Chapter 8: Inspection, Damage Assessment and Repair

Inspecting propellers

The propeller is one of the most highly stressed parts of the aircraft, so scheduled inspections are detailed. Every inspection follows the propeller manufacturer's manual, the aircraft maintenance programme and all applicable airworthiness directives (ADs) and service bulletins (SBs), and is recorded in the propeller logbook.

• Visual inspection — clean the blades first, then inspect the whole propeller with good lighting and a magnifier: blades, hub, spinner, de-ice boots, retention, safetying and oil/grease leaks.
• Tactile inspection — feel for blade play (shake and twist the blades; some radial and angular play is normal on certain hubs, within limits), roughness and loose parts. Check that blades move freely (where possible) and that nothing is loose in the hub.
• Tachometer accuracy — an inaccurate tachometer can let the propeller run in a restricted range or overspeed. Check it at regular intervals (e.g. at each 100-hour/annual inspection).
• Look at records: hours since overhaul, calendar time limits, AD/SB status, repairs and blade-angle settings.

Wood propeller inspection

• Look for cracks (especially near the hub and at lag screws of metal sleeves), dents and bruises, warping, glue-line separation (delamination), and defects in the finish that let moisture in.
• Charring of the wood between the propeller and the flange shows the hub bolts have been loose. Elongated bolt holes mean the propeller must be rejected or overhauled.
• Metal tipping: check for looseness, cracked solder joints, loose screws and rivets, cracks and erosion. Separation between the leading-edge strip and the tip cap shows the cap is moving outward. Check that tip drain holes are open.
• A fine line in the fabric sheathing or finish may show a crack in the wood beneath. Check hub bolt torque and safetying.

Metal blade inspection

• Metal blades fail by fatigue starting at stress raisers: sharp nicks, cuts, scratches and corrosion pits. Inspect the full length of the leading and trailing edges, especially near the tips, the face and back, and the shank.
• Any transverse crack in an aluminium alloy blade is cause for rejection. No repairs are permitted to the shank (retention area) of adjustable-pitch blades beyond the manufacturer's limits. Multiple deep nicks may also mean rejection.
• NDT methods:
  • Dye penetrant (visible or fluorescent) — confirms suspected surface cracks in aluminium and steel.
  • Etching — a caustic etch followed by a nitric acid neutralising rinse reveals cracks and grain flow; used at overhaul. Chromic acid anodising shows cracks as dark lines as the solution bleeds out.
  • Eddy current — finds surface and near-surface cracks, e.g. in blade shanks and hub bores, without removing paint.
  • Ultrasound — internal flaws, e.g. in blade roots and composite structures.
  • Magnetic particle — steel parts (steel blades, hubs, pins, counterweight clamps).
• Corrosion: check under de-ice boots, around the blade clamps and retention, and in hubs. Remove light corrosion and restore protection; deep pitting may exceed limits.

Composite blade inspection

• Inspect visually for nicks, gouges, erosion of the coating and the erosion sheath, cracks, loose material, debonds and lightning strike damage (burns at the tip and along conductors).
• Find delaminations and debonds by coin tapping: a hollow or dead sound shows a void. Ultrasound or phased-array inspection gives more detail.
• Erosion of the polyurethane coating is common and can be repaired with approved coatings; damage into the structural plies needs assessment against the manual.

Ground strike and sudden stoppage

• A propeller strike (ground, object, or a sudden engine stoppage) can bend blades, crack hubs, damage blade retention and twist the crankshaft or reduction gear, even if blades look only slightly damaged.
• The propeller must be removed and inspected by an approved propeller shop (usually equivalent to an overhaul), and the engine manufacturer's sudden-stoppage inspection must be carried out (e.g. crankshaft run-out, gear inspection).
• Whenever a propeller is removed, inspect the hub cone seats, cones and mating parts for wear, galling and corrosion.

Repair and treatment schemes

• Minor repairs (within the manual's limits and usually allowed on the aircraft) include dressing out small nicks and scratches on the leading and trailing edges, face and back; light corrosion removal; touch-up of protective finishes; repairs to de-ice boots; small composite coating repairs.
• Dressing: use files, stones and fine emery cloth to remove the damage completely and form a smooth, shallow blend running along the edge — many times longer than it is deep — with no sharp corners. Work along the blade, not across it. Polish, inspect with dye penetrant, then restore protection (chemical conversion coating such as Alodine and paint). Record the repair.
• Repairs must keep the blade within minimum width and thickness limits at each station. Large repairs can change balance and natural frequency.
• Major repairs — done only by an approved propeller repair station/overhaul shop: cold straightening of bent blades (within strict limits), shortening blades to remove tip damage (all blades must be shortened to match, then rebalanced and re-tracked), repitching, blade surface grinding, re-anodising and shot peening, and any repair to the shank or hub.
• Overhaul at the manufacturer's time or calendar limit (TBO): complete disassembly, cleaning, NDT, dimensional checks, replacement of bearings, seals and life-limited parts, refinishing, reassembly, blade-angle setting and balancing.
• Cleaning: use mild soap and water or an approved solvent with a brush or cloth. Never use acids or caustic materials, and avoid pressure washing seals. Keep cleaning fluids out of the hub. A thin film of oil or corrosion preventive on steel/aluminium blades helps in harsh environments.
• Servicing: grease hubs at the specified interval with the specified grease (pump grease in through one fitting with the opposite fitting removed, to avoid over-pressurising seals); check and charge the air/nitrogen dome to the pressure for the ambient temperature (with blades on the start locks/as specified).