Module 17A: Propeller (A/B1) — Study Material

1. Module Overview

Module 17A covers the theory, construction, operation, maintenance, and inspection of propellers fitted to aeroplanes with piston engines (B1.2 licence category). The syllabus is divided into key areas: fundamentals (blade theory, angles, forces), construction (fixed-pitch, variable-pitch, constant-speed, feathering, reversing), control systems (governors, pitch change mechanisms), and maintenance practices (inspection, troubleshooting, documentation, and installation).

The knowledge levels for this module range from Level 1 (overview of fundamentals) to Level 3 (detailed theory of constant-speed systems, governor operation, and maintenance procedures). This study material synthesises the core knowledge required to meet the Part-66 syllabus and pass the module examination.

2. Key Concepts Explained in Detail

2.1 Propeller Fundamentals

Blade Angle and Pitch

The blade angle (β) is the angle between the chord line of the blade section and the plane of rotation. Because the rotational speed of a blade section increases with radius, the blade is twisted so that the angle of attack remains relatively constant along its length. The blade angle is therefore greatest at the root and smallest at the tip.

Geometric pitch is the theoretical distance a propeller advances in one revolution if it were moving through a solid medium.
Effective pitch is the actual distance travelled per revolution, which is less than geometric pitch due to slip.
Slip is the difference between geometric and effective pitch, expressed as a percentage of geometric pitch.

Blade Stations

Blade angles are measured at a reference station, typically 75% of the blade radius (measured from the centre of the hub). This is the industry-standard reference point because it represents the average effective angle of attack across the blade. Checking blade angle at this station ensures the propeller is set to the manufacturer's specification, which is critical for performance and synchronisation on multi-engine aeroplanes.

Forces Acting on a Propeller

Thrust: The forward force produced by the aerodynamic reaction to the mass of air accelerated rearward.
Centrifugal force: Acts radially outward on each blade, tending to straighten the blade and pull it out of the hub.
Aerodynamic twisting moment: Tends to rotate the blade towards high pitch (coarse) due to the centre of pressure being aft of the blade axis.
Centrifugal twisting moment: Tends to rotate the blade towards low pitch (fine) due to the blade's mass distribution.
Torque: The resisting force from the engine that must be overcome by the propeller.

Propeller Efficiency

Efficiency (η) is the ratio of thrust power to shaft power. It is affected by blade angle, airspeed, and RPM. Maximum efficiency occurs when the blade angle of attack is at its optimum value. At high airspeeds, a fine pitch angle causes the blade to stall; at low airspeeds, a coarse pitch angle causes excessive drag.

2.2 Propeller Construction

Fixed-Pitch Propellers

Wooden: Laminated hardwood (e.g., birch, mahogany) bonded with waterproof adhesive. The leading edge is often protected by a brass or stainless-steel sheath. Wooden propellers are lightweight and inexpensive but are susceptible to moisture ingress, which causes internal decay and delamination. Any crack, regardless of size, is a rejectable defect because it indicates internal structural failure that cannot be reliably repaired.
Metal: Aluminium alloy forgings, machined to the final blade profile. Metal propellers can be repaired by blending out minor nicks and dents, provided the damage is within the limits specified in the manufacturer's Component Maintenance Manual (CMM). Any crack is unacceptable and requires removal from service. Stop-drilling or welding is not permitted on propeller blades.

Variable-Pitch and Constant-Speed Propellers

Variable-pitch: Allows the pilot to select blade angle manually, but does not automatically maintain RPM.
Constant-speed: Uses a governor to automatically adjust blade angle to maintain a selected RPM regardless of throttle setting or airspeed. The governor senses engine RPM via a flyweight mechanism and adjusts oil pressure to the pitch change mechanism.

Pitch Change Mechanisms

Oil pressure to increase pitch: Hydraulic pressure from the governor moves the blades towards high pitch (coarse). A loss of oil pressure causes the blades to move to low pitch (fine) due to centrifugal twisting moment. This design is not fail-safe for feathering.
Oil pressure to decrease pitch: Hydraulic pressure moves the blades towards low pitch (fine). A loss of oil pressure causes the blades to move to high pitch (coarse) due to counterweights or aerodynamic twisting moment. This design is fail-safe and is used on feathering propellers.

Counterweight Systems

Counterweights are attached to the blade shanks and are positioned such that centrifugal force on the counterweights tends to rotate the blades towards high pitch (feather). When hydraulic pressure is lost, the counterweights move the blades to the feather position, reducing drag on a failed engine. This is a critical safety feature on multi-engine aeroplanes.

Feathering

Feathering is the rotation of the blades to an angle of approximately 90° to the airflow, aligning the blade edge-on to minimise drag. This is used on multi-engine aeroplanes when an engine is inoperative. The feathering system may be manual (pilot-operated) or automatic. After feathering, the propeller stops rotating, reducing drag and preventing further engine damage.

Reversing

Reversing rotates the blades to a negative pitch angle, producing thrust in the opposite direction to slow the aeroplane after landing. Reversing is only available on propellers designed for this purpose and requires a high-pressure oil system to overcome the centrifugal twisting moment.

Spinner

The spinner is an aerodynamic fairing that covers the propeller hub and blade roots. It reduces drag and provides a smooth airflow over the hub. A dent or out-of-round condition creates an aerodynamic imbalance, which induces vibration at high RPM, leading to stress on the propeller, engine, and airframe. The spinner must be repaired or replaced if damaged.

Rubber Bushings

Some propeller hubs use rubber bushings to mount the blades. These bushings dampen vibrations and allow slight blade movement, reducing stress on the hub and blade roots. Hardening or cracking of the rubber indicates loss of damping and requires replacement.

2.3 Propeller Control Systems

Governor Operation

The propeller governor is a hydraulically actuated, flyweight-controlled device that regulates oil pressure to the pitch change mechanism. The governor consists of:

Flyweights: Rotate with the engine and are connected to a pilot valve. At the governed RPM, the flyweights are in equilibrium. If RPM increases, the flyweights move outward, raising the pilot valve and directing oil to the pitch change mechanism to increase pitch (on an oil-to-increase-pitch system). If RPM decreases, the flyweights move inward, lowering the pilot valve and releasing oil pressure to decrease pitch.
Pilot valve: Controls the flow of oil to and from the propeller.
Relief valve: Limits maximum oil pressure in the system.
Speeder spring: Provides the force against which the flyweights act. Tension is adjusted by the cockpit propeller control lever.

Governor Malfunctions

Sticking flyweights: If flyweights are stuck in the closed position, the governor cannot sense an overspeed condition and cannot move the blades to increase pitch. This allows RPM to rise above the governed limit.
Blocked oil screen: A blocked governor oil screen restricts oil flow to the propeller, causing a loss of pitch control. On an oil-to-increase-pitch system, this results in a decrease in pitch and an uncommanded RPM increase.
Relief valve stuck closed: The governor cannot dump oil pressure, causing an over-pitch condition and a decrease in RPM.
Air in the system: Air is compressible, so air in the governor oil system causes a spongy or delayed response to throttle changes. This is a common troubleshooting point, especially after maintenance or when the system has been drained.

Pitch Lock

Some propellers are fitted with a pitch lock that prevents the blades from moving to a lower pitch if oil pressure is lost. This prevents a propeller overspeed on a failed engine.

2.4 Propeller Maintenance and Inspection

Track Check

Propeller track is the check that all blade tips lie in the same plane perpendicular to the axis of rotation. A track gauge is placed on a smooth surface (e.g., engine cowling or wing) and the clearance between each blade tip and the gauge is measured as the propeller is rotated. The difference between the maximum and minimum readings must be within the limits specified in the AMM. An out-of-track condition causes vibration and stress on the propeller, engine, and airframe.

Blade Angle Measurement

Blade angle is measured at the 75% station using a universal propeller protractor. The propeller is rotated so that the blade is horizontal, and the protractor is placed on the blade face. The reading is compared to the manufacturer's specification. Blade angle is critical for performance, synchronisation, and proper operation of the constant-speed system.

Blade Damage Assessment

Nicks and dents: Minor nicks on metal propeller blades are acceptable if within the limits published in the manufacturer's maintenance manual (AMM or CMM). If within limits, they are blended out smoothly to prevent stress concentrations, followed by a crack inspection (e.g., dye penetrant). If the damage exceeds the limits, the blade must be repaired or replaced.
Cracks: Any crack in a metal propeller blade is unacceptable and requires removal from service. Stop-drilling or blending is not permitted for propeller blades.
Wooden propellers: Any crack is grounds for rejection. Wooden propellers cannot be repaired for structural defects.
Moisture ingress: A dark stain extending from the hub into the blade shank indicates moisture ingress and possible internal decay or delamination. This is a rejectable defect for wooden propellers.

Balancing

Blending removes material, which changes the weight distribution of the blade. After blending, the propeller must be re-balanced to prevent vibration and stress. Balancing is performed using a propeller balancing stand or a dynamic balancer. Static balance is checked by suspending the propeller horizontally; dynamic balance is checked during a ground run using vibration analysis equipment.

Oil Leaks

Oil leaks from the propeller hub are identified by oil streaks or stains originating from the hub, blade roots, or spinner. Accumulated dirt on these streaks is a classic sign of a leak. Minor oil seepage at the propeller dome can be normal for constant-speed propellers, but it must be monitored. The correct initial action is to clean the area and monitor for excessive leakage. Any significant leak from the propeller dome can indicate a failing seal or internal damage, which could lead to loss of pitch control. The aeroplane must be grounded and inspected.

Corrosion and Seal Deterioration

Long-term inactivity can lead to seal drying and corrosion. Cycling the propeller (operating the pitch change mechanism) redistributes hydraulic oil, lubricates seals, and prevents sticking. This is a standard practice found in aircraft maintenance manuals.

Fretting Corrosion

Fretting corrosion at counterweight pivot points is a critical finding. The correct maintenance action is to dismantle the assembly to inspect for cracks, typically using magnetic particle inspection (MPI) on steel counterweights. Lubrication or polishing is not acceptable as it can hide cracks.

Blade Tip Erosion

Blade tip erosion is typically caused by abrasive particles in the air, especially during taxiing when the propeller is close to the ground. This is a common maintenance finding and is addressed by repainting or applying protective coatings.

2.5 Propeller Installation and Removal

Removal Sequence

The correct removal sequence ensures safety and proper reinstallation:

Disconnect the battery and ensure the ignition is off.
Remove the spinner and any retaining hardware.
Mark the propeller and flange orientation to maintain balance and track.
Remove the mounting bolts and nuts.
Carefully slide the propeller off the flange, supporting its weight.
Protect the flange and propeller from damage.

Installation Sequence

Inspect the flange and propeller for damage.
Align the propeller with the flange using the orientation marks.
Install the mounting bolts and torque to the specified value.
For castellated nuts, torque to the specified value, then if the cotter pin hole is not aligned, tighten (never loosen) to the next alignment position, ensuring the maximum torque is not exceeded.
Install the cotter pins.
Install the spinner and check for proper fit.
Perform a track check and functional test.

Torque Application

For castellated nuts, the correct procedure is to torque to the specified value, then if the cotter pin hole is not aligned, tighten to the next alignment position. Backing off the nut is not acceptable as it reduces the clamping force and can lead to loosening.

2.6 Documentation and Certification

EASA Form 1

When a propeller is removed for repair and then returned to service, it must be accompanied by an EASA Form 1 (Authorised Release Certificate) issued by an approved maintenance organisation (Part-145 or Part-M Subpart F). This certifies that the work has been performed in accordance with approved data and the component is fit for use.

Certificate of Release to Service (CRS)

The installation of a propeller is a maintenance task that requires a CRS (e.g., EASA Form 53) issued by the certifying staff. The CRS must reference the Form 1 and the installation task. An EASA Form 1 alone is not sufficient for the installation; a logbook entry is not a CRS.

Component Maintenance Manual (CMM)

The CMM is the approved source for detailed servicing, inspection, and repair data for a specific propeller model. It contains dimensional limits, repair procedures, and inspection intervals. The Aircraft Flight Manual (AFM) contains operational data, the Engine Maintenance Manual (EMM) covers the engine, and Airworthiness Directives (ADs) are mandatory instructions.

3. Important Formulas and Regulations

3.1 Formulas

Geometric pitch (inches) = 2π × r × tan(β), where r is the radius at the reference station and β is the blade angle.
Slip (%) = (Geometric pitch − Effective pitch) / Geometric pitch × 100.
Propeller efficiency (η) = Thrust power / Shaft power = (T × V) / (P × 2π × N), where T is thrust, V is true airspeed, P is torque, and N is rotational speed.

3.2 Regulations

Part-66 (Regulation (EU) No 1321/2014, Annex III): Sets out the basic knowledge requirements for aircraft maintenance licences. Module 17A is mandatory for the B1.2 category.
Part-145 (Regulation (EU) No 1321/2014, Annex II): Requires that maintenance be performed in accordance with approved data and that components be released with an EASA Form 1.
Part-M (Regulation (EU) No 1321/2014, Annex I): Governs the continuing airworthiness of aircraft and components.
AMC/GM: Acceptable Means of Compliance and Guidance Material provide interpretation and acceptable methods of compliance with the regulations.

4. Common Relationships Between Concepts

Blade angle and RPM: Increasing blade angle (coarse pitch) increases the load on the engine, decreasing RPM. Decreasing blade angle (fine pitch) decreases the load, increasing RPM. The governor maintains a constant RPM by adjusting blade angle in response to changes in throttle setting or airspeed.
Oil pressure and blade movement: In an oil-to-increase-pitch system, oil pressure moves the blades to coarse pitch. In an oil-to-decrease-pitch system, oil pressure moves the blades to fine pitch. The direction of blade movement on loss of oil pressure determines the fail-safe behaviour.
Counterweights and feathering: Counterweights move the blades to feather when oil pressure is lost, providing a fail-safe feature. This is essential for multi-engine aeroplanes to minimise drag on a failed engine.
Track and vibration: An out-of-track condition causes vibration, which stresses the propeller, engine, and airframe. Track must be checked after installation and at scheduled inspections.
Balancing and vibration: Blending removes material, changing the weight distribution. Re-balancing is mandatory after blending to prevent vibration.
Governor and pitch control: The governor senses RPM via flyweights and adjusts oil pressure to the pitch change mechanism. Any malfunction in the governor (sticking flyweights, blocked screen, air in the system) results in loss of RPM control.

5. Typical Exam Focus Points

Blade angle measurement: The 75% station is the reference point for measuring blade angle. Understand why the blade is twisted and why the reference station is used.
Track check procedure: Know how to perform a track check using a track gauge and the significance of an out-of-track condition.
Governor malfunctions: Be able to diagnose common governor failures, including sticking flyweights, blocked oil screens, and air in the system. Understand the effect of each failure on RPM.
Fail-safe designs: Understand the difference between oil-to-increase-pitch and oil-to-decrease-pitch systems, and the role of counterweights in feathering.
Blade damage assessment: Know the limits for nicks and dents, the procedure for blending, and the rejection criteria for cracks (metal) and any crack (wooden).
Oil leaks: Identify the signs of oil leaks and the correct initial action (clean and monitor vs. ground the aeroplane).
Documentation: Understand the difference between an EASA Form 1 (component release) and a CRS (installation release). Know when each is required.
Torque application: Know the correct procedure for castellated nuts and cotter pins — tighten, never loosen, to align the cotter pin hole.
Feathering and reversing: Understand the purpose of feathering (drag reduction on a failed engine) and the blade angle at the feather stop (approximately 90°).
Long-term storage: Know the procedure for preventing corrosion and seal deterioration (cycling the propeller).
Spinner damage: Understand why a dented or out-of-round spinner must be repaired (aerodynamic imbalance and vibration).
Rubber bushings: Know their function (vibration damping) and the consequence of hardening or cracking (replacement).

Summary

Module 17A requires a solid understanding of propeller theory, construction, control systems, and maintenance practices. The key to passing the exam is to understand the relationships between blade angle, oil pressure, governor operation, and fail-safe design, and to be able to apply this knowledge to practical maintenance scenarios. Always refer to the manufacturer's CMM or AMM for specific limits and procedures, and ensure that all maintenance is documented in accordance with Part-145 and Part-M requirements.