ATPL Aircraft General Knowledge: What's it all about?

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• Number of questions in exam: 80

• Exam duration: 2 hours

• Pilot Theory Online difficulty rating: Medium/Hard

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In the journey towards getting your ATPL, mastering the AGK syllabus is essential. This guide provides an overview of the subject, discussing design, functions, and maintenance requirements. It covers everything from the airframe and hydraulics to flight controls, anti-icing systems, and engines, providing foundational knowledge for understanding modern aircraft.

System Design: Loads, Stresses, and Maintenance

Loads and Stresses

Aircraft structures must withstand a variety of loads and stresses, including aerodynamic forces, weight from fuel and passengers, and stress from manoeuvres. Designers use high-strength materials like aluminium alloys, titanium, and carbon composites, and engineers calculate safety margins to prevent structural failures. Fatigue life is a key consideration, as constant loading and unloading can lead to metal fatigue over time, which is mitigated through regular inspections and maintenance.

Key points:

• Stress: The distribution of force per unit area (usually measured in Pascals or Newtons per square meter). Stresses can lead to strain (deformation) or even failure of structural components.

• Strain: The measure of deformation in response to stress being applied to a material.

  
  

Maintenance

Aircraft undergo various maintenance checks, categorized as A, B, C, and D checks, which differ in frequency and depth. These checks involve inspecting structural integrity, testing systems, and replacing components as necessary to ensure airworthiness. This systematic approach is crucial for identifying issues before they compromise safety.

Key points:

• Hard time maintenance: Components or systems are replaced, overhauled, or inspected based on fixed intervals. These intervals are typically defined in terms of flight hours, flight cycles (take-offs and landings), or calendar time (e.g., annually). Engines are a good example of components that are maintained like this.

• On condition maintenance: A more flexible approach where components are inspected and maintained based on their actual condition, for example the tyres or display units (screens).

Airframe

The airframe is the primary structure of the aircraft, encompassing the fuselage, wings, and tail. Modern airframes are made of lightweight, durable materials like carbon fibre composites, which provide strength without compromising fuel efficiency. The airframe must be aerodynamically designed for optimal performance, and it integrates various systems that contribute to both safety and comfort.

Key points:

• Monocoque: A type of construction where the outer skin carries most of the stresses. This design has no internal framework, and the skin is reinforced to handle all loads.

• Semi-Monocoque: More common in modern aircraft, this structure includes a combination of a skin and internal framework. The internal framework adds strength and allows the skin to bear the stresses.

Hydraulics

Hydraulic systems play a critical role in aircraft operation, providing the necessary force to operate high-power-demand systems, including the landing gear, flight controls, and brakes. Hydraulics work by transmitting force through incompressible fluid in pipes, allowing powerful and precise movements.

Key points:

• Reservoirs: Containers that holds the hydraulic fluid. They ensure that there is an adequate supply of fluid at all times and phases of flight.

• Pumps: The pumps moves the hydraulic fluid and supply pressurised fluid to components. They are usually powered by the aircraft's engines or the electrical system.

• Actuators: Hydraulic actuators convert hydraulic energy into mechanical movement to operate components like flight control surfaces and the landing gear.

Landing Gear, Wheels, Tyres, and Brakes

Landing Gear

The landing gear supports the aircraft during ground operations and absorbs the impact of landing. It includes wheels, shock absorbers, and retraction mechanisms. Landing gear is typically retractable in larger aircraft to reduce drag and improve aerodynamics.

Key points:

• Tricycle Gear: The most common configuration, where the aircraft has a nose gear and two main landing gear units located under the fuselage or wings. This configuration offers great stability during take-off, landing, and taxiing.

• Taildragger Gear: An older configuration where the aircraft has main gear located forward, and a tailwheel at the rear. This is more common in older or aerobatic aircraft and provides better performance on rough fields but less stability during ground operations.

Wheels

There are many different types of wheel configuration, depending on the type of aircraft.

Key points:

• Single Wheels: Common on smaller aircraft and some light aircraft configurations, single wheels are used primarily on the main landing gear.

• Dual Wheels: Found on larger aircraft, dual wheels are mounted in tandem (side-by-side) to distribute the weight more evenly and increase the load capacity.

• Multiple Wheels: For very large aircraft, especially those with higher gross take-off weights, there may be three or more wheels on each main gear, typically found on aircraft like the Airbus A380 and the Boeing 747.

Tyres

Aircraft tyres are specially designed to handle the high loads and speeds involved in take-off, landing, and taxiing. They’re constructed with multiple layers for durability and include a fusible plug to prevent explosion under extreme heat, as during a hard landing or brake failure.

Key points:

• Tyre air: Aircraft tyres are almost always filled with nitrogen, as it is more temperature stable.

• Speed limitations: Aircraft tyres have a speed limit, this is 225mph in the 737s that I fly.

Brakes

Modern aircraft use hydraulic brakes, with large commercial airliners featuring advanced braking systems that can include anti-skid functions, which prevent wheel lockup during high-speed braking. Carbon brakes are increasingly common due to their durability and heat tolerance.

Key points:

• Anti-skid: This system prevents the wheels from locking up during braking, ensuring maximum braking efficiency and safety.

• Autobrakes: This system automatically applies the wheel brakes during landing or rejected take-off, reducing pilot workload and ensuring consistent deceleration.

Flight Controls

Flight controls allow the pilot to manipulate the aircraft’s attitude and trajectory. Fly-by-wire technology is increasingly used in modern aircraft, where electronic signals replace mechanical linkages, offering more precise control, safety limitations and reduced weight.

Key points:

• Primary controls: Elevators, ailerons, and rudder control pitch, roll, and yaw, respectively.

• Secondary controls: Flaps, slats, and spoilers aid in lift generation and drag control, particularly during take-off and landing.

Pneumatics – Pressurisation and Air Conditioning Systems

Pressurisation

Aircraft are pressurized to maintain a cabin altitude that ensures passenger comfort and safety at high altitudes.

Key points:

• Outflow Valves: Regulate cabin pressure by controlling the amount of air leaving the cabin.

• Pressure Relief Valves: Prevent over-pressurization by releasing air if the pressure exceeds safe limits.

Air Conditioning

In flight, the air in the cabin comes from the compressors of the aircraft's engines. This air needs to be conditioned in regard to temperature, and pressure, before it can enter the cabin.

Key points:

• Ground air conditioning units: Sometimes used when on the ground. These units supply conditioned air (either hot or cold) to the aircraft via a hose connected to the aircraft's air conditioning system.

• Recirculation fans: These circulate air that has already been conditioned, to reduce the load on the aircraft's packs

Anti-Icing and De-Icing Systems

Anti-icing

Ice can really ruin your day as a pilot. Almost all transport aircraft have complex anti-icing systems, to keep the aircraft clear of ice in almost all phases of flight

Key points:

• Thermal anti-ice: Hot air from some of the compressor stages of the engine heats large parts of the aircraft, like the engine cowlings and the leading edge of the wings.

• Electric anti-ice: Small components like the pitot probes and TAT probes are heated electrically.

De-icing

Removes ice after formation.

Key points:

• De-icing boots: These are fitted mainly to turboprop aircraft. The boots inflate/expand which cracks the ice, making it fall off. The boots are typically fitted to the leading edges of the wings and tail.

• Fluid de-icing: Before take-off, contamination on the aircraft almost always has to be removed. Ground service providers do this with de-icing/anti-icing fluid or in some circumstances, hot air.

Fuel System

Aircraft fuel systems are responsible for storing and delivering fuel to the engines. They include fuel tanks, pumps, filters, and valves to manage fuel flow efficiently. Fuel is typically stored mainly in the wings to help maintain balance and reduce structural stress.

Key points:

• Jet A: The freezing point of jet A is typically -40°C

• Jet A-1: The freezing point of jet A is typically -47°C

• Jet B: The freezing point of jet A is typically -60°C

Electrics

Modern aircraft rely heavily on electrical systems for navigation, communication, lighting, and more. Redundant systems ensure that critical functions remain operational even if one power source fails.

Key points:

• Alternators/Generators: Produce power from engines.

• Batteries: Provide backup power in emergencies.

• Electrical Buses: Distribute power across systems.

Piston Engines

Piston engines, commonly used in smaller general aviation aircraft, operate similarly to car engines, using a series of combustion cycles to generate power. Fuel is mixed with air, compressed, and ignited to produce energy, which drives a propeller. Although less powerful than turbine engines, piston engines are efficient at lower altitudes and speeds.

Key points:

•  Piston: A cylindrical piece of metal that moves up and down inside a cylinder, to generate pressure and power.

• Cylinder: Contains the piston.

• Crankshaft: Converts the linear motion of the pistons into rotational motion

Turbine Engines

Turbine engines, used in most commercial aircraft, are more powerful and efficient at high altitudes. They work by compressing incoming air, mixing it with fuel, and igniting the mixture to produce a high-speed exhaust. This thrust propels the aircraft forward.

Key points:

• Turbojet: The simplest form, where all the air is passed through the engine and exhausted at high speed to produce thrust.

• Turbofan: The most common engine used in commercial aviation. A portion of the air is bypassed around the core of the engine, creating additional thrust with lower fuel consumption.

• Turboprop: Similar to a turbofan but uses a turbine to drive a propeller, typically used in smaller regional aircraft.

• Turboshaft: Similar to a turboprop but the power is used to drive a shaft, commonly used in helicopters.

Protection and Detection Systems

Protection systems

In AGK, this can refer to fire protection, and rain protection. Fire protection is a much bigger part of the syllabus.

Key points:

• Fire protection: The 737 has two fire bottles fitted, which can be fired into either engine (one bottle in each engine or both bottles in one engine). The bottles contain Halon 1121.

• Rain protection: Some aircraft have a rain repellent system which sprays a hydrophobic fluid onto the windscreen, to help with seeing out of the windscreen in heavy rain. This is rarely used in practice.

Detection systems

How to we know when to fire the extinguishing agents into the engines, without some form of indication that there's a fire? By trusting out detection systems! Note, we have fire detection (and protection) in many parts of the plane, not just the engines.

Key points:

• Overheat detection: Fire loops in certain parts of the engine sense an increase in temperature, and trigger a warning when a certain temperature is reached.

• Smoke Detection: Simply smoke alarms! These are fitted in the holds of the aircraft, where your suitcases are kept. They trigger alarms when they detect smoke.

Oxygen Systems

Oxygen systems ensure that both passengers and crew can maintain normal bodily functions at high altitudes where oxygen levels are low.

Key points:

• Passenger Oxygen: Masks that deploy automatically in case of depressurization.

• Crew Oxygen: Special masks with a dedicated supply to allow the crew to manage emergency situations effectively.

  
  

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