Aircraft General Knowledge - Electrics, Systems & Engines Flashcards
What are the ideal properties of aircraft materials?
Low density corrosion resistant High strength High stiffness Good Fatigue performance High operating temp Low cost Ease of fabrication
What is plastic and elastic deformation?
Elastic - a material endures temporary stress but returns to original shape
Plastic - permanent deformation of shape post stress
What is ultimate stress?
It is the stress at which a structure fails
Where are the design requirements for an aircraft set out?
CS23 - light aircraft
CS25 - aircraft >5700kg MTOW
What is an alloy of aluminium and its properties?
Duralumin - 3-4% copper, 0.5-1.5% magnesium, 0.5-1% manganese
Fatigue tolerant Heat conductive High stiffness Low cost High strength and stiffness
Low operating temp
Poor corrosion resistance
Name and describe the 2 types of corrosion?
Oxidation - dry corrosion - reaction between material and environment without intervention of an electrolyte
Electrolytically - wet corrosion - one surface becomes anodic (+) the other cathodic (-) and occurs as a reaction in metals in an attempt to become more stable. A substance like impure water allows current to flow and a PD is produced adding material to the cathodic and removing from anodic.
What is the difference between these fuselage structures:
Truss
Monocoque
Semi-monocoque
Truss - wooden longerons (struts) run the length of the fuselage held apart by compression struts and were cross braced by wire. The fuselage was often covered by fabric. The frame takes the full load.
Monocoque - the skin of the aircraft takes all load bearing and is maintained by circular formers, longitudinal stringers and doublets to reinforce weaknesses cut into the skin e.g doors and windows.
Semi-monocoque - uses monocoque and frame structures to allow the skin to only be part load bearing. Formers and bulkheads to support the load bearing skin with stringers to support tensile and compression loads. The skin still takes the majority of the load but is supported by the frame structure.
Describe a wings internal structure?
A wing Is made up of:
Spars running the length of the wing usually 2-3. There is likely to be corrugated skin joining the spars. The outer skin would go on top to produce a torsion box (spar/spar, rib/rib, skin/skin and sometimes stringers make a box which compartmentalise the wing)
Ribs running from leading to trailing
Stringers run along the wing
What is aerodynamic flutter? When is it most likely? And what can be done to reduce it?
Flutter is an undamped oscillation caused by an aerodynamic imbalance.
A light wing is most susceptible to high frequency flutter. Fuel in outboard tanks, engines forward and upset ailerons can all act to push the wings down and can reduce the onset of flutter.
What is a fail safe or damage tolerant structure?
A fail safe structure is were there are parallel load paths that allow a component to fail but not make the structure fail allowing replacement at next service
A Damage tolerant structure is were the load is spread over a greater area
The monocoque structure depends on what most heavily for strength?
It’s shape
What are the classifications for failure conditions quantitatively and qualitatively?
Classification Qualitative Quantitative
Minor Probable <10^-3
Major Remote <10^-5
Hazardous Extremely remote <10^-7
Catastrophic Extremely improbable <10^-9
Define strain on a material?
Strain is defined as deformation caused by the actin of stress - it is the quantifiable effect of stress
Define buckling?
Buckling is the bending or deformation caused by compression load
What is the design load limit and the safety factor placed on top? What is the term given with applied safety factor?
The design load limit (DLL) is the designers expected load the aircraft will see in service based on 2.5g for transport aircraft. Aviation regulation then demands a 1.5 or 150% safety factor applied to this. This is termed the design ultimate load (DUL)
What does the S-N(Wohler) curve depict?
The graph depicts amplitude of load against amount of cycles to failure. It shows the greater the amplitude of loading the fewer cycles required for a component to fail and vice versa.
What is used to reduce the susceptibility of duralumin to corrosion?
A pure layer of aluminium covering the duralumin core called Alclad
What is a a stress concentration factor?
Stress concentration factors tend be down to a shape or material being imperfect e.g. A square window has weak corners and is a stress concentration factor. Likewise an impure alloy will have a stress concentration factor at the source of the impurity and may cause a component to fail early.
What electrical systems does a: light aircraft, a twin turbine and a jet airliner have? Why is it called a system and not a battery?
Light aircraft - 14v DC system
Twin turbine - 28v DC system
Jet airliner - 200v/115v AC system
These are systems because in reality these aircraft do not have the stated rated voltage battery. The systems pertain to the generator or alternator along with the battery and as such in a light aircraft there is a 12v battery which is recharged by a 14v alternator. Likewise in a turbine, it has a 24v battery and a 28v alternator to charge it. In each case the alternator is 112% more than the battery.
What type of electrical circuits is used in aircraft systems and why?
Parallel circuits are used because in parallel circuits if a load or electrical component fails then the other loads do not fail with it.
What is the difference between conventional flow and reality?
Until recently, most electrical scientists believed that electricity flows from positive to negative (conventional flow). However, in reality it actually flows from negative to positive
What are the key contributors to resistance in an electrical circuit ?
- wire length
- wire thickness (the smaller in cross section the higher the resistance )
- material property (insulator vs. Conductor)
- temperature (the greater the temp the larger the resistance)
What is the speed of electricity and how long does it take to travel round the circumference of the earth?
3500miles/second and approximately 7seconds to travel 21600nm
What is a positive and negative temperature coefficient resistor?
Positive - the greater the temp the larger the resistance (thermistor e.g self regulating heaters, over current protection)
Negative - the opposite occurs
What is a semi conductor and some of its properties?
A semi conductor in its natural state is an insulator. however, have unique properties such as their temp rises their resistance falls - NTC opposite to that of metals. Likewise, they may only allow conduction in one direction and insulation in another. Some of these materials are silicon.
What are kirchoffs first and second laws?
Law one: the total current flow into a point of a circuit equals the current flow out of that point
Law two: if all the voltage drops are added together in a closed circuit, they will equate to the total voltage applied to the closed circuit.
How are ammeters and voltmeters connected in an electrical circuit and what do the both detect? How are ammeters connected up in an aircraft?
Ammeter is always connected in series and has low resistance
Voltmeter is always connected in parallel and has high resistance
Both work by detecting current, using a coil in a magnetic field.
The ammeter in an aircraft is connected between the busbar and the battery in series. This allows measurements of current drawn from the battery which will be negative.
Name 2 common aircraft ammeters?
Zero centre and zero left ammeters
Zero centre - shows positive and negative charge (negative indicates battery discharging)
Zero left - only shows positive charge
What is the purpose of the maximum zero fuel weight (MZFW)?
To ensure that bending forces at the wing root do not exceed maximum designed loads in flight (g)
What are the purpose of stringers?
Stringers are used to reinforce the aircraft skin to prevent buckling and support against tensile forces. They also give the fuselage its shape.
What are the magnetic lines in a magnetic field called?
Lines of flux
When is an electrical field produced?
When a positive and negative charge attract an electrical field is produced. It is caused by voltage and creates static electricity.
What factors influence the strength of the electrical field?
The proximity of poles that attract I.e. Positive and negative the closer together the greater the charge
The amount of charge (Coloumb) or an increasing amount of electrons will increase the charge
What is direct current and its advantages? Name some components that generate DC electricity.
Direct current is a form of electricity which flows in one direction. It is simple to setup and can be stored in batteries for future use.
Batteries, DC generators, AC generators (alternators) via a rectifier (converts AC to DC)
What are the primary purpose for batteries in an aircraft?
Starting the engines and APU
In emergency situations such as electrical failure (
by law an aircraft must have 30minutes battery power)
To stabilise voltage during the switching of loads
What are the 3 main components in a battery?
Anode (-)
Cathode (+)
Electrolyte (insulating solution that prevents electrons flowing from the anode to the cathode the shortest way I.e. Inside the battery)
What is the difference between a primary and secondary cell? What are the primary voltages in a small aircraft and a turbine aircraft?
Primary cell is the most basic form of battery and is non-rechargeable. 1.5v and not used in aircraft.
A secondary cell is a battery, made up of numerous cells in series, that is rechargeable. In a light aircraft the terminal voltage of a lead acid battery is 13.2v with each cell producing 2.2v. The nominal voltage is 12v under load. In a turbine aircraft a 24v battery is used.
What are the 2 main batteries used in aviation? Explain their advantages and disadvantages
Lead acid battery
Advantages:
Cheaper
Predictable life expectancy
Disadvantages
Heavier
Corrosive electrolyte inside that can cause damage if battery split
Slow charge rate
Cannot be left for a long period after being discharged as this may caused sulphation rendering the battery useless
Voltage reduces over time
Nickel cadmium (NiCad)
Advantages:
Lighter than lead acid
Maintain charge even if discharged and left for a period of time
Fast charge rate
Have a wider range in operating temperature (operates better in colder conditions)
Disadvantages:
Susceptible to thermal runaway upon quick recharge, that can cause the battery to catch fire as the battery materials have a negative thermal coefficient
Expensive
When a NiCad battery fails it is unpredictable as it maintains voltage all the way until it fails.
How is the life of a lead acid battery measured and what are the ranges?
Given a lead acid battery loses charge as the sulphuric acid corrodes the lead plates turning them into lead sulphate the voltage will drop throughout its life.
To measure this a hygrometer is used to measure the specific gravity of the sulphuric acid. Therefore:
a brand new lead acid battery fully charged should have - 1.25 to 1.30
A discharged battery at low charge - 1.20 to 1.24
A fully discharged battery - 1.17
What are the main materials, number of cells and voltages for NiCad and lead acid batteries?
Lead acid: Anode - lead Cathode- lead peroxide Electrolyte -sulphuric acid Cells - 6 Nominal voltage - 12v(13v)
NiCad: Anode - iron Cathode - nickel cadmium Electrolyte - potassium hydroxide gel Cells - 10 Nominal voltage - 12v(13v)
What is a metal alloy and a composite?
Alloy - 2 or more metals with desirable properties to produce a new material
Composite - 2 or more materials brought together to give new properties you didn’t have before.
Give 2 construction designs for composite materials and their advantages?
Composite - matrix and fibre
Constructs: honeycomb and sandwich structure e.g. Fibreglass, carbon fibre, Kevlar
Advantages: lightweight, good rigidity and strength
Disadvantages: susceptible to core damage under concentrated loads
Describe 2 forms of maintenance
Hard time maintenance - paced urge under which an item must be removed from service before its scheduled maintenace period for inspection or repair. Labour intensive due to detailed inspections and likely un-utilised aircraft time
On-condition maintenance - procedure whereby upon inspection or functional check of items performance may result in item removed from service before potential fail. This include visual inspection, tests or other means without disassembly or overhaul.
What is the maximum pressure difference between outside air pressure and inside the pressurised fuselage ?
9psi
What is a cantilever wing?
A cantilever is a structure that is self supporting and supported at one end only. Therefore, a cantilever wing has no external bracing or support.
How are wing spars produced?
They are either fabricated taking many components to make one I-beam or extruded using a malleable metal and pushed through a hole or model to shape it.
If extruded it is called a girder.
What kind of doors and windows do we have on a modern transport jet?
The plug type - the open inside and when pressurised are pushed into their seals causing a tighter seal.
What are the consequences of a nosewheel landing and a tail strike?
Nose wheel:
Damage to the nose wheel assembly specifically the drag struct which holds the nose undercarriage vertical
Damage to the forward pressure bulkhead
Tail strike:
Damage to the rear pressure bulkhead
Damage to the empennage I.e. Tailplane and rear control surfaces etc.
What is the structure of the cockpit windscreen ? As such what loads is it subjected to?
Three layers of glass with a vinyl (polycarbonate) inter layer. Within the layer is also a heating element
Impact loads specifically air flow/resistance, bird/insect strikes and precipitation. It must also pressurisation/ compression loads.
What is hydrostatic pressure? Give the equations for pressure
It’s the pressure of a fluid as an effect of its height. The higher the fluid the higher the pressure. It acts at 90 degrees to a surface.
Pressure =force/area
What are and the differences between a passive and a active hydraulic system ?
A hydraulic system is one which allows a fluid to be compressed that applies a small input and a large output therefore a mechanical advantage.
A passive system is one without a pump and pressure is applied when a force is applied. These systems are reminiscent of braking systems and tend to be confined to small aircraft and cars. An active system is one that utilises a pump and is continually doing work whilst a system is active. These systems are categorised as high and low pressure systems.
What are the properties of hydraulic fluid and where is it stored?
Name the fluids
Properties: Virtually incompressible Chemically inert Stable or has good storage properties Non-corrosive Lubricant Reasonably priced and readily available Non-sludging/foaming
Hydraulic fluid is stored in a reservoir
Mineral oil (oil based so flammable) - DTD580/OM15 (to be replaced by def stan a1-48) Skydrol (synthetic oil which is phosphate ester based)
Describe a some hydraulic seals and their purpose. What are the combined with to prevent extrusion?
O-ring - cheap and bi directional seal but susceptible to extrusion
Square- cheap and bi directional seal but susceptible to extrusion
Chevron - can only be used in the direction of the ‘v’
U-ring
Seals are paired with backing rings to prevent extrusion(rolling out) of the seal. These tend to be only for dynamic seals I.e. Those used to seal moving frictions surfaces
What are the ramifications of air being present in the hydraulic fluid? What are the actions you would take to rectify this?
When air is present in the hydraulic fluid systems for example in a braking system, when the force is applied it will feel spongy.
This is because a gas is very compressible and therefore, makes the fluid more compressible. As such, the hydraulic system must be bled normally whilst simultaneously adding the correct hydraulic fluid in the reservoir until you get air free oil.
What is the difference between an open centred and closed hydraulic system?
An open centred system can only provide hydraulic pressure to one system at a time. The closed system is therefore found in more modern jet transport aircraft which often have many pumps allowing multiple systems to be operated at the same time. These systems tend to operate to 3000psi
What is the effect of placing batteries in series vs. Parallel?
Series:
Voltage is the sum of all batteries
Same capacity (Amp/hr)
Parallel:
The nominal voltage of 1 battery and current of 1.
The capacity is the sum of all batteries
By law what are the requirements in electrical failure?
There must be emergency power from batteries to last 30minutes and 10minutes for emergency lighting
What is a dipole and uni pole system? How are these used in an aircraft and what are the advantages and disadvantages of the one used in an aircraft?
A dipole system is one that is a closed circuit using conductive wiring often used in an aircraft that is made not of a conductive skin e.g. Wood.
In a modern jet airliner, a uni pole system is used whereby electrical components are wiring to the positive terminal but the circuit is completed by conductivity being carried out via the outer skin of the aircraft. The primary advantage of this is saving on weight by cutting out half the wiring required.
The primary disadvantage is that damaged wiring connecting the skin to the negative terminal can cause a short to the earth disabling systems and become a fire risk.
What is a busbar and how is it wired in an aircraft ?
A busbar is a heavy copper bar that is used to extend electrical power and connect multiple loads. There maybe many busbars in an aircraft and can be connected together much like an extension cable in a home.
Loads are connected to a busbar in parallel which means if a load fails, other components still receive electrical power.
Describe what happens to pressure, temperature and velocity of an incompressible fluid flowing through a divergent and convergent duct?
Pressure x velocity/ temp = constant
When flowing through a convergent duct: velocity increases, temp and pressure decrease
When flowing through a divergent duct: velocity decreases, pressure and temp increase
What is the definition of a piston engine aircraft?
An engine where the working fluid expanded in a cylinder against a reciprocating piston.
What are the four strokes of a constant volume engine?
Induction - mixture added to the combustion chamber
Compression - the mixture is compressed and ignited
Power - pressure energy pushes the piston downwards to produce mechanical energy
Exhaust - exhaust gases are expelled via an inlet valve as the piston comes back up
What is meant by TDC and BDC? What is swept volume?
Top dead centre - the point at which the piston in a cylinder is at maximum linear travel
Bottom dead centre - the point at which the piston in a cylinder is at minus linear travel
The volume between the two is defined as swept volume.
Swept volume is the piston cross sectional area multiplied by the stroke
How many degrees does the crankshaft rotate through 1 4 stroke cycle? How many revolutions of the crankshaft does it take to travel through 1 4 stroke cycle?
720 degrees and 2 crankshaft rotations
What are the differences between a petrol and Diesel engine?
A petrol engine requires a spark plug to initiate combustion whereas a Diesel engine uses high compression to initiate compression therefore, no spark plugs.
What are the differences between the theoretical and practical 4 stroke engine cycle?
The theoretical otto cycle has all events occurring at TDC and BDC I.e. Valves opening/closing and all strokes. However, in reality this is very inefficient due to inefficient crank angle and doesn’t optimise the finite volume to optimise peak pressure I.e. Volumetric efficiency
The practical cycle optimises these inefficiencies by using valve lag and lead to optimise volume of air in the cylinder and the expelling of exhaust gases. It also advances/ retards ignition to maximise peak pressure and exploit ineffective crank angle between compression and power strokes
What are adiabatic and isochoric processes in terms of a piston engine?
Isochoric processes are those that have a pressure change with no change in volume. E.g combustion post spark
adiabatic process is one that has a change in pressure with no heat energy added or removed. E.g. Mechanical work post combustion expanding volume of cylinder.
What is the difference between a single acting and double acting actuator? What are the variances of model?
A single acting actuator has one jack ram acting on a piston with an empty volume the other side. There maybe a spring on the jack ram side to operate the actuator in the opposite direction
The double acting actuator works in both ways often used with a 4 port selector valve to allow hydraulic fluid to be pumped in the higher volume side
There are balanced and unbalanced double acting actuators - balanced ones have 2 jack Rams attached to a single equal area piston. These give the same force either side due to the same volumes. An unbalanced variant is where there is a larger volume on the other side of the piston to where the jack ram is.
What is an active vs. Passive hydraulic system? What are the standard pressures for a low and high active hydraulic system?
An active hydraulic system is one that is powered and doesn’t rely on an input to work. They have pumps. A passive system relies on a force being applied to work and therefore have no pump.
Standard pressures for a low pressure hydraulic system are up to 2000psi but can be as low as 1000psi. For a high pressure hydraulic system standard pressures are 3000psi but can be as high as 5000psi
What is an engine drive pump and a rotary actuator? When are both used?
An EDP is a gear type pump found in most light aircraft and can be used as backup pumps in larger aircraft. The EDP is also known as the constant volume pump and by this it means that a constant amount of hydraulic fluid fed to the teeth is consistent not same volume in/out
A rotary actuator is another form of actuator that can provide mechanical advantage to services requiring a rotary motion. These are made up of a number of small Pistons attached to an angled swash plate that is immovable and as such the force causes a rotary motion. These actuators are often used for flaps and undercarriage. They also can be used to drive backup electrical generators
Name and describe each valve.
Rotary Selector valves - 2 ror 4 port selector valves that have multiple fluid paths.
Spool/pilot valves - sliding valve type that has land and grooves that blocks and allows oils flow respectively
Non-return valves - made up of a ball and Spring and have many variants e.g. PRV, TRV and check valves that are construction ally the same but serve different purposes. They all do not allow fluid to flow in its opposite direction unless they are pressure or temperature calibrate (PRV/TRV)
Shuttle valves - often used as redundancy if one hydraulic flow to a system fails shuttle valves engage upon one failing due to the pressure difference ensuring hydraulic fluid is supplied to the system.
Pressure maintain/priority valves - in the event of hydraulic failure these valves prioritise primary flight controls and wheel brakes ensuring that there is sufficient pressure to maintain priority services.
Restrictor valves - restrict flow in either one or both directions with the aim of reducing the speed of an operation e.g. Retraction flaps or undercarriage
Flow control valves more sophisticate form of restrictors that tend to be positioned upstream of hydraulic motors
Throttling valves - another more sophisticated restrictors
Sequencing valves - ensure that one hydraulic operation is completed before another commences. They can be hydraulic or mechanical and are fitted to the pressure supply circuit.
Hydraulic fuses - valves that are flow rate sensitive and are used to isolate a leak.
Modulators are valves with a piston inside that contains a very small opening through the middle that is often used in anti skid systems to provide fine control of brake pressure.
What does PLANE/PLANK calculate?
Indicated horse power or the theoretical power of a piston engine:
Pressure Length of stroke Area of piston Number of cylinders RPM/2
What is volumetric, mechanical and thermal efficiency?
Mechanical is the indicated horsepower the frictional horse power
Volumetric is the ratio of mass of air induced into the cylinder by comparison to the mass of air in ISA conditions
Thermal efficiency is the ratio of work done by the propeller crankshaft by comparison to the heat lost in combusting the fuel
What are the volumes in a cylinder? How are these used to calculate compression ratio?
The total volume = swept + clearance
Swept = TDC-BDC volume
Clearance =TDC - remainder
Compression ratio = total volume: clearance or (swept -clearance)/clearance
What is detonation? How is detonation avoided in high compression ratio engines like diesels?
Detonation is when the mixture reaches it auto ignition point changing the steady burn 60-80ft per second to in excess of 1000ft per second closer to an explosion. This can produce severe engine and piston damage
The reason petrol engines cannot reach a higher compression ration is because there is minimal way of stopping the fuel detonating at higher compression ratios. Diesels avoid this because fuel as it is injected in auto ignites under the extreme pressure and temperature of the compressed air in the cylinder. Detonation is impossible as detonation requires a large mixture to all be at the auto ignition point, instead the fuel is injected at its auto ignition point causing an immediate burn.
What are the following components made of:
- crankcase
- piston
- cylinder head and barrel
- crankshaft
Crankcase - magnesium
Piston - aluminium alloy
Crankshaft - steel alloy
Cylinder head and barrel - aluminium alloy and steel respectively
What valves are used by the inlet and exhaust valve? What are the differences between the 2?
Poppet valves
Inlet valve tends to be larger to allow as much air into the system (LP Inside vs out), the exhaust valve tends to get hotter than the inlet as it vents hot exhaust gases therefore tends to be made hollow and filled with sodium to absorb some of the heat up the valve stem.
What is the importance of tappet clearance?
Tappet clearance is important in the appropriate operation of the inlet and exhaust valves to ensure they are not late or early in their opening or closing. Likewise it allows for thermal expansion of metals at high temps
Most modern engines have hydraulic tappets that allow hydraulic fluid to set the best tappet clearance automatically rather than relying upon manual calibration
What is cracking and re seating pressure?
Pressure relief valves and their variants that are pressure sensitive will be cracked or their ball unseated at a specific pressure, when the system pressure drops and the ball is re seated by the string this is the feasting pressure
A pressure refile if valve that can support the full flow rate of the pump are called full flow pressure relief valves.
What is cavitation?
It is the process whereby very low pressure is produced in the hydraulic system causing high temps and ‘hammering’ of services and their uneven movement.
When must a automatic cut off (ACOV) valve be used and with what other systems and why?
An ACOV valve is used with a constant volume/displacement pump as it cannot regulate its out put and therefore system pressure. Therefore, there must be protection mechanisms in place to protect against over or under pressurisation to prevent cavitation
An accumulator is also used alongside an ACOV and a full flow relief valve that exceeds the ACOV pressure setting.
An accumulator pre charged with nitrogen to 1500psi with the hydraulic system depressurised. With the system operating the accumulator pressure gauge will read?
System pressure.
This is once hydraulic fluid exceeds the charge pressure of 1500psi.
At start up, a variable volume EDP is set at?
Maximum stroke by spring pressure
When is a RAT (ram air turbine) used?
As an emergency means of keeping hydraulic pressure to the primary flight controls.
Low hydraulic pressure sensors would be fitted where?
The outlet to the EDP
What hydraulic systems are used in light and large aircraft? Describe their salient component parts
In a light aircraft generally using one hydraulic service namely undercarriage deployment and retraction will use a hydraulic power pack. It is made up of a self contained: reservoir, linear actuators (3 - rear and nose gear), shuttle, non-return valves and pressure relief valves (and TRVs) as well as a single electrical motor. In case of electrical failure free fall control is provided to deploy the undercarriage.
In larger aircraft, extensive closed centred systems are used to run multiple hydraulic systems often comprising of 4 or more services. These are often duplicated and triplicated using PTUs, have numerous hydraulic EDPs per system as well as back up electrical pumps. Emergency pumps such as RATs are also present. Numerous pressure maintaining, NRVs and other valves make up the system. The systems are finely interlinked and therefore each hydraulic system is often colour coded to identify it
Which cooling mechanism is more efficient and why? Liquid cooled or air cooled
Liquid cooled is more efficient as you have greatest temperature control. This is due to increasing/ decreasing flow of liquid to the amount of heat generated in a specific state of flight. An air cooled engine has no such control and Cylinder Head temperature varies with 4 factors (of which 3 are in the pilots control):
- power produced by engine
- mass flow flowing over the engine
- mixture ratio
- cooling air temperature
When should the engine oil be checked in a wet and dry sump system and why?
Dry - immediately after shutdown
Wet - after at least 20 minutes post shutdown
In a wet sump, the oil cools and lubricates using splash lubrication. As the dipstick takes a measure from the sump, gravity needs time to work the oil back into the sump.
In a dry sump, because oil is stored in a tank, the most oil is in that tank during operation hence the ‘dry sump’ therefore, checking immediately after shutdown will be most accurate
What are the primary properties of the engine oil ?
Primary properties:
Lubrication
Cooling
Secondary properties:
Corrosion protection
Cleaning
And hydraulic operations
In a dry sump system are the 2 pumps the same and if not why not?
The dry sump system contains the pressure pump and the scavenge pump. To maintain a dry sump, the scavenge pump must output more volume than the pressure pump otherwise oil will begin to pool in the sump.
Precisely, the scavenge pump is approximately double the capacity of the pressure pump
What does the ignition switching system do? How is each magneto selected?
Each magneto is controlled by its own ignition switch. The switch Isolates the magnetos ignition circuit by connecting the primary circuit to earth.
The magneto is turned off by earthing the primary circuit and therefore, turned on by removing the earth connection. This is fail safe in the air as a broken wire does not affect the ignition but means that a broken wire on the ground leaves the mags live.
What are the problems of a small and excessively large gap within a spark plug?
A spark plug with too large a gap may not spark at all whereas, a small gap may produce a weaker spark and therefore, effect engine output
What are the three mechanisms to provide a larger spark upon engine start?
Impulse magneto - a spring is wound, which upon release dramatically increases the spin of the magnet inducing a larger voltage therefore, larger spark.
High tension booster coil - commonly used on larger engines, the HT booster coil induces a large current in the secondary winding while the battery powers the primary winding. A trembler interrupter is used to interrupt the primary winding circuit to produce high voltage pulses in the secondary winding
Low tension booster coil- requires the battery to work and feeds boosted battery power to the primary winding to improve slow speed power generation
What is a magneto and How does a magneto generate electrical power?
A magneto is an AC generator, transformer and distribution system providing high voltage current to spark plugs.
It Rotates a permanent magnet within a ferro-magnetic core
What is spark plug fouling?
Fouling of the spark plug occurs when deposits like carbon, lead and oil clog the gap between the spark plug electrodes inhibiting or delaying a spark.
What valve or selector operates an actuator and returns fluid back to the reservoir simultaneously?
- pressure reducing valve
- a selector
- shuttle valve
- TRV
A selector because it provides fluid to an actuator and simultaneously allows fluid to return from the actuator
The system pressure of a hydraulic system fitted with a variable volume ‘swash plate’ pump is controlled by?
A control piston because it is a constant pressure/variable volume pump, it is self regulating not requiring an ACOV. Therefore, the output of constant pressure pumps is regulated by an internal control piston which alters the stroke of the piston.
What is a depressurising solenoid?
The depressurising solenoid is energised by the selection from the cockpit to offload the engine driven hydraulic pump on engine start
A hydraulic fluid has what of these properties:
- high viscosity at all temps
- high flame point, low flash point
- low viscosity at all temps
- low flame point and flash point
Flash point - the point at which there is enough vapour to support a momentary flame
Flame point - the lowest temperature at which there is enough vapour to support a continuous flame
Therefore, a hydraulic fluid must have high of each.
It cannot have a high viscosity at all temp ranges due to excessive pressures. Therefore, it muse have low viscosity at all temps
When does a one way check valve has its flow stopped.
When input pressure is less that output pressure
What is a floating separator used for?
To separate gas and liquid inside of an accumulator
What is the affect of over or under charged gas within an accumulator?
The effects for both tend to be the ACOV operating more often and rapid pressure fluctuations.
What are the main pumps used in a light aircraft vs modern transport aircraft?
Light aircraft:
- spur gear constant volume pumps
Larger aircraft
- constant pressure piston pumps (with constant volume pumps sometimes used as backup)
What is a hydraulic fuse and where is it used in a system?
A hydraulic fuse is a one time operated device which remains open until system is reset. It is a flow rate sensitive device which tend to be used to stop leaks and are often used in exposed pipes and brake systems.
They are always placed upstream side of the hydraulic actuator I.e. On the supply side.
What is the primary purpose for the valve spring and what is the tappet clearance measured between?
The primary purpose of the springs is to close the valve which are opened by the rocker arm. The tappet clearance is measured between the valve stem and rocker pad
What will the MAP gauge on a normally aspirated engine read at full throttle?
Slightly less that atmospheric pressure
Where does valve overlap occur?
The inlet valve closes before top dead centre on the exhaust phase and exhaust closes after top dead centre on the induction phase
What is the purpose of a condenser within the ignition system?
A condenser is a capacitor. It stores a brief amount of electrical potential to assist in the breakdown of primary current and prevent arcing at the contact breaker points.
Where does the primary circuit of the magneto receive its initial current from?
The magneto primary circuit normally derives its voltage by induction, it needs no battery to start.
As RPM increases what happens to the spark of the ignition system?
It is advanced so that peak ignition comes close to the start of the power stroke.
Where is the oil cooler located on a wet sump system?
After the pressure pump (and micronic filter) but before the engine
How is engine displacement calculated?
Displacement = piston area x no cylinders x stroke length
What is the difference between a reversible and non-reversible control system?
A reversible control system is a power assisted system in which the pilot is connected directly to the controls but control movements can be hydraulically assisted in larger aircraft
A non-reversible control system is one in which the pilot is not connected to the control system, but instead is connected to a power control unit and an artificial feel unit either by cable runs or electronically in fly by wire systems.
What is a fly by wire control system?
Fly by wire removes any connection from a control column to the PCU. It is placed in a powered control system in which pilot control column actions are sent in analogue form to actuator control electronics.
The ACE computes the control deflection sends it in digital format to another ACE which then converts it to an analogue signal to be issued to the PCU.
What is a gear change system and what control surface is it most often fitted to?
A gear change system is in an effort to match large control deflections at low speed whilst safe guarding against over stress at high speed.
This type of system is most used on the rudder.
On an aircraft fitted with powered leading edge and trailing edge high lift devices what is retracted first?
Leading edge devices are retracted after trailing edge devices.
Therefore, trailing edge devices first.
What are the primary causes for detonation? If detonation is occurring how can you know and what is a remedy?
Over weak mixture
Low RPM and high power settings
Applying heat when engine config. Is in high power situations
Fuel usage with low anti-knock qualities
Detonation causes auto ignition of the fuel vapour with air and as such sounds like a knocking in the engine. The best solution is to increase RPM and ensure fuel mixture is rich in an attempt to bring the temperature down. If climbing, a shallower angle of attack to allow more airflow over the cylinders
What does the 100LL or 100/130 stand for in aviation gasoline naming convention?
LL - low lead (tetra ethyl lead added to aviation gasoline to lessen chance of detonation)
100 is the octane of the fuel at lean mixture
100/130 - the 130 refers to the octane of the fuel when fully rich
What is the relationship with CHT and EGT as mixture weakens?
Due to the slower burn cylinder head temp will increase as there is more time for the burn to transfer its heat energy to the engine body. Conversely as mixture weakens the burn temperature decreases therefore, exhaust gas temp decreases.
What is the stochiometric combustion mixture? Why is this not used in normal operations?
This is also known as the chemically correct mixture or ideal ratio. Is optimal combustion ration is 15 parts air 1 part fuel. It represents maximum power and heat released from combustion.
This is not used in Normal operations because it is a relatively weak mixture by carburettor and piston engine requirements. This mixture runs very close to high temps and as such detonation risk. Hence, why in piston engines it tends to be 12 parts to 1.
