Eleanor Finch
Aircraft propeller repair is a complex and highly regulated area of maintenance. While the FAA does not provide a specific definition of a 'major repair', it is generally understood that a repair constitutes any maintenance that restores a type-certificated product to a condition for safe operation. Propeller repairs are considered major when they involve significant alterations to the propeller's original specifications, such as repairs to steel blades, changes to the weight or balance, or structural repairs using welding, plating, or metalizing methods. These repairs require specific certifications and inspections to ensure the propeller's performance, structural integrity, and airworthiness are not compromised. Corrosion and physical damage are common issues, and even minor repairs must be carefully executed to prevent impairing the propeller's functionality.
Characteristics of what constitutes a major repair to an aircraft propeller
| Characteristics | Values |
|---|---|
| Repairs to steel blades | Straightening or any other repairs |
| Separation or disassembly | Crankcase or crankshaft of a reciprocating engine with integral supercharger |
| Separation or disassembly | Crankcase or crankshaft of a reciprocating engine with spur-type propeller reduction gearing |
| Special repairs | Structural engine parts by welding, plating, metalizing, or other methods |
| Corrosion | Surface corrosion, pitting, and intergranular corrosion |
| Physical damage | Nicks, dings, and cracks on blades and the hub |
| Major repairs | Performed by an FAA Certified Repair Station or a certificated mechanic with a powerplant rating |
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What You'll Learn
Major repairs require an FAA-approved repair station
Repairs and alterations are the two types of maintenance that are performed on aircraft. Repairs are done to restore a type-certificated product to a "condition for safe operation", whereas alterations involve adding or removing from the type-certificated product's configuration.
A certificated mechanic with a powerplant rating can approve and return to service a powerplant or propeller, or any related part or appliance, of an aircraft with a special airworthiness certificate. This is only possible after performing and inspecting a major repair or major alteration, provided the work was performed in accordance with instructions developed by the manufacturer or a person acceptable to the FAA.
The FAA has laid down certain guidelines to determine what constitutes a major repair. According to the FAA’s Advisory Circular AC 20-37E, "Limited minor repairs may be made on propellers by appropriately rated maintenance technicians either on the aircraft or when the propeller is removed." Minor dents, cuts, scars, scratches, and nicks may be removed as long as their removal does not weaken the blade, substantially change weight or balance, or impair performance.
The most common forms of physical propeller damage include nicks, dings, and cracks on both the blades and the hub of the propeller. The most insidious cause of propeller damage is corrosion, which includes external corrosion that is visible on the blades, as well as internal corrosion that affects the components within the hub of a variable-pitch propeller.
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Repairs to steel blades
Steel blades are more robust than aluminium blades in terms of erosion and impact resistance. The leading edge of a composite steel blade is made of stainless steel, which is three to four times harder than that of an aluminium blade. Composite steel blades can withstand more impact, and while they may suffer some damage, they can usually still fly until the next service date.
Aluminium blades, on the other hand, are more susceptible to damage. Repairs involve removing the damaged area and using metalworking tools to restore a smooth finish. However, repeated repairs will result in a thinner blade, and the blade will need to be replaced after two or three overhauls.
Composite steel blades, while very durable, are not immune to damage. The most common types of physical damage to propellers include nicks, dings, and cracks on the blades and the hub. Composite blades can be repaired and restored to their original shape and aerodynamics. Repairs to composite blades involve replacing material lost to gouges and other damage.
Before attempting any repairs, it is crucial to identify the type of blade, whether it is made of Kevlar or carbon fibre. The Hartzell Propeller Composite Propeller Blade Field Maintenance and Minor Repair Manual provides detailed instructions on composite propeller repairs and can help determine whether repairs can be done independently or require professional assistance.
It is important to note that even minor damage to a propeller can escalate quickly under the intense forces of flight. Therefore, regular inspections and maintenance are essential to ensure the safety and performance of the aircraft.
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Corrosion and physical damage
Intergranular corrosion is a less common form of corrosion that occurs within the metal structure itself. It is often caused by issues in the metal casting, although it can also be caused by trapped moisture under decals or around bolt holes. To prevent corrosion, it is important to keep the aircraft dry and completely dry the propeller after each use. Investing in quality propeller covers can also protect the propeller from corrosive agents, oxygen, and moisture.
In addition to corrosion, propeller blades are susceptible to straightening or steel blade repairs. These issues require major repairs, as classified by the FAA. Physical damage to the propeller hub can also occur, requiring repair or replacement of the hub. Regular inspections are crucial to identify any visible damage, and propeller shops offer detailed inspections using ultrasonic, eddy current, dye penetrant, and magnetic particle methods. Implementing a strict inspection and maintenance schedule is the best way to prevent corrosion and ensure the propeller's integrity.
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Powerplant major repairs
Separation or disassembly of the crankcase or crankshaft
This is considered a major repair if it involves a reciprocating engine equipped with an integral supercharger or other propeller reduction gearing besides spur-type.
Special repairs to structural engine parts
Any repairs to the structural engine parts that require welding, plating, metalizing, or other similar methods are considered major repairs.
Changes to the engine's basic design
Any changes to the basic design of the engine's fuel, oil, cooling, heating, electrical, hydraulic, de-icing, or exhaust systems are considered major alterations.
Changes to the empty weight or balance
Any alterations that result in an increase in the maximum certified weight or centre of gravity limits of the aircraft are also considered major.
Powerplant major alterations
Powerplant major alterations refer to changes in the powerplant that are not listed in the aircraft specifications issued by the FAA. These may include alterations to the engine's configuration, weight, balance, structural strength, performance, or other qualities affecting airworthiness.
It is important to note that the person performing the maintenance, such as a mechanic or repairman, is responsible for determining whether a repair or alteration is major or minor. Repairs are generally considered maintenance that restores a type-certificated product to a "condition for safe operation," while alterations involve adding to or removing from the product's configuration.
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Airframe major alterations
- Elements of an airframe: This includes alterations to spars, ribs, fittings, shock absorbers, bracing, cowling, fairings, and balance weights. Any changes to these structural components would be considered major as they play a crucial role in the aircraft's structural integrity and performance.
- Hydraulic and electrical actuating systems: Changes or repairs to these systems are considered major. These systems are responsible for the movement of various aircraft components, such as flaps and landing gear, and any alterations must be carefully evaluated to ensure they meet safety standards.
- Changes to empty weight or balance: Any modifications that result in an increase in the maximum certified weight or centre of gravity limits of the aircraft are major. This is because altering the weight and balance can significantly impact the aircraft's flight characteristics and stability.
- Changes to basic aircraft systems: Major alterations include changes to the fundamental design of fuel, oil, cooling, heating, cabin pressurization, electrical, hydraulic, de-icing, or exhaust systems. These systems are critical for flight operations and passenger comfort, and any modifications must adhere to strict regulations.
- Changes to the wing or control surfaces: Alterations that affect the flutter and vibration characteristics of the aircraft are considered major. This includes changes to fixed or movable control surfaces, such as ailerons or flaps, which play a crucial role in the aircraft's aerodynamic performance and stability.
It's important to note that the distinction between major and minor alterations is crucial in aircraft maintenance. Minor alterations require "acceptable" data as per FAA guidelines, whereas major alterations require explicit approval from the FAA, a designated engineering representative (DER), or the aircraft manufacturer. This approval process ensures that any significant changes to the aircraft meet the necessary safety and performance standards.
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Frequently asked questions
Repairs to a propeller that involve any repairs to, or straightening of, steel blades are considered major repairs. This also includes any special repairs to structural engine parts by welding, plating, metalizing, or other methods.
Minor repairs to an aircraft propeller include the removal of minor dents, cuts, scars, scratches, and nicks. These repairs do not weaken the blade, substantially change weight or balance, or impair performance.
A certificated mechanic with a powerplant rating can perform major repairs to an aircraft propeller.
