The propeller hub is the structural heart of any propeller assembly. It clamps the blades, transmits engine torque to thrust, and — in variable-pitch designs — houses the pitch-change mechanism that makes the entire controllable-pitch system work. Because it endures cyclic loading, centrifugal force, and torsional stress every single flight hour, the hub demands a level of overhaul scrutiny that goes well beyond a casual wipe-down. For an Aviation Maintenance Technician (AMT) working toward a Powerplant certificate, understanding what happens during a hub overhaul — from disassembly through dimensional inspection to reassembly — is both an FAA knowledge test requirement and a genuine safety imperative.
This article walks through the complete overhaul process in detail, covering why each step exists, what technicians look for, and how the governing regulations and manufacturer data interact. Every step described here is grounded in the FAA's Aviation Maintenance Handbook series and 14 CFR Part 43 requirements.
Regulatory Framework for Propeller Overhaul
Propellers are classified as propeller assemblies under 14 CFR Part 35, and their maintenance must comply with 14 CFR Part 43. A major overhaul of a propeller hub is considered a major repair or alteration when it involves machining, welding, or reassembly of a controllable-pitch hub. Such work must be performed by or supervised by an FAA-certificated powerplant mechanic, a certificated repair station, or the original manufacturer under an approved data source. The approved data hierarchy — manufacturer's Overhaul Manual, FAA-approved data, or Airworthiness Directives (ADs) — governs every dimensional limit and procedure step. The technician must never substitute personal judgment for published limits; if a specification is not met, the component is either repaired per approved data or replaced.
Disassembly and Initial Inspection
Before any part is cleaned or measured, a thorough incoming inspection documents the condition of the hub as received. The technician notes blade track (the path each blade tip traces), any visible cracks, corrosion, impact damage, or blade looseness. Photographs and written records support later determinations about whether a defect was pre-existing or introduced during overhaul.
Disassembly follows the sequence specified in the manufacturer's overhaul manual — never a generic procedure. For a typical constant-speed propeller hub (such as a Hartzell or McCauley design), disassembly involves:
- Removing the propeller from the engine flange and placing it on an approved fixture that supports the hub without distorting it.
- Draining residual oil from the pitch-change cylinder and dome (on hydraulically actuated designs).
- Removing the blade retention nuts or clamp rings using torque-reversal in the sequence specified to avoid distortion of the hub barrel.
- Carefully withdrawing each blade from the hub barrel, keeping the blade-specific shims, O-rings, and bearing races together and tagged to that blade position — they are not interchangeable.
- Disassembling the pitch-change mechanism: piston, dome, spring, and associated seals.
Every fastener, retaining ring, and seal is discarded if the manual calls for mandatory replacement at overhaul. This is common for O-rings, lip seals, and snap rings — using them a second time risks leaks and pitch-change failures that are invisible until in-flight.
Cleaning Methods and Their Limits
Proper cleaning is not cosmetic — contaminants mask cracks and corrosion. Steel hub components are typically cleaned with alkaline degreaser or solvent, while aluminum components require solvents that will not cause intergranular attack. The FAA Aviation Maintenance Handbook (FAA-H-8083-32) cautions against using caustic cleaners on aluminum, as they can cause intergranular corrosion and surface pitting that appears only as a slight dullness yet structurally weakens the part. After cleaning, all passages — particularly oil transfer tubes and blade-bearing grease passages — must be blown clear with dry, filtered compressed air and verified open with a probe or light.
Non-Destructive Inspection Techniques
Once clean, each hub component undergoes non-destructive inspection (NDI). The method depends on the material and geometry of the part:
- Dye penetrant inspection (DPI): The standard for aluminum hubs and aluminum alloy components. The technician applies penetrant, allows dwell time per specification, removes excess, applies developer, and examines under a black light or white light depending on the penetrant type. Any indication that does not disappear with a second cleaning is treated as a relevant indication — a crack — until proven otherwise by the manufacturer's disposition guidance.
- Magnetic particle inspection (MPI): Used for ferrous (steel) hub components such as retention bolts, pitch-change pins, and steel clamp rings. The part is magnetized, ferrous particles (wet or dry) are applied, and the inspector looks for particle accumulation along field lines that indicate a discontinuity. After inspection, parts must be thoroughly demagnetized to prevent interference with aircraft magnetic compasses or avionics.
- Eddy current inspection: Increasingly common for detecting subsurface cracks in aluminum hubs and blade sockets without surface preparation. A calibrated probe is swept over the area of interest; the instrument's meter or audio signal changes when conductivity changes due to a crack or corrosion pit.
- Visual inspection with magnification: A 10x loupe or binocular microscope is used in areas the penetrant cannot reach, such as threaded bores and recessed blade-socket radii.
Any crack found in a hub barrel, flange, or blade retention area is cause for rejection of the entire hub. Cracks are not repairable by welding in an aluminum hub; the structural geometry and grain flow of the casting or forging cannot be restored. Steel hubs may have limited weld-repair provisions, but only under manufacturer-approved procedures with post-weld heat treatment and re-inspection.
Dimensional Inspection and Wear Limits
After NDI, every hub component is measured against the manufacturer's dimensional limits. Key measurements include:
- Blade socket bore diameter: Excessive wear allows blade wobble, creating fretting corrosion and fatigue cracking at the blade shank. Limits are typically within a few thousandths of an inch of the new-part dimension.
- Blade retention bearing race condition: Ball or roller bearings that support blade rotation during pitch change must be free of spalling, brinelling, and corrosion pits. Brinelling — small indentations from static overload — causes rough pitch-change operation and accelerated wear. Suspect brinelling whenever an aircraft has experienced a prop strike, even a ground strike at idle.
- Flange pilot bore: The bore that centers the hub on the engine crankshaft flange must be within tolerance for concentricity. An out-of-tolerance bore produces vibration that is felt as a low-frequency shimmy and can damage engine bearings.
- Pitch-change cylinder bore: Scored or oval cylinder walls allow oil bypass, reducing governor response speed and causing erratic RPM control.
- Thread condition: All threaded features are inspected with the correct thread gauge. Damaged threads are repaired only by the manufacturer's approved insert process — never by retapping to a larger size without approval.
Reassembly and Lubrication Requirements
Reassembly reverses the disassembly sequence with equal precision. All replacement parts — seals, O-rings, bearings, and mandatory-replace fasteners — must be new, FAA-approved parts with proper traceability documentation. Bearings are installed with the correct press fit, and seal lips are lubricated with the fluid they will be sealing (typically propeller oil) rather than a general-purpose grease that could swell the seal material.
Torque values for blade retention nuts, dome bolts, and flange bolts come directly from the manufacturer's overhaul manual and are applied in the specified sequence. Improper torque sequence on a multi-blade hub can distort the barrel and change blade track, producing vibration. After assembly, the hub is pressure-tested with oil or air at the specified pressure for the specified time to confirm seal integrity before the propeller is returned to service.
Why Hub Overhaul Intervals Matter
Manufacturers establish time between overhaul (TBO) limits for propeller hubs based on fatigue analysis and service experience. These limits are expressed in flight hours or calendar time. Exceeding TBO is not a gray area: 14 CFR Part 43 and the applicable Airworthiness Directives make compliance mandatory for type-certificated aircraft. Some ADs specifically address hub overhaul after prop strikes, regardless of hours remaining on the TBO clock — because the internal fatigue state of the hub cannot be determined by external inspection after a sudden stoppage event.
Common Test Traps
- Confusing major and minor repairs: Hub overhaul involving disassembly, machining, or replacement of structural components is a major repair — it requires approved data and an appropriately rated technician or repair station. Blade cleaning and minor dressing are minor repairs.
- Assuming visual inspection is sufficient: Cracks in propeller hubs often originate below the surface or inside blade sockets where visual alone will miss them. NDI is mandatory, not optional, during overhaul.
- Reusing O-rings and seals: Even if they look serviceable, seals that are specified as mandatory-replace items must be replaced. The FAA knowledge test frequently tests this judgment.
- Overlooking prop strike requirements: A prop strike — including a ground strike — may mandate immediate hub teardown per the applicable AD or manufacturer's service bulletin, regardless of how minor it appears externally. Failing to comply is an airworthiness violation.
- Mixing blade-specific components: Shims, blade clamps, and bearing races that are fitted to a specific blade and socket position must be returned to that position. Interchanging them changes blade track and pitch geometry.