output arm pushes the attached component in a direction perpendicular to the cable run. Triangular or other multi-arm bellcranks are used when a single pivot point must serve multiple control paths simultaneously.
Bellcrank Construction and Mounting
Most bellcranks in certificated aircraft are machined or forged from aluminum alloy or steel and are treated to resist corrosion. The pivot hole is bushed with a bronze or steel bushing to minimize wear. The bellcrank pivots on a bolt that passes through anchor brackets bolted or riveted to the airframe structure. The pivot bolt must be properly torqued, with a cotter pin or other approved locking device installed, but the bellcrank itself must rotate freely — binding at the pivot is a serious airworthiness concern.
Cable attach points on the bellcrank arms typically use clevis pins and castellated nuts secured with cotter pins, or threaded studs with self-locking hardware as specified in the manufacturer's maintenance manual. Pushrod ends often attach via rod-end bearings (also called rose joints or heim joints) that allow for small angular misalignment without binding. Inspectors should check all attach points for freedom of movement, proper hardware security, and absence of corrosion or elongation of holes.
Rigging Principles
Rigging is the process of adjusting a control system so that all components are in their correct neutral (or specified) positions simultaneously and move through the correct range of travel when the cockpit controls are operated. Bellcranks are central to rigging because even small angular errors in their neutral position will cascade into incorrect control surface travel and asymmetric response.
Setting Neutral
Every rigging procedure begins with establishing a known reference point. For most aircraft this means using rigging pins — precision-ground pins specified in the maintenance manual that slip through alignment holes in the bellcrank and matching holes in the surrounding structure when the bellcrank is exactly at neutral. With the rigging pin inserted, the bellcrank cannot rotate, which allows the technician to adjust cable tensions and pushrod lengths with confidence that the pivot is in the correct geometric position. Never substitute a drill bit or common bolt for the specified rigging pin; the diameter and tolerance matter.
Cable Tension
Cables attached to bellcrank arms must be tensioned to the values given in the aircraft's maintenance manual, typically measured with a tensiometer. Cable tension is affected by temperature: aluminum airframes expand with heat, slackening cables; steel cables also expand but at a different rate. Many manuals provide a tension-versus-temperature chart so the technician can correct measured tension to the standard reference temperature (commonly 70°F / 21°C). Standard rigging procedure typically requires the rigging pins to be installed, holding the control surfaces and bellcranks locked in their neutral position, while cable tension is measured and adjusted; the pins are removed afterward to verify the system moves freely through its full range.
Control Travel
After neutral is established and cable tensions are set, the technician must verify that the control surfaces move through the correct angular travel in both directions. Travel is measured with a protractor or inclinometer placed on a specified location on the surface or its horn. The maintenance manual specifies both the total travel and any permissible tolerance (e.g., aileron up 20° ± 2°, down 14° ± 2°). If travel is incorrect, the length of pushrods or turnbuckle adjustments must be re-examined.
The geometry of the bellcrank arm lengths directly affects the ratio of cockpit control travel to surface travel. Changing the attach point location on a bellcrank arm is never an approved field fix — the arm length is an engineered value. Adjustments are made only through the turnbuckles and adjustable pushrod end fittings that the designer has provided for that purpose.
Checking for Binding and Interference
With the system fully rigged, cycle the cockpit controls through full travel in both directions while an assistant watches the bellcranks, cables, and pushrods for any binding, chafing against structure, or interference with adjacent systems. Binding can cause control forces higher than certified or, worse, jamming. The FAA requires that no part of a control system shall contact any fuel line, electrical wire, or structure in a way that could cause inadvertent operation or failure.
Why Rigging Accuracy Matters
Incorrect bellcrank neutral position or improper cable tension has real safety consequences. If the ailerons are not rigged to their proper neutral, the aircraft may exhibit a persistent roll tendency that the pilot must constantly correct — an aerodynamic imbalance that adds workload and degrades safety margins. More critically, if a bellcrank pivot is over-torqued or binding, a pilot may be unable to move a control surface when needed. A broken or jammed bellcrank in any primary flight control system is a catastrophic failure mode. This is why rigging is performed following any control system repair, cable replacement, or major structural work, and must be signed off in the maintenance record with a return-to-service statement.
Key Numbers and Rules
- Rigging pins must match the specification in the manufacturer's maintenance manual — diameter and fit tolerance are critical.
- Tensiometer selection must match the cable diameter and type being measured; using the wrong tensiometer gives false readings.
- Temperature correction for cable tension is required; most manuals reference 70°F (21°C) as the standard temperature.
- Control surface travel is always measured in degrees at the surface, using a calibrated protractor or digital inclinometer referenced to a flat surface specified in the manual.
- Turnbuckle safety wire or clip must be installed after any tension adjustment; the commonly cited general practice (per AC 43.13-1B) is no more than three threads of a turnbuckle barrel exposed at each end after adjustment, though the specific manufacturer maintenance manual governs.
- Pivot bolt must be safetied per the manufacturer's specification — usually a cotter pin through a castellated nut, or a self-locking nut where approved.
- All rigging must be documented in the aircraft maintenance records per 14 CFR Part 43.
Common Test Traps
- Rigging pin substitution: The exam may present a scenario where no rigging pin is available and ask what to substitute. The correct answer is that no substitution is permitted — the proper rigging pin must be obtained.
- When to measure cable tension: Standard practice is to measure and adjust cable tension with the rigging pins installed, locking the bellcranks and surfaces in neutral. The pins are removed afterward so the technician can verify the system moves freely through its full range without binding.
- Bellcrank arm length and mechanical advantage: A longer output arm produces greater surface travel for a given input motion but reduces the force delivered. The FAA may ask which change to arm length increases or decreases mechanical advantage.
- Temperature effects on cable tension: High temperature causes cables to go slack (the airframe expands faster than the cables in aluminum aircraft). Students often confuse this and select that high temperature increases tension.
- Thread exposure on turnbuckles: A commonly cited general practice is no more than three threads exposed on either side of the barrel after adjustment and safetying, though the applicable manufacturer maintenance manual should always be checked for the specific aircraft. Exceeding this limit is a common distractor answer presented as acceptable.