Module 11B: Piston Aeroplane Aerodynamics, Structures and Systems

1. Module Overview

This module provides the certifying staff (B1.2) with a comprehensive understanding of the design, construction, operation, and maintenance of piston-engined aeroplanes. It integrates aerodynamic principles with the detailed study of airframe structures and all associated systems. The syllabus is designed to ensure that the technician can perform inspections, troubleshoot faults, and carry out repairs and replacements to the high standards required by EASA regulations, using approved data. The module covers everything from the fundamental theory of flight to the specific components of flight control, landing gear, hydraulic, pneumatic, electrical, and powerplant systems.

The knowledge levels required are:

Level 1 (Overview): A general familiarisation with the principles and components.
Level 2 (General Knowledge): A broader understanding of system functions, components, and their interrelationships.
Level 3 (Detailed Theory): A deep, detailed knowledge of system operation, maintenance practices, and troubleshooting, enabling the certifying staff to make independent decisions.

2. Key Concepts and Detailed Theory

2.1 Aeroplane Structures (Aerodynamics & Structures)

The airframe is the core of the aeroplane, and its integrity is paramount. The primary structure carries the main flight and ground loads, while secondary structures are non-load-bearing or carry only minor loads.

Primary Structure: Includes the fuselage, wings, empennage, and their main spars, ribs, and skin. Damage to primary structure is a serious airworthiness concern.
Secondary Structure: Includes fairings, wheel spats, and non-structural cowlings. Damage here is less critical but must still be assessed.
Tertiary Structure: Includes small items like inspection panels and minor fairings.

Wing Construction:

Spars: The main longitudinal members that carry the bending loads. The main spar is the most critical structural element. Corrosion pitting here beyond allowable limits (as defined in the SRM) is a serious defect.
Ribs: Transverse members that maintain the aerofoil shape and distribute loads to the skin and spars.
Skin: The outer surface provides the aerodynamic contour and, in a stressed-skin design, carries a significant portion of the shear and torsional loads as part of the wing's torsion box.
Struts: On strut-braced high-wing designs, the strut transfers a portion of the wing lift load directly to the fuselage, relieving the bending moment at the wing root.

Fuselage Construction:

Monocoque/Semi-Monocoque: The skin and stringers form a stiff shell that carries the loads. The fuselage skin is primary structure, and any crack, especially emanating from a rivet hole near a cut-out, is a fatigue issue requiring immediate reporting and an approved repair.
Truss (Tubular Steel): A framework of welded steel tubes. Surface corrosion on non-load-bearing tubes can be removed by mechanical means (e.g., abrasive paper) if the remaining wall thickness is within limits. After removal, a corrosion inhibitor and protective finish must be applied.

Corrosion:

Surface Corrosion: The most common form on aluminium alloys, appearing as a white, powdery deposit (aluminium oxide). It is caused by the breakdown of the protective oxide layer and can be treated by mechanical removal and re-protection.
Intergranular Corrosion: Occurs along grain boundaries and is not easily visible. It is a serious condition that can lead to structural failure.
Stress Corrosion Cracking: Requires a tensile stress and a corrosive environment.
Filiform Corrosion: Appears as worm-like threads under a coating.

Damage Assessment:

All damage must be assessed against approved data, typically the Structural Repair Manual (SRM) or the Airworthiness Manual (AMM). These documents define Allowable Damage Limits (ADL) . For example, a dent in an aluminium leading edge is typically limited to a specific depth (e.g., 2 mm) for a given diameter. If damage exceeds these limits, a repair or replacement is required. Stop-drilling is a common temporary measure for cracks to prevent propagation until a permanent repair is performed.

2.2 Flight Control Systems

Flight controls are divided into primary and secondary controls.

Primary Flight Controls: These are essential for controlling the aeroplane's attitude and are directly controlled by the pilot. They include the ailerons (roll), elevators (pitch), and rudder (yaw).
Secondary Flight Controls: These assist the pilot or improve performance. They include trim tabs, balance tabs, flaps, and slats.

Trim Tabs:

Purpose: A small movable surface on the trailing edge of a primary control surface. It produces an aerodynamic force that assists the pilot in maintaining a set attitude, reducing the force required on the control column.
Balanced Tabs: A type of tab that moves in the opposite direction to the primary control surface. This creates an aerodynamic force that assists the pilot in moving the primary control, reducing the control hinge moment and thus the pilot's effort.

Control Cables:

Control cables are critical components. Their condition is strictly controlled.

Inspection: Cables must be free from corrosion, kinks, and broken wires.
Rejection Criteria: For primary flight control cables, any broken wire is generally cause for replacement, as a single broken wire can lead to rapid fatigue failure. For secondary control cables (e.g., trim tabs), the typical limit is 3 broken wires in any 6-inch length. Splicing is not permitted for primary control cables.
Tensioning: Cable tension is measured with a tensiometer at designated points (straight runs) with the system rigged in the neutral position. The tension is specified in the AMM for a specific temperature. Incorrect tension leads to control freeplay or overstress. If tension is low, it is adjusted using turnbuckles.

Rigging:

Control surface deflections are specified in the AMM. If the deflection is incorrect (e.g., aileron travel is 15° up and 10° down instead of the specified 20° up and 15° down), the rigging must be adjusted, typically by altering the length of the pushrod.

Free Play:

Excessive free play in a control system (e.g., elevator, rudder) can be caused by worn hinges, cable stretch, or a mis-rigged actuator. All potential sources must be inspected and rectified.

Stall Warning Systems:

These are mandatory equipment. They can be vane-type or pressure-sensor type. A damaged system must be repaired or replaced and tested per approved data. If the vane moves freely but the horn does not sound, the fault is likely in the electrical circuit (e.g., broken wire, faulty switch).

2.3 Landing Gear Systems

Landing gear can be fixed or retractable, and configured with a nosewheel (tricycle) or tailwheel (conventional).

Tailwheel (Conventional) Gear: The tailwheel is often steerable and linked to the rudder pedals to maintain directional control during ground operations, particularly in the tail-low attitude of taxi and takeoff.
Nosewheel Gear: The nosewheel is steerable and often equipped with a shimmy damper, a hydraulic device that dampens the oscillatory motion of the nosewheel, preventing severe vibration.

Retractable Gear Systems:

Uplock Mechanism: Holds the landing gear securely in the retracted (up) position.
Downlock Mechanism: Holds the gear in the extended (down) position.
Squat Switch: A safety device that prevents gear retraction when the aeroplane is on the ground. The circuit is only completed when the oleo strut is extended (in the air).
Retraction Test: Must be performed with the aeroplane's weight and CG within the limits specified in the AMM to avoid overloading the gear actuators and structure.

Oleo Struts:

These are hydraulic shock absorbers. A leaking oleo strut indicates worn seals. The correct repair is to replace the seals and recharge the strut with the specified fluid and nitrogen per the AMM.

Troubleshooting:

If the gear does not extend, the first step is to check the hydraulic fluid level. Low fluid can cause air to enter the system, leading to cavitation and incomplete gear retraction.
If the gear motor runs but the gear does not move, the most likely cause is a mechanical fault, such as the uplock hooks not releasing.
A 'gear unsafe' indication could be a false indication due to faulty bulbs or wiring; the AMM troubleshooting chart typically starts with the simplest and most common cause.

2.4 Hydraulic and Pneumatic Systems

Hydraulic Systems:

Used for high-force applications like landing gear and flaps.

Pressure Relief Valve: Opens at a preset pressure to prevent damage to components from over-pressurisation.
Thermal Relief Valve: Relieves pressure caused by fluid expansion when the fluid heats up (e.g., during prolonged ground operation), preventing damage to seals and components.
Low Fluid Level: Can cause air to enter the system, leading to pump cavitation and incomplete actuator movement.

Pneumatic/Vacuum Systems:

Primary Source: An engine-driven compressor or vacuum pump.
Vacuum System Purpose: To spin the gyros in the attitude indicator (artificial horizon) and heading indicator (directional gyro).
Suction Relief Valve: Regulates the vacuum pressure to a preset value (typically 4.5 to 5.5 inches of mercury) to ensure the gyroscopic instruments operate correctly.
System Faults: If the vacuum gauge indicates normal pressure but the attitude indicator is faulty, the most probable cause is a defective gyro rotor (e.g., worn bearings). A leaking vacuum line would reduce the vacuum pressure.

2.5 Electrical Power Systems

The electrical system provides power for starting, ignition, instruments, lights, and other accessories.

Bus Bar: A common connection point that distributes electrical power from a source (e.g., battery or generator) to multiple circuits.
Split Bus Bar: Provides redundancy by isolating essential loads on separate buses, each powered by its own source (e.g., one from the battery, one from the alternator).
Ammeter: Indicates the charge or discharge state of the battery.
A discharge indication with the engine running means the battery is supplying current because the alternator output is insufficient. This can be caused by a broken or slipping alternator belt, a faulty regulator, or an open field circuit.
A charge indication means the alternator is supplying more current than the system is using.
Circuit Breakers: Protect circuits from overcurrent. A jammed actuator will cause the motor to draw excessive current, tripping the circuit breaker.

2.6 Powerplant (Piston Engines)

Lubrication Systems:

Wet Sump: The oil is stored in a sump in the engine crankcase.
Oil Cooler: Regulates oil temperature, preventing overheating (which reduces viscosity and lubrication) and overcooling (which can cause sludge formation).
Low Oil Pressure at High RPM: A classic cause is worn bearings, which allow more oil to escape, reducing system pressure. A relief valve stuck open would cause low pressure at all RPMs.

Compression Testing:

Purpose: To assess the condition of the cylinders, piston rings, and valves.
Procedure: The piston must be brought to exactly TDC on the compression stroke. This ensures both valves are closed, so the applied air pressure will not push the piston down and rotate the propeller, which is a safety hazard. It also ensures an accurate reading.
Interpreting Results: High leakage (e.g., 40%) can be caused by worn rings, leaking valves, or a cracked head. The source can be identified by listening for air escaping from the exhaust (exhaust valve), intake (inlet valve), or crankcase (piston rings).

Fuel and Induction Systems:

Boost Pump: Used primarily for starting and to prevent vapour lock in the fuel feed lines.
Carburettor Heat: Used to melt or prevent ice formation in the venturi and throttle plate by directing hot air from a heat exchanger around the exhaust system.
Fuel Vent System: The vent line is designed with an angled outlet facing into the airflow to create a slight positive pressure at the vent, which helps prevent fuel siphoning from the tank while still allowing the tank to breathe.

Exhaust Systems:

A crack in the exhaust manifold can allow exhaust gases to escape, reducing back pressure and causing a slight loss of power. More critically, it can allow carbon monoxide to enter the cabin, which is a serious safety hazard.

Propellers:

Fixed-Pitch: The blade angle is fixed. Engine RPM changes directly with throttle position, and manifold pressure follows the same trend.
Constant-Speed: The blade angle is automatically adjusted by a governor to maintain a selected RPM. The governor varies oil pressure to the propeller hub. Oil streaks from the hub are common; minor seepage is acceptable, but significant leaks indicate seal wear.
Wooden Propellers: The varnish finish is critical to prevent moisture absorption. A crack in the varnish must be treated seriously; the propeller should be removed from service and inspected/repaired by an approved facility. Small nicks on metal blades within limits can be dressed out to prevent stress risers.

2.7 Instrumentation (Pitot-Static and Gyroscopic)

Pitot-Static System:

Static Port: Provides ambient static pressure to the altimeter, VSI, and airspeed indicator.
Blocked Static Port: Traps the static pressure inside the instruments at the pressure when the blockage occurred.
During a climb, the trapped pressure is higher than ambient, so the altimeter reads lower than actual altitude, and the VSI will erroneously indicate a climb.
Functional Test: Applying a vacuum to the static port reduces the pressure, simulating an increase in altitude. The altimeter should read a higher altitude, and the VSI should indicate a climb.
Cleaning: Static ports must be kept clear. Approved cleaning methods include using a soft brush or low-pressure air. Sharp tools can damage the port.

Gyroscopic Instruments:

Attitude Indicator (Artificial Horizon): Driven by the vacuum system. If the vacuum pressure is normal but the instrument is faulty, the most likely cause is a defective gyro rotor.

2.8 Fire Protection

Firewall: A fire-resistant bulkhead that separates the engine compartment from the cockpit/cabin. Its primary purpose is to contain an engine fire and protect the occupants.
Fire Detection System: Provides a warning to the flight crew. It does not extinguish the fire.
Fire Extinguishing Agent: Halon (or its replacements) is commonly used in engine nacelle systems because it is effective and leaves no residue.

2.9 Cabin Heating and Ventilation

Heat Source: Many piston aircraft use a shroud around the exhaust manifold to heat outside air for cabin heating.

3. Important Formulas, Regulations, and Procedures

Regulatory Framework: The module is based on Regulation (EU) No 1321/2014, Annex III (Part-66) , specifically Appendix I for the basic knowledge syllabus. Maintenance must be performed in accordance with Part-145 requirements, using approved data such as the AMM (Aircraft Maintenance Manual) and SRM (Structural Repair Manual).
Cable Tension: Tension is specified in the AMM in units of force (e.g., lbf or N) and is temperature-dependent. It is measured with a tensiometer.
Broken Wire Limits:
Primary control cables: Any broken wire is cause for replacement.
Secondary control cables: Typically 3 broken wires in any 6-inch length.
Vacuum System Pressure: Typically regulated to 4.5 to 5.5 inches of mercury (inHg).
Compression Test: The piston must be at TDC on the compression stroke to ensure both valves are closed, preventing propeller rotation and ensuring an accurate reading.
Landing Gear Retraction Test: The aeroplane's weight and CG must be within the limits specified in the AMM.

4. Common Relationships Between Concepts

Aerodynamics and Structures: The aerodynamic forces on the wing (lift) create structural loads (bending, torsion). The structure must be designed to carry these loads without excessive deformation. Damage to the structure (e.g., a dent) can alter the aerodynamic flow and reduce the structural margin.
Flight Controls and Rigging: Correct rigging (cable tension, control surface deflection) is essential for correct aerodynamic response and acceptable control forces. Excessive free play can lead to control flutter.
Powerplant and Systems: The engine drives the alternator (electrical), vacuum pump (pneumatic), and hydraulic pump (hydraulic). A failure in the engine drive (e.g., a broken belt) will affect all these systems.
Pitot-Static and Instruments: The altimeter, VSI, and airspeed indicator all rely on the pitot-static system. A blockage in the static port will affect all three instruments, leading to erroneous readings.
Landing Gear and Hydraulics: The landing gear is often hydraulically actuated. Low fluid level, pump failure, or a mechanical jam can all prevent gear extension or retraction.

5. Typical Exam Focus Points

Identification of Primary vs. Secondary Structure and Controls: Knowing which components are critical for airworthiness.
Corrosion Types and Treatments: Recognising different corrosion forms and the correct action for each (e.g., surface corrosion on aluminium vs. steel).
Control Cable Inspection Criteria: Knowing the exact rejection limits for broken wires in primary and secondary cables.
Pitot-Static System Faults: Understanding the effect of a blocked static port or pitot tube on the altimeter, VSI, and airspeed indicator.
Landing Gear System Components: The function of uplocks, downlocks, squat switches, shimmy dampers, and the procedures for retraction tests.
Hydraulic and Pneumatic System Components: The function of pressure relief valves, thermal relief valves, and suction relief valves.
Piston Engine Maintenance: The correct procedure for a compression test and the interpretation of results. The causes and effects of low oil pressure.
Propeller Maintenance: The difference between fixed-pitch and constant-speed propellers, and the specific maintenance concerns for wooden propellers (varnish integrity).
Troubleshooting Logic: The ability to systematically identify the most likely cause of a system failure, starting with the simplest and most common causes (e.g., checking fluid levels, inspecting belts, checking bulbs).
Use of Approved Data: Emphasising that all maintenance actions must be performed in accordance with the AMM, SRM, or other approved data. Never perform an unapproved repair.