Systems All

What type of electrical system do large aircraft primarily use?

AC

- AC alternators common for lighter aircraft

- AC generators used when weight is less vital

- Some aircraft require DC (AC gets converted)

Why do large aircraft systems primarily use AC?

- Their high demand for electrical power

- Lots of wiring needed

- AC wiring (high voltage/low current) is smaller in diameter

Why is AC wiring better than DC wiring (aside from diameter)?

AC's higher voltage/lower current loses less voltage so better for longer wires (more potential to get to the end)

What are the components of a DC generator?

- Rotating conductor (armature)

- Stationary bar/electromagnet

- Split rings (pulsating DC) and brushes (commutator)

What are the components of an AC generator?

- Rotating conductor (armature)

- Stationary bar/electromagnet

- Slip rings (AC)

What are the components of an alternator?

- Stationary conductor (stator)

- Rotating bar/electromagnet (rotor)

- Slip rings

What defines AC and DC?

Brushes and rings

What is the downside of split and slip rings?

Arcing and energy losses

What is the main difference between an alternator and a generator?

Alternator load current comes from the stator (outside windings) while the generators load current comes from the armature (rotating part) and has to flow through the commutator

What is the process of a generator?

Generator -> AC wild -> CSD, GCU, IDG or VSCF -> AC/DC conditioned

How do you start an alternator?

- DC current energises rotor coil for magnetic field

- Provided by battery

What is a 3-phase alternator?

- 3 coils

- Individually produces single-phase AC

- Produces 120V AC across each phase (208V AC across two phases)

- Generally 400Hz

What is the voltage for large aircraft?

110V, 120V or 210V

How is a generator controlled?

Components for

- Current limitations

- Over voltage protection

- Prevention of reverse flow of current from battery through alternator/generator

How is a generator operated over a range of speeds?

Hydraulic or mechanical constant speed generator drive unit (CSGD) or variable speed constant frequency output (VSCF)

What is a Constant Speed Drive (CSD)?

- Has a design input speed

- At lower/higher input speeds, hydraulic link adds/subtracts speed

- Normally between the engine gearbox and generator

What is an Integrated Drive Generator (IDG)?

- Generator and CSD together

- Lighter

- Aka Integrated Constant Speed Drive (ICSD)

Can the CSD/IDG be disconnected in flight?

- Yes, by a switch

- If fault in generator/gearbox

- Can't be reset

What is a Variable Speed Constant Frequency System (VSCF)?

- No CSD

- Generator produces 'wild' frequency power

- Conditioned to give constant frequency AC

- Less moving parts

- Cyclo-converter system or DC link

What is a Generator Control Unit (GCU)?

- For generator to be starter

- Controls starter motor-generator

- Like VSCF by more functions

What is AC wild power?

- Power that has frequency variation

- Some electrical equipment can use it

What are the controls of a generator?

- Sensing if frequency drops (so current doesn't increase)

- Sensing voltage (so it stays within limits)

- Real and reactive load equalisation (for parallel generators otherwise they might overheat etc)

What is a rectifier?

- Converts AC to DC

- One or more semi-conductor diodes

- Bridge rectifier has diodes stopping current going one way

What is a transformer-rectifier?

- Rectifies AC to DC

- Reduces AC voltage to required level (28V)

What is an inverter?

Converts DC to AC

What is an inductor?

- Like resistor in AC circuit (changes voltage and current)

- In DC circuits, filters voltage fluctuations

- Stores energy in magnetic field and current flows through wires

- If a spike occurs, magnetic field cuts through itself and opposes change in current

What is the purpose and the three basic types of large aircraft AC power distribution systems?

Allows load sharing and redundancy

- Split-bus system (twins)

- Parallel system (three/four engine)

- Split/parallel system

What is a split-bus system?

Generator 1 and 2 each supply bus 1 and 2 respectively, is one of then fails, it is isolated by the generator breaker and a bus tie will close so that the other generator can feed both busses

What is a parallel system?

All generators are connected to a common bus and share the load equally. If a generator fails, it gets isolated and a bus tie breaker selected so the bus stays working

What is a compound system?

- Two RH alternators in parallel

- Two LH alternators in parallel

- LH and RH busses kept isolated by a split-system breaker

How is emergency power supplied?

- DC from battery inverted to AC

- Ram Air Turbine (RAT), wild frequency)

- APU (constant speed gas turbine)

- Pneumatic air system

- Hydraulic system

What is the ground power supply?

- Electrical power for starting

- Also for maintenance

- Battery cart

How does a 28V DC plug and socket work?

- Has two longer pins and one shorter pin

- Two pins must be fully engaged before the power turns on

- Shorter pin activates a solenoid to generate power so there is no sparking

Why are large aircraft primary flight controls often split?

To lower control authority and provide redundancy

What is done to flight controls to prevent damage (specifically wing warping)?

The amount they travel can be reduced with speed or with ailerons, out ailerons are locked at high speed

What is it called when the flight control is the whole of the surface?

All-flying tailplanes, all moving wingtips and all moving fins

What are the types of secondary flight controls?

- Trim tabs

- Balance tabs

- Servo tabs

- Anti-servo tabs

- Spring tabs

- Variable incidence tailplanes

What is the purpose of a trim tab?

Trims the controls via trim wheel in the cockpit, holding controls in the desired position

What is the purpose of a balance tab?

Reduces control loads (aids control)

What is the purpose of a servo tab?

Assists movement (manually connected to control stick)

What is the purpose of anti-servo tabs?

Increases control feel making it more difficult to move the controls

What is the purpose of spring tabs?

Like a servo tab but the spring varies the load

What are the auxiliary controls?

- Spoilers (can be part of primary flight controls)

- Speed brakes

What are high lift devices?

- Flaps

- Slats

- Slots

- Leading edge flaps (Kruger, droop)

How are primary flight controls mainly mechanically actuated?

- Push-pull control rod system

- Cable and pulley system

- Combination of the above

What helps the pilot to move the flight controls?

- Aerodynamic balancing and/or tabs

- Power actuators (hydraulic jacks) either power assisted or power operated

What are power assisted controls?

- Still have direct mechanical link

- As pilot's control moves, control surface moves and pilot/spool valve directs hydraulic fluid to an actuator

- Actuator assists moving control

- Valve resets to a neutral/locked position

How will shockwaves affect the control system for power assisted?

Causes buffeting which is fed back through the control system

How do power operated controls work?

Pilot control directly linked to actuator control valve. Actuator provides all effort needed. Pilot has no actual feel of control forces, it requires artificial feel

What is a power control unit (PCU)?

A pilot/spool valve and actuator incorporated together

What happens if there is a complete hydraulic failure?

Hopefully emergency hydraulic systems are available, but if not, differential engine control

What is a fly-by-wire system?

Pilot controls/movements transmitted (with synchro's or digital data bus) electronically to actuators. Like a power operated system but incorporates control valves to direct fluid to actuators (uses a feedback system)

What is artificial feel?

"q" feel system, based off existing dynamic pressure. Computer calculates forces based on control surface position and airspeed, uses hydraulic pressure to oppose pilot movements

What is actuator gust snubbing (gust locks)?

Large aircraft can't fit external control locks so they have internal control column locks or a gust suppression system fitted in the control actuator. The flight control is free to move but won't hit the primary stop with force (causing damage)

What is the take-off configuration warning system?

An audible warning if the aircraft isn't configured for take-off and the thrust levers are advanced to take-off. It uses position sensors and the thrust management computers

When will a stall warning typically go off?

Warning of approach stall, the margin is typically 5% above the stall airspeed

How does the stall warning work?

Stick shaker, stick pusher and audible warnings only cancelled when AoA if reduced

What is the pitch limit indicator (PLI)?

Estimates the aircraft pitch at which the stick shaker will operate

What are the inputs for the stall warning?

- N1/EPR (Engine power)

- AoA

- WOW switch

- ADC

What is the overspeed warning system?

- Warns if airspeed will exceed Vmo/Mmo

- Airspeed/mach system

- Master warning lights, OVSPD light, EICAS overspeed message, aural warnings

- Remains active until the aircraft airspeed is reduced

What is envelope control?

In a fly-by-wire system, the control is by computers and they can be designed to keep the aircraft inside the flight envelope, limiting pilot input and prevent overstress, overspeed, excessive AoA and excessive altitude. AFCS prevents structural and aerodynamic limitations being exceeded

What are the inputs for envelope control?

- Control wheel

- Control column

- Rudder pedals

- Speed-brake lever

- Pitch trim switches

- Position sensor (feedback)

- ADIRU/ADAHRS

What is the automatic flight control system (AFCS)?

Combines flight director and autopilot. Gets navigation information from the FMS/IRS/GPS, transmits to servos which get feedback from flight control position and displays aircraft attitude via flight director

What are the modes and controls of AFCS?

Auto land, yaw damper, auto trim system, auto throttle, all controlled within design envelope (Green is active, white is armed)

What are the inputs for the AFCS?

- ADC (speed and mach number)

- Encoding altitude (altitude)

- Rate gyro (yaw damping)

- IRS (heading)

- FMS (track)

When will the autopilot disengage?

- When pilot input is detected

- When switch is activated

- If turbulence is sever

- System malfunction

What is the autoland system?

Fully automated landing with crew supervising and only intervene if a system fails, in an emergency or after landing

What is the alert height (AH)?

Height above the runway, based on aircraft type and its fail operational automatic landing systems. Above it, an approach must be discontinued if a specific failure occurs

What are the AFDA status annunciations (ASA)?

- Land 2

- Land 3

- No auto land

What is land 2 for autoland?

AFDS redundancy reduced (one autopilot unavailable)

What is land 3 for autoland?

All autopilots engaged and all system operating normally

What are the two failure modes for autoland?

- Fail operation

- Failure passive

What is fail operation?

- Must have at least two autopilots engaged for the approach

- Failure of one you can still autoland

- No decision height approach

- Land 3, AFDS has redundancy below AH

- At AH check ASA and discontinue if failure

What is fail passive?

- Single autopilot approach

- Failure won't cause immediate deviation from desired flight path

- Immediate control must be assumed and carry out missed approach unless visual

- With land 2 annunciated, AFDS has redundancy below AH

What is the lowest allowable DA for a fail passive system?

50ft

Why is a yaw damper required?

To control dutch roll and limit pilot input as airspeed increases

What are the inputs for the yaw damper?

- Primary control system

- IRS rate gyro

- ADC

- Gust pressure transducers

- Rudder ratio changer

What is the automatic trim system?

Automatically adjusts pitch trim with autopilot engaged, confined to pitch trim. Trim servomotor operates in parallel with autopilot pitch control servo. Limited to half the rate of manual trimming, it typically adjusts tailplane incidence

What makes the automatic trim system required?

Fuel burn, changing speed, changing configuration, any CoG changes

What are the components to the automatic trim system?

- Pitch trim servomotor

- Stabiliser trim panel

What is the pitch trim servomotor?

Separate hydraulic screw type actuator or electric motor actuator

What is the stabiliser trim panel?

Provides pilot trim switches and ability to isolate 2 hydraulic systems

Where are fire protection systems built-in to aircraft?

In designated fire zones and potential fire zones

What are the requirements for a fire detection system?

- Warns immediately of fire/over heat

- Red light and audible signal

- Indicates extinguished fire

- Indicates re-ignition of fire

- Shows location

- Durable and resistant to damage

- Auto re-sets when fire is extinguished

When would smoke and toxic gas detection be used?

When type of fire anticipated is expected to produce more smoke before sufficient temperature change

What will a smoke and toxic gas detection system be?

Generally in cargo/baggage hold and lavatory

- Light refraction type (photoelectric cell)

- Ionisation type

- Solid-state type

What is the light refraction type of smoke detector?

Smoke interferes with light beams and reflects it into a lens and photocell which sets off alarm

What is the ionisation type smoke detector?

Ions align creating a circuit with electricity from battery. As smoke particles enter alarm, ion path is broken up and weakens circuit, setting off alarm

Where would detection systems typically be located?

- Powerplant

- Cargo hold

- APU

- Lavatory

What are the three systems for fire detection?

Thermo-switch

Thermocouple

Tubular type

- Fenwal

- Kidde

- Pneumatic type (Systron-donner)

- Lindberg system

What is a thermo-switch system?

Aka a spot detector, a circuit with one or more thermal switches connecting in parallel. Outer casing expands with heat (bimetallic design) and elongates, the two contacts inside then meet and close the circuit activating the warning system. Will reset itself

What is a thermocouple system?

Two dissimilar metals with one end fused together, placed in an open frame, as airflows over the probe, the exposed wires at one end create a hot junction and the other end is under insulating material which creates a cold junction. The temperature difference creates a current which sets off a sensitive relay. If temperature rise is slow, cold junction heats up along with the hot junction

What is a fenwal system?

Continuous loop, small, lightweight, flexible inconel tube, pure nickel wire centre conductor, space between conductor and tubing wall has ceramic insulating material and eutectic salt packing which has a low melting point. When heated, current flows between the wire and the tube wall (salt melts and drops resistance) which signals control unit. Will correct itself once the fire is out

What is a kidde system?

Continuous loop system, inconel tube with thermistor material embedded with two electrical conductors. When heated, thermistor drops resistance, sensed by electronic control circuit monitoring system (wire conductors). Second wire is the grounding wire, grounded to outer shell

What is the pneumatic system (systron-donner)?

Very similar to Lindberg pneumatic fire detection system

Continuous-length system, stainless steel tube with two separate gases and a gas absorption material (wire). Tube will normally have helium gas under pressure. Titanium centre wire has hydrogen gas, wrapped in helical shape. Gaps between turns release hydrogen gas under certain temperatures and once corrected, titanium reabsorbs hydrogen. Constant volume means pressure increases with temperature (helium gas in tube exerts more pressure and closes pneumatic switch)

What is the role of fire extinguishing agents?

- Displaces oxygen (CO2/dry powder)

- Chemically combines with oxygen to prevent combustion

Why aren't dry chemical extinguishers used?

They cause damage by toxic and corrosive chemicals

What are the two types of airframe fire extinguishing systems?

- Conventional (older)

- High Rate of Discharge System (HRD System)

How does a conventional fire extinguishing system work?

Perforated ring and distributor nozzle, when activated, agent bottles are opened and gas flows through lines to selected engine, flowing out of the perforated ring to smother fire

What is an HRD system?

Spherical bottles under high pressure, actuated by explosive charges (squibs) to flood compartment and displace oxygen. Propels extinguishing agent at high speeds