Aircraft Lubricants, Fuel and Synthetic Oil

Lubricants and Fuel

• The correct oil specification for an engine type can be located in the Aircraft Operators Manual, AMM, Engine Maintenance Manual, and the Engine oil tank service placard.
• The primary purpose of lubricant is to reduce friction between moving parts and assist in engine cooling; lubricants also seal and cushion moving parts, clean the engine interior, and aid in cooling.
• An oil cooler is a heat exchanger that transfers heat from the oil to the outside air.
• Synthetic lubricants are typically used in turbine engines.

Volatility and Pour Point

• Volatility measures the speed at which a chemical element or compound evaporates.
• Pour point refers to the lowest temperature at which oil will still flow because of gravity.

Gas Turbine Oil

• Turbine synthetic oil operates within a range of -50 degrees Celsius to +200 degrees Celsius.
• Synthetic oils are 3,000+ PSI strength, while petroleum-based oils are somewhat less than 500 PSI regarding film strength .
• An oil's optimum viscosity is 2.5 – 4.0 Centistokes.

Flash Point

• Flashpoint is the temperature at which oil emits flammable vapors; oil giving off flammable vapors at low temperatures have a low flash point.
• Gas turbine engine oil flash points register above 250 degrees Celsius.
• Synthetic oils are rated using Kinematic Viscosity Rating with a rating of 5-7 cST, instead of SAE for petroleum.
• A low viscosity index suggests viscosity experiences large changes with temperature variations.
• Diester refers to man-made extracts of mineral, vegetable, and animal oils.

Synthetic Oils

• Synthetic oils are produced by synthesizing raw materials to create a base stock, differing from crude oil refining.
• Potential disadvantages of inadvertently mixing oil brands include chemical shock and oil foaming.
• Type I and II oils are straw-colored when new, darkening with use; Type III is slightly darker when new, with color change due to an oxidation inhibitor reacting with oxygen.
• Excessive oxidation often triggers viscosity increases or sludge formation in the oil.
• Reciprocating engines consume more oil than turbine engines.

Cautions for Synthetic Lubricants

• Never use silicone-based grease on oil system O-rings.
• Clean up spills right away.
• Refer to the MSDS before using and put on PPE to reduce the possibility of skin absorption.

Wide-Cut Fuel

• Wide-cut fuels consist of a hydrocarbon mixture spanning the gasoline and kerosene boiling ranges.
• Disadvantages of wide-cut fuels include greater evaporation losses at high altitudes and an increased risk of fire during ground handling; if wide-cut fuel is used, planes are less survivable in crashes.
• The most commonly used aviation turbine fuels are Jet A (USA) and Jet A-1 (Worldwide, except USA).
• Jet A-1 possesses a lower maximum freezing point (-47 degrees Celsius compared to -40 degrees Celsius for Jet A), making Jet A-1 more apt for long international flights, as well as polar route use during winter.
• Approved turbine fuels are identified via white letters on a black background, with color bandings of grey (Jet A-1) or yellow (Jet B).
• One Calorie represents the amount of heat needed to raise the temperature of one gram of water by one degree Celsius.
• Jet A-1 yields more energy per given volume than AVGAS.
• Specific gravity measures the density of a volume of fuel versus the same volume of pure water at the same temperature.
• Jet A-1 is 0.8, and AVGAS is 0.72, regarding specific gravities of fuel.

High Volatile Fuels

• Fire hazards, vapor locks, and fuel loss due to evaporation are disadvantages of high volatile fuels.
• Modern fuels are becoming less volatile although the flash point and vapor pressure must be routinely monitored.
• When the flash point decreases, it creates a need for significantly higher vapor pressure.
• AVGAS has a -40 degrees Celsius flashpoint, while Jet A-1 registers at 38 degrees Celsius.
• Fuel should be drained into a clear glass container to check for contamination.
• Water exists in both dissolved and free forms.
• Using water-finding paste will change green to purple upon water exposure
• Water may serve as an incubator for microbial growth in a turbine engine, resulting in microscopic animal and plant life.
• Anti-icing and anti-microbial agents (PRIST) classify as common fuel additives.

Fuel Spillage

• Airport fire service, vacated ramps, passenger and crew unloads, and engine shutdowns may be required with major spillage.

Lubrication Systems

• The primary function of the lubrication system involves supplying oil to the engine parts subject to friction and heat loads from engine rotation and gas path exposure.
• Pressure relief valve systems and full-flow systems classify as the two basic types of self-contained lubrication systems used in gas turbine engines today.

Lubrication Systems Classifications

• Wet sump and dry sump systems classify as two engine lubrication systems.
• APUs primarily use wet sump systems.
• Scavenge pumps route oil back to the sump or gearbox after lubricating the bearings, draining to low-ying locations.
• Turbine engines use primarily dry sump lubrication systems.
• In dry sump arrangements, oil is stored in a separate oil reservoir mounted either internally within the engine, externally on the engine, or within the aircraft.
• A pressure lubrication system comprises an oil reservoir, pressure and scavenge pumps, pressure relief valve, several oil filters, and cooler/vent lines.
• Sheet aluminum or stainless steel are often used to construct the oil reservoir, mounted either internally or externally.

Reservoir Pressurization and Scupper Drains

• Reservoir pressurization suppresses oil foaming, which in, turn, prevents pump cavitation.
• The oil reservoir is usually pressurized through an adjustable relief valve in the reservoir vent line; typically adjusted to maintain a reservoir pressure of approximately 3-6 PSI.
• A 10% tank capacity or 0.5 gallon is the standard size for an oil reservoir expansion space, whichever registers greater.
• Scupper drains serve to catch spillage during servicing and overflows after the cap opens.
• Dipsticks are the best method when checking the oil level because sight glasses can grow cloudy and unreliable.
• Pressure pumps and scavenge pumps classify as the two types of oil pumps.

Pressure and Scavenge Pumps

• Pressure pumps deliver oil under required pressure to parts and scavenge pumps return excess oil from lubrication points back to the oil tank.
• The three types of lubrication pumps are gear, vane, and gerotor.
• Vane pumps are more tolerant to debris and lighter than gear and gerotor pumps.
• Scavenge pumps have a greater capacity than pressure pumps.
• Pressure and scavenge pumps are often enclosed in a single housing within turbine engines.

Valves used in Turbine Engine Systems

• Pressure relief valves, check valves, thermoplastic bypass valves, and pressure bypass valves classify as the four most common valves used in turbine engine systems.
• Pressure relief valves limit and adjust maximum pressure within the oil system; whenever oil pressure exceeds the spring pressure, the spring opens and bypasses the oil.
• Check valves prevent oil seepage through the pressure pump and into the engine when stationary and exists as a ball and spring or cone and spring combo.

Thermostatic Bypass Valves

• Thermostatic bypass valves are fitted to cooler outlets to sustain oil temperature while quickly bypassing oil to lubrication points upon a cold start; when the oil is cold, the valve opens and enables oil to bypass the cooler.
• Bypass valves are attached to the oil system to prevent engine oil starvation if a component grows blocked.

Types of Oil Filters

• Laminated paper, cleanable screen, screen and spacer, and thread filters classify as the commonly used oil filters..
• Laminated paper filters are made using a pleated paper element assembled around a core steel and are fit for low-pressure systems.
• Screen filters are leveraged in both scavenge and pressure systems.
• Thread filters are also known as 'Last Chance' filters.
• Complete oil flow can be permitted even if the filter grows entirely blocked by enlisting an oil bypass valve that automatically lets oil bypass to the filter once it is plugged/clogged.
• Differential oil pressure assesses pressure in the oil pressure system versus vent pressure.

Oil Coolers

• Fuel and air cooled oil coolers classify as the two basic types of oil coolers.
• Fuel-cooled oil coolers are more widespread.
• The thermostatic bypass valve is in the open position and enables bypassing of the cooler, the valve is kept at the cooler inlet to ensure oil is cold.
• Air is cooled when flowing through the matrix and utilized primarily on turboprop engines in air-cooled coolers.
• Oil jets distribute oil to the sections needing lubrication after it goes through both the pump and the filter.
• Mist and Vapor Lubrication Oil Jets offer a wider area for lubrication in larger engines from a single oil jet.
• Centrifugal breathers are the most common type of vent system.
• The required pressure within the tank and bearing chambers is 5 to 7 PSI.
• Vents in vent systems open with decreases in altitude.
• The aneroid capsule starts to close at 8,000 ft and is fully closed by 20,000 ft.

Magnetic Particle Chip Detectors

• The presence of small fuzzy particles or grey metallic paste suggests normal engine wear; metallic chips or flakes show notable internal wear and necessitate additional investigation.
• Magnetic Particle Chip Detectors can be found on MCDs in the engine forward bearing sump, engine rear bearing sump, the accessory gearbox, and the transfer gearbox with its location labeled for identification depending on which scavenge system it is connected to.
• Quick disconnect bayonet fitting attachments are the common fittings used in these systems.
• Higher pressure differential means more oil flows to the bearings.
• In full-flow systems, the oil flow correlates directly to how fast the engine and oil pump operate.
• PSI measures oil pressure on the gauge.
• Turbine engine pressure gauges are typically connected to the oil system downstream of the main oil filter.
• Apart from emergencies, low-pressure warnings can come on when starting the engine.

Fuel Systems

• Fuel systems provide fuel in a suitable form for combustion, regulating flow to meet necessary rates for stable running during startups, acceleration, and all other engine conditions.
• Fuel flow meters show the amount of fuel consumed per hour, enabling the pilot to calculate available flight time with accuracy.

Fuel Flow and Valves

• Fuel flow spins a small turbine wheel, and a digital circuit reads the number of revolutions over a set period to compute fuel flow rate in operation.
• Motor-driven gate and solenoid-operated valves classify as the two valve types.
• Solenoid valves are quicker to open/close against motor-driven valves.
• High-pressure cocks in HP shut-off valves remain open from idle to maximum throttle, then close during shutdown.
• Pressurizing and dump valves become essential when engines use duplex fuel nozzles with dual outlets, which divide the fuel flow into primary and main manifolds.
• Pressurizing valves situated on the main manifold open from above idle through maximum and at maximum fuel flow levels, the main manifold feeds 90%, while the primary supplies about 10%.
• Drain valves run by a pressure differential
• Fuel accumulates at the bottom of the combustion chamber after shutdown or a false start.

Fuel Heaters

• Fuel heaters keep the fuel system protected against ice formation and thaw ice that forms on the fuel filter screen thanks to the hot air obtained from the compressor segment of the engine.
• A fuel filter must be capable of removing tiny particles up to 10 microns in measurement units.
• The two common types of engine-driven fuel pumps are the spur gear and piston varieties.
• A spur gear pump leverages an impeller at the pump entrance to initially increase pressure.
• Piston-type pumps leverage the cam-plate inclination and rotor movement to supply a reciprocating motion to the plungers, which in turn yields the pumping action.

Differential Pressure Switches and Power Control in Turbine engines

• Differential pressure switches are enlisted in fuel systems to sense for icing on the fuel filter and light up a cockpit warning light upon reaching the set amount.
• Control of power and thrust in gas turbine engines involves the amount of fuel injected into the combustion chamber.
• If there is too much fuel [rich], the turbine experiences damage triggered by excess heat, the compressor stalls or surges which causes a rich blowout; insufficient fuel or lean causes a lean die-out.
• Electro-hydromechanical fuel control units sense inputs using electronic engine control to set the hydromechanical portion of the FCU that in turns restricts fuel flow to the engine.
• Fuel Control Units (FCU) help achieve easy starting, easy acceleration, and steady running across all kinds of operating conditions.
• Optimal air-to-fuel sits at 15:1 in an atomized spray.

Engine Systems and Variable Effectors

• Ancillary functions include oil cooling and hydraulic control within various engine systems.
• Aircraft altitude, air temperature and airspeed are the three components that affect air-to-fuel ratio.
• Air density and fuel flow decrease, remaining proportional, as altitude increases
• Compressor inlet pressure, compressor inlet temperature, compressor speed and engine burner are engine systems the FCU can regulate to conform to specific operating conditions.
• After starting the engine on a hydromechanical FCU, flyweights in the governor activate and then overcome initial speeder spring tension to maneuver the roller cage upward and compress the metering valve opening.

Air Pressure, Mass Airflow and Servo Valves

• Decreases in inlet air pressure and mass airflow occurs if the aircraft ascends; these readings increase if descending.
• Pilot servo valves slow pilot servo control rod transitions, preventing drastic shifts of fuel.
• A differential pressure valve registers a lessened differential as fuel increases, leading the sensor to shut the valve for differential maintenance.
• The spring cap slides up the pilot servo valve rod and releases pressure on the flyweight speeder spring amidst hydromechanical FCU deceleration processes.
• Turbine engines require stabilization at idle over a length of time prior to shutdown to achieve regulated cooling of the turbine and propel complete scavenging of propeller control oil, particularly in turboprop applications.
• P2 means Fuel pressure immediately downstream of the metering head.
• The Tt2 sensor causes a variation of Px bleed in line, along with varying air density at idle spots, to reduce idle stall concerns triggered by over or under fueling and ceases authority above idle.

RPM Control systems

• RPM control, acceleration and deceleration control, and maximum//minimal flow control classify as the components of a simple RPM system offers.
• The essential regulation parameter is RPM, while compressor discharge pressure and inlet air temperature stands as parameters for any metering valve.
• The bellows assembly opens slightly, along with decreased RPM and closes in tandem with any RPM increase.
• Hydro-pneumatic systems depend on the position of the power lever, N1 RPM, pressure emitted from the compressor, and external temperature during computing for accurate fuel rates as opposed to hydromechanical controls alone.
• Bimetal discs sense inlet duct temperature.

Controlling Governor Sections

• Evacuated bellows control acceleration and deceleration bellows impacts on deceleration.
• An FCUs starting mechanism requires a closed fuel shut-off valve, idled power lever, expanded governor spring, underspeed flyweight operations, atmospheric pressure levels registered by burner, and decreased multiplication linkage.
• Prolonged engine life, economic fuel consumption, enhanced consistency, lessened crew fatigue, and lowered care expenditures are part of the benefits linked to Electric Motor Control engine systems.
• Electronic Engine Control (EEC) leverages both supervisory and Full Authority Digital Engine Control (FADEC) systems.

Redundancy for EEC and thrust setting

• ECs use A and B channels for redundancy measures with one active, and others ready for use on standby.
• Fan-speed setting is a core factor behind EEC thrust measures.
• P and D valves sharply cut off combustion, prevent fuel boil amid aftereffects of rest engine heat.
• Adjusting screws enable fuel flow regulation from the main manifold: tuning in to delay, or back to accelerate.
• Prevention around manual hand-draining leverages returns of excess fuel to aircraft supplies or pushes residuals by enlisting air bleeds into dump ports.
• Common system faults seen when troubleshooting fuel issues feature engine motor issues, start failures, and erratic readings against targeted pressure ratios.

Direct Parameters impacting FCU

• Power lever angle, RPM, air temperature, air pressure, burner pressure and fuel density play impact FCU readings.
• Elevated altitudes coupled with minimal air pressure results in low air mass measures.
• The flow adjusts to vary compensation given to differing fuel specific gravities under vibration of the fuel differential switch, which helps regulate optimal fuel flow readings.
• Speed governors, differential pressure regulators, acceleration governors and pressure sensors.
• The degree of fuel flow through the throttle valve stays balanced and similar to orifice readings. High inertia factors cause high rotations among large engine classes.

Air System purpose

• Air systems enable temperature regulation of crucial engine portions: combustors, turbines.
• Introduced airflow is spent around 20% by combustion within combustors, with remaining bypassed among snout.
• Almost 40% of exhausted air cools the engines, and 40% causes secondary/vortex thrusts.

Thermal Readings

• Thermal performance rests among measures surrounding high turbine temperatures
• Turbine entry temps are tied to the efficiency of blade and disc endurance towards hostile conditions.
• "Pre-swirl" nozzles reduce entry temps and pressures to cool blade discs.
• Blades exist using differing methods, with effectiveness judged by the blades methods.
• Turbine discs leverage annular spaces around the components for external movements controlled by inter-stage safeguards.
• Engine bays/pods are cooled with atmospheric relief.

Airflow Purpose

• Airflow in external compartments offers a safety factor related to nacelle relief surrounding vapours that could be flammable.
• A decision on any sealing option is correlated against heat from surrounding forces, temperatures, available space and service.
• Carbon, brush, hydraulic, ring, or labyrinth shields regulate the various points of contact.

Sealing and Air Pressures

• Sealing air pressures leveraged against diverse compressor points will match measurements of compressor pressure itself.
• Labyrinth seals work over restricted flows from one surface, and reduce oil leaks through airflow that travels externally relative to chambers within components.
• Fluid and abradable lined, continuous groove inter-stage air, and thread labels are used throughout oil safeguards.
• Carbon shields must withstand hotspots.
• Ring faces need movement where contact with shafts can occur.
• Hydraulic models face connections alongside various members where some measure of airflow can exist.
• Brush guards may withstand wear without losses throughout operations.

Bleeding and Ice Control

• Pressure used within turbine units for ice or heat operations are referred to commonly among functions used in airframe work.
• N2 (High-Pressure points) leverage ice safeguards among helicopter frames.
• Pressure signals for regulators show operational signals.
• On-ground ice forms at 10 degrees Celsius or less with rain, sleet, or snowfall.
• Air used to offset ice pressure comes with passing values.
• Two electric components with cutoff points are standard.
• Exhaust instruments shift readings against temperature measures.
• Coils adapt to varied airflow points.

Lights and Bleeding

• Motors need fully open lights for blue instrument measures