11.10 Fuel Systems

Introduction to Aircraft Fuel Systems

• Aircraft fuel systems store and deliver fuel to engines and APUs under all operating conditions
• Fuel systems consist of tanks, pumps, filters, valves, lines, and monitoring devices
• The fuel system ensures consistent, contaminant-free fuel flow during all flight phases
• Fuel load is a significant weight factor, requiring a strong supporting structure
• Fuel systems prevent pumps from extracting from multiple tanks simultaneously to avoid air introduction

Fuel System Layout

• Fuel tank location, size, shape, and construction vary based on aircraft type and use
• Tanks are made of materials that resist chemical reactions with aviation fuel
• A sump and drain at the tank's lowest point allow for contaminant and water collection
• Tank tops are vented to the atmosphere
• Baffles in most tanks (except smallest) minimize fuel surging during aircraft movement
• Expansion space allows for fuel volume increase due to temperature changes
• Jettison valves in some tanks allow fuel dumping to reduce landing weight
• The fuel storage system includes several tanks, usually within the wings
• Auxiliary tanks in the fuselage or horizontal stabilizer increase range
• Trim tanks store fuel and are used for aircraft trim
• Vent/surge tanks in wingtips vent fuel tanks properly

Fuel Lines and Fittings

• Aircraft fuel lines are rigid or flexible, depending on location and application
• Rigid lines are made of stainless steel or aluminum alloy, connected with MS or AN fittings
• Stainless steel lines protect against debris, abrasion, and heat damage
• Flexible hoses connect vibrating components like the engine and airframe
• Flexible fuel hoses have a synthetic rubber interior, fiber braid wrap, and synthetic exterior
• Some flexible fuel hoses feature a braided stainless-steel exterior
• Protective fire sleeves, secured with steel clamps, are often wrapped around fuel lines
• Leaking fittings can be re-tightened initially, but overtightening must be avoided
• Damaged fittings or fuel lines should be replaced after depressurizing and inspection
• All metal fuel lines and components must be electrically bonded and grounded for static electricity flow
• Bonded clamps secure rigid fuel lines at specified intervals per the maintenance manual

Types of Aviation Fuel

• Aircraft engines must burn the fuel type specified by the manufacturer only
• Mixing different fuel types is strictly prohibited
• Two basic types of fuel: AVGAS for reciprocating engines and jet fuel for turbine engines

Aviation Gasoline (AVGAS)

• AVGAS is specifically formulated for use in aircraft reciprocating engines
• AVGAS is a hydrocarbon compound refined from crude oil via fractional distillation

Jet Fuel

• Jet fuel is designed for use in turbine engines
• It cannot be mixed with AVGAS or used in reciprocating engine fuel systems
• Jet fuel is kerosene-type, with a higher flash point, making it less flammable

Turbine Engine Fuel Types

• Three basic turbine engine fuel types available: Jet A1, Jet A, and Jet B
• Jet A1 is the most popular worldwide
• Jet A1 and Jet A are fractionally distilled within the kerosene range, with low volatility and vapor pressure
• Flashpoints for Jet A and Jet A1 range from 43 °C to 65 °C (110 °F to 150 °F)
• Jet A freezes at –40 °C (–40 °F), and Jet A1 freezes at –47 °C (–52.6 °F)
• Jet B is a blend of kerosene and gasoline, used in colder climates for better cold-weather performance, freezing point of approximately –50 °C (–58 °F)
• Jet A1 and Jet A density is 0.81 kg/litre at 15 °C (59 °F)

Fuel Colour Code

• Dyes help monitor and identify fuel types
• 100LL AVGAS is dyed blue and is most readily available, mainly used in the US
• Some 100 octane or 100/130 octane fuels may still be available and is dyed green
• 82UL (unleaded) AVGAS is dyed purple and used for low-compression engines
• 115/145 AVGAS is dyed purple and designed for large, high-performance reciprocating engines from the World War II era, it's available only by special order from refineries.
• All grades of jet fuel are colourless or straw-coloured, distinguishing them from AVGAS
• Jet fuel fill hose nozzles are purposefully too large to fit into an AVGAS tank fill opening

Fuel Tanks Introduction

• Aircraft fuel tanks are a major component of aircraft fuel systems
• Tanks are classified as internal or external, and by construction method or intended use

Fuel Tanks

• Three types of aircraft fuel tanks: rigid removable, bladder, integral
• Fuel tank type depends on aircraft type, design, intended use, and age
• Most fuel tanks are made of non-corrosive material and ventilated via a vent cap or line
• Aircraft fuel tanks have a low area called a ‘sump’ for contaminant and water settlement
• Baffle plates are used in most tanks to control fuel movement during flight maneuvers

Rigid Removable Fuel Tanks

• Rigid removable fuel tanks are built with various materials and strapped into the airframe structure
• Tanks are riveted or welded and include baffle plates and other fuel tank features
• Materials include 3003 or 5052 aluminum alloy or stainless steel
• Seams are riveted and welded to prevent leaks
• Early tanks were made of thin steel coated with a lead/tin alloy called terneplate
• Removable tanks are secured to the airframe with padded straps

Bladder Fuel Tanks

• Bladder tanks are made of reinforced flexible material, used instead of rigid tanks
• Bladder tanks have similar features to rigid tanks but do not require a large opening for installation
• The tank is rolled up and placed into the structural bay/cavity through a small opening, then unrolled
• Bladder tanks are attached to the structure with dedicated fastening devices, lying smooth and unwrinkled
• Support boards, often fiberglass, prevent chaffing on raised internal structures
• Bladder tanks are used on all sizes of aircraft, they are strong and have a long life
• Leaks can be patched following the manufacturer's instructions

Integral Fuel Tanks

• Transport category and high-performance aircraft use wing/fuselage structure to form a sealed fuel tank
• A fuel-resistant two-part sealant seals the tank
• Sealed skin and structure provide the greatest volume and lowest weight
• Wings are the most common location and are known as "wet wings"
• Baffles prevent fuel movement during aircraft maneuvers
• Baffle check valves allow fuel to move inboard but prevent outboard movement
• Access panels allow inspection and maintenance of system components inside the tank, engineers enter the fuel tank to conduct maintenance
• Wing access aluminum panels are sealed with an O-ring and aluminum gasket for electrostatic bonding, an outer clamp ring secures the panel

Nitrogen Generating System (NGS)

• NGS improves safety by reducing fuel tank explosion risk
• It replaces oxygen with nitrogen to reduce fuel-air mixture flammability

Historical Background of NGS

• Aircraft accidents linked to fuel tank explosions highlighted the need for NGS
• The explosion of TWA Flight 800 in 1996 spurred research into reducing explosion risks
• NGS aims to keep oxygen concentration in the tank's ullage below combustion levels

How the NGS Works

• NGS produces nitrogen-enriched air (NEA) by removing oxygen from compressed air
• The Air Separation Module (ASM) uses hollow fiber membranes to separate nitrogen and oxygen

Detailed Process of NGS:

• The system sources compressed air, primarily from the aircraft’s engines (known as bleed air)
• The air is cooled by passing it through heat exchangers
• Cooled air is passed through filters
• Air enters the ASM, where oxygen and nitrogen are separated.
• Oxygen molecules diffuse through the fiber's walls more rapidly than nitrogen molecules and is vented out of the system.
• Nitrogen-enriched air is directed into the aircraft's fuel tanks
• The oxygen that is separated from the incoming air during the process in the ASM is vented overboard

Nitrogen Output

• Nitrogen is constantly fed into tanks to displace outside air entering due to pressure equalization
• Pressure valves regulate flow to prevent overfilling
• Nitrogen is quickly distributed without fans

NGS Operation Modes:

• Low-flow mode conserves bleed air and prolongs ASM life, used during climb and cruise altitude
• High-flow mode increases nitrogen flow to maintain oxygen content at a maximum of 12% on landing, used during descent when air pressure rises

Identification and Classification of Fuel Leaks

• Aircraft fuel system integrity is critical, and any defects or leaks must be addressed
• Fire, explosion, or fuel starvation risks make fuel system defects a top priority
• Follow manufacturer's maintenance instructions

Location of Leaks and Defects

• Knowledge of fuel system operation is essential for troubleshooting
• Maintenance manuals provide troubleshooting flow diagrams
• Defects can be located by tracing fuel flow from the tank to the engine
• Close visual inspections are needed for suspected leaks, look for connection points of two fuel lines or a fuel line and a component that could have an internal leak
• Leaks produce marks/stains and a stronger smell

Gaskets, Seals, and Packings

• Leaks can be repaired by replacing gaskets or seals
• New gaskets/seals must be installed when a component is replaced or reassembled
• Ensure mating surfaces are clean, and grooves allow a tight seal. Inspect new gaskets and seals for any damage

Fuel Tank Repairs

• All fuel tanks (rigid, bladder, or integral) can develop leaks
• Conduct all fuel tank repairs following the manufacturer's instructions

Fuel Leak Classification

• Four classifications describe aircraft fuel leaks: stain, seep, heavy seep, running leak
• Monitor the surface area of collected fuel for 30 minutes for classification

Fuel Leak Classifications

• Stain: less than 3/4 inch (19 mm) in diameter
• Seep: 3/4 to 1.5 inches (19 to 38 mm) in diameter
• Heavy seep: 1.5 inches to 4.0 inches (38 to 101 mm) in diameter
• Running leak: pools and drips from the aircraft

Fuel Tank Safety

• TWA Flight 800 was caused by an explosion of the Center Wing Fuel Tank (CWT)
• Contributing factors included a small amount of residual fuel in the center tank and continuous heat-up by the air conditioning packs.
• The NTSB concluded that the explosion was likely a short circuit outside the CWT

Key Recommendations

• The NTSB recommended FAA examine design and maintenance practices
• FAA issued SFAR 88 and amendments to 14 CFR, requiring fuel tank system design reviews, modification reviews, and improved maintenance
• SFAR 88 requires permanent fuel tank safety from manufacturers
• Operators must introduce improved maintenance programs
• Critical Design Control Configuration Limitations (CDCCL)

Critical Design Control Configuration Limitations (CDCCL)

• CDCCL refers to fuel system design features which must be maintained
• Features in an aircraft/component that must be retained during any modification, change, repair, or scheduled maintenance characterize CDCCL.
• The definition of CDCCL includes only information necessary to ensure the safety of fuel tank systems.
• They are mandatory and cannot be changed without FAA approval.

Ignition of Fuel Vapours

• Sealing electrical system components allows for pumps, fuel gauging elements, and wiring to be immersed in fuel tanks.
• Protection from possible ignition sources from these components is required.

Primary Causes of Fuel Vapour Ignition:

• Electrical arcs
• Filament heating
• Friction sparks
• Hot surface ignition or auto-ignition

Requirements

• Components in or adjacent to a fuel tank must meet standards preventing flammable fluid vapor ignition

Fuel Tank Entry and Closure

• Drain all fuel and follow safety procedures before entering a fuel tank
• Purge fuel vapors and use a breathing apparatus
• Position an additional safety person outside the tank

Fuel Tank Venting

• Vapor concentration must be below the lower explosive limit (LEL)
• Adequate oxygen level inside the tank is necessary
• Continual venting removes volatile fuel, vapours and vapours from cleaning solvents
• A fuel tank must be certified as 'safe'

Gas Detectors

• Fuel tank entry procedures minimize risks with numerous precautions
• Entry is limited to approved personnel
• Atmosphere is continuously monitored for toxic concentrations
• Adequate oxygen level must be maintained
• Gas detectors measure combustible vapour concentration
• Alarms trigger when concentration reaches explosive hazard levels (between LEL and UEL)

Fuel Tank Entry Requirements

• Fuel tanks are classified as “confined spaces”.

• Fuel tank entry is a team task, team members must be trained, certified, and authorized for the task which are the confined space entrant and the standby person.

• Specialised safety equipment is required to enable maintenance personnel to enter large aircraft fuel tanks, this includes:

  • Two separate filtered air supplies, each with a minimum air supply of 170 l/min
  • One for the confined space entrant’s use – primary rig
  • One for the standby person’s use – standby rig
  • Face mask respirator for each person

• Breathing equipment is only used to sustain life; it does not remove the risk of an explosion

Standby Person Duties

• Check all cylinders (bottles) are fully charged.
• Ensure hose connections are secure.
• Maintain and monitor the air supply and confined space entrant.
• Control hose movements and prevent tangles.
• Switch air supply if warning sounds or confined space entrant is in distress, and assist in the exit of the entrant.
• Monitor gauges: if primary air supply pressure falls below 85 psi (586 kPa), switch to emergency air and remove entrant from the fuel cell.

Fuel Tank Closure

• A written record of all maintenance items taken into a fuel tank should be maintained and updated and ensure that this record shows that there are no unwanted items left in the tank
• Prior closing, ensure that all maintenance items and unwanted materials are removed as they could cause damage to the fuel system or ultimately be the cause of a fire or explosion
• Prior to closing certain areas should be paid attention to, such as:
  • Fuel and water drain holes and paths.
  • Fuel Quantity Indicating Systems (FQIS) components.
  • Fuel scavenge system components.
• Prior to closing the fuel tank, it is important that an approved person conducts an inspection to ensure that there are no unwanted items remaining within the tank.

Fuel Supply Systems Introduction

• The fuel feed system is the heart of the fuel supply system since it delivers fuel to the engines.
• Manufacturers have their own fuel system designs, but all basic fuel system requirements and functions are similar.
• Aircraft fuel systems must store and deliver clean fuel to the engine(s) at a pressure and flow rate which can sustain operations, regardless of the aircraft operations.

Fuel Supply Systems

• The following fuel supply systems used on single reciprocating engine aircraft and jet aircraft are:
  • Gravity feed
  • Pressure feed
  • Injection feed

Gravity Fuel Feed System

• Gravity fuel feed systems use gravity to supply fuel to the engine fuel control mechanism.
• High-wing aircraft containing a fuel tank in each wing are common.

Pressure Fuel Feed System

• Low-wing aircraft cannot use gravity to feed fuel to the engines so an engine-driven and/or electric pump must be used.
• Electric pump supplies fuel pressure for starting and acts as backup

Injection Fuel Feed System

• Several high-wing, high-performance single-engine aircraft combine gravity flow with fuel pump(s)
• Fuel injection systems spray pressurised fuel into the engine intake or directly into the cylinders

Jet Aircraft Fuel Feed System

• Large jet aircraft have complex fuel systems with components not found in reciprocating engine aircraft fuel feed systems.
• They are separated into the following fuel subsystems:
  • Storage
  • Vent
  • Distribution
  • Feed
  • Indicating
• Most large jet aircraft use integral fuel tanks that use the bulk of the wing’s structure as a fuel tank.
• Centre wing or fuselage fuel tanks are also common.

Fuel Pumps Overview

• Hand-operated Pumps
• Vane-Type Fuel Pumps
• Centrifugal Pump
• Ejector Pumps

Hand Operated Pumps

• Older reciprocating engine aircraft use a hand-operated pump (wobble pump), which is a double acting pump which delivers fuel with each stroke of the pump handle.
• These vane-type pumps back up the engine-driven pump and transfers fuel or primes the engines during start-up.

Vane-Type Fuel Pumps

• Vane-type fuel pumps are used as engine-driven primary fuel pumps and as auxiliary or boost pumps.
• It is a constant displacement pump, moving a constant volume of fuel with each revolution of the pump.
• In engine-driven applications, the vane-type pump is driven by the accessory gearbox.
• Most vane-type pumps contain an adjustable pressure relief component which uses the pressure that builds up at the outlet of the pump to lift a valve off its base to regulate the flow and this excess fuel returns to the inlet side of the pump.

Centrifugal Pump

• Centrifugal pump is driven by an electric motor and is located just outside of the bottom of the tank, with the inlet of the pump extending into the tank, it’s the most common pump on aircraft, especially used on large high-performance aircraft.
• When the pump is mounted outside the tank, a pump removal valve is installed so the pump can be removed without draining the fuel tank
• Centrifugal boost pumps ensure positive pressure throughout the fuel system, preventing vapour lock.

Ejector Pumps

• Some aircraft use ejector pumps to assist in ensuring that liquid fuel is always at the inlet of the centrifugal pump.
• A small-diameter line circulates the pump outflow back into the section of the tank where the pump is located.
• The fuel is directed through a venturi that is part of the ejector which forms low pressure as the fuel rushes through the venturi

Fuel Filters

• Aircraft fuel systems have filters and strainers to ensure fuel is free from contaminants
• There must be a provision in place to drain a sample of fuel from tanks and the main strainer
• A mesh screen inside a wing fuel tank protects the boost pump inlet

Common Filter Types

• Micron filter
• Wafer screen filter
• Plain screen mesh filter
• The use of each of these individual filters is dictated by the filtering treatment required and its location.
• A porous cellulose material can remove foreign matter measuring from 10-25 microns.
• Fuel temperature can also be monitored for the possibility of a blockage caused by frozen water.

Dumping, Venting, and Draining Introduction

• Aircraft have maximum take-off weight and a maximum structural landing weight.
• If an aircraft lands overweight, it might suffer structural damage

Dumping/Jettison

• A dumping/jettison system allows the flight crew to dump fuel overboard to prevent exceeding the aircraft's maximum landing weight if there is an emergency.
• The jettison system ensures the aircraft is light enough but also has adequate fuel for a go-around.
• Minimum amount of fuel is kept so that the aircraft can reach the next airport.
• Jettison nozzle valves open to dump fuel via the jettison nozzles.
• Jettison control switch are guarded to prevent inadvertent operation.

Fuel Vent Systems

• Fuel vent systems are designed to prevent the build-up of pressure in the fuel tanks
• It also allows proper flow of fuel from the tanks regardless of the attitude or the quantity of fuel on board
• Over-pressurising the tanks can cause structural damage.

Surge Tanks

• Surge fuel tanks are normally empty and are designed to contain fuel overflow and prevent fuel spillage

Vent Float Valves

• A vent float valve consists of a valve body with a pad valve, a float, and a connection to a vent line.

Operation of a Vent Float Valve

• When the fuel level in the tank is low, the weight on the float keeps the valve in the open position.
• When the fuel level rises due to a high quantity or surging of the fuel, the float rises, and the valve closes.

Flapper Valves

• Flapper valves are fitted in the tank baffles
• These valves prevent fuel from travelling outboard during aircraft manoeuvres but allow fuel to drain inboard during normal operation.

Draining

• The purpose of a drain system is to drain water from the fuel tanks.
• Draining also provides access to the fuel tanks for maintenance

Manually Operated Drain Valves

• For safety reasons, drain valves are operated manually
• Some drain valves are located in difficult positions and are therefore designed to be opened electrically

Electrically Operated Drain Valves

• Electrically operated drain valves rely on a solenoid, which opens the valve when activated

Fuel Transfer Introduction

• It is important that the flight crew, as well as engineers, can move fuel between the various fuel tanks.
• The fuel transfer system is a series of fuel lines and valves that permits the movement of fuel from one tank to another onboard the aircraft.

In-flight Fuel Transfer

• To use fuel from another fuel tank during flight, due to a fuel imbalance in the tanks, the cross-feed valves must be selected to the ‘OPEN’ position by the flight crew on the flight deck.
• Some aircraft are equipped with auxiliary or reserve fuel tanks.

Ground Manual Fuel Transfer

• Ground manual fuel transfer is used to transfer fuel from one tank to another when the aircraft is on the ground
• Note: The defuel valve can only be opened when the aircraft is on the ground

Fuel Cross-feed Valve

• The fuel cross-feed valve allows fuel to flow between the left and right engine fuel feed manifolds - with the two engine fuel feed manifolds connected, one fuel tank supplies fuel to both engines.
• The valve position is shown with the alignment marks.

Actuator

• The actuator is a 28 Vdc motor, which has a manual override lever that allows the valve to operate without electrical power.

Fuel System Indications and Warnings Introduction

• Fuel indicating systems on aircraft monitor a variety of factors such as: To display current conditions To notify of unsatisfactory conditions
• Most aircraft include annunciator lights that illuminate when an event demands attention

Fuel Indicators, Gauges, and Warnings

• Fuel Flow Indicator: One for each engine and is used as the main method for monitoring fuel delivery to the engines
• Fuel temperature gauges and fuel filter bypass warning lights are provided as standard
• Fuel quantity gauges for all tanks, using a capacitance-type fuel quantity indication system and fuel totaliser
• The fuel quantity is indicated as a mass in kilograms, pounds (lbs), or tons Ratiometer Type, Electric
• Electric fuel quantity indicators are common
• The calibrated dial indicates the corresponding quantity of fuel

Capacitance Type

• Large and high-performance aircraft normally employ electronic fuel quantity systems
• They contain no moving parts in the tank units.
• Variable capacitance transmitters are installed in the fuel tanks and extend from the top to the bottom of each tank
• Signals provided for all tanks

Capacitors

• A capacitor is a device that stores electricity, the amount stored depends on:
  • The area of its plates
  • The distance between the plates
  • The dielectric constant of the material separating the plates

Mechanical Type

• Several aircraft with capacitance-type fuel indicating systems also use a mechanical indication system to cross-check fuel quantity indications and determine the amount of fuel onboard, for example a fuel measuring stick.

Ultrasonic Type Level Sensors

• Ultrasonic level sensors gauge the fuel level by measuring the distance from the transmitter, usually located at the top of a fuel tank, to the surface of the fuel inside the tank
• The sound waves’ time of flight is the direct function of how much space exists between the liquid surface and the top of the tank, i.e., ullage. Fillage becomes the natural measurement and ullage a derived factor

Fuel Selector Panels

• The fuel selector panel controls all fuel input to the engines and is achieved by using booster pumps and cross-feed switches

Fuel Temperature Gauges

• Monitoring the fuel temperature is critical and informs the flight crew when the temperature is causing ice to form in the fuel system and the fuel filter in particular

Fuel Pressure Gauges

• Monitoring fuel pressure gives the flight crew an early warning of a fuel system malfunction and is critical

Pressure Warning Signal

• Visual and audible warning devices are used along with fuel pressure gauge indications to draw the flight crew’s attention to certain situations

Valve-in-transit Indicator Lights

• An aircraft with multiple fuel tanks uses valves and pumps to move fuel to the engines or to jettison fuel
• Valve-in-transit lights notify the flight crew when a valve is opening or closing.
• Annunciator lights that show the valve position as OPEN or CLOSED, are also used.

Refuelling Systems Introduction

• The refuelling system of an aircraft consists of pipes, valves, controls, and indicators
• There are two methods of refuelling: gravity (overwing) refuelling and pressure refuelling.

Gravity (Overwing) Refuelling

• Gravity (overwing) refuelling is performed with an external pump and hose into the wing tanks by means of overwing filler ports.

Pressure Refuelling

• The normal method to refuel an aircraft is by pressurised refuelling either from a fuel truck (bowser) or an underground fuel storage supply system.

Ground Refuel/Defuel Selector Panels

• Refuelling control panels can be found at the wing leading edge, close to the refuelling couplings or on the fuselage and may require the use of a stand or step ladder for access

Main components of a typical refuelling system:

• The refuelling couplings
• The refuelling valves
• The refuelling manifold
• The diffusers
• For easy access, refuelling couplings are on the underside of one or both wings, or on the fuselage

Refuelling Modes

• Three modes of pressure refueling:
  • Manual
  • Automatic
  • Override

Refuelling Valves

• The refuelling valves in each tank control the fuel flow into the tanks during the refuel operations
• On older aircraft, these valves were manually operated, or motor-driven, but now they tend to be solenoid-operated valves

Refuelling and Defuelling Safety Precautions

• As part of the maintenance process, it may be required to refuel and defuel an aircraft prior to commencing any maintenance task and care must be taken when refuelling/defuelling an aircraft so that the operation is carried out as safely as possible and that all AMM procedures are followed.

Aircraft fuel Bowser Position and Safety Zones - Examples

• "No Smoking” signs must be displayed at a minimum distance of 15 m (50 ft) from fuelling equipment and aircraft tank vents
• Fire appliances must be readily available when all fuelling operations are taking place
• Whenever possible, aircraft must be fuelled in the open, and not in a hangar
• The correct type and grade of fuel are stated in the maintenance manual and are marked adjacent to the filler point(s).
• Engines must not be operated during refuelling and defuelling

Defuelling Methods

• Two methods of defuelling exists, being pressure defuelling and suction defuelling.
• Pressure defuelling occurs when an aircraft is defuelled using tank boost pumps, and it is the preferred method for defuelling an aircraft

Precautions

• Precautions must be taken to provide a path to equalize the potential for static electricity

Bonding - How to take precaustions

• Prior to making any fuelling connection to the aircraft, the fuelling equipment is bonded to the aircraft
• Bonding connections are electrically and mechanically secure

Longitudinal Balance Fuel Systems Introduction

• Longitudinal stability is the stability of an aircraft in the longitudinal or pitching plane
• Longitudinal balance fuel systems aid in maintaining the aircraft’s center of gravity

Longitudinal Balance Fuel Systems

• The center of gravity of the aircraft can change as the aircraft uses fuel in flight
• On some aircraft, fuel can be transferred to maintain the CG, so trim is not required for cruise flights
• Flight crew initiate system operations by entering all variables. Before a flight
• Aircraft Automatic longitudinal CG control minimise aircraft drag during the cruise by moving fuel to the trim tank