021 - Aircraft General Knowledge

Structural Parts

• Spars = carry the wings bending moments.

• Skin = carries the pressurisation loads.

  • Gives Fuselage aerodynamic shape.

• Ribs = maintain the shape of the wings.

  • Maintains Wing aerodynamic shape.

• Stringers = help the skin to absorb the loads.

• Longerons = endures twisting and torsional loads.

Static vs Dynamic Loads

• Dynamic = loads that change in value over time.

• Static = constant/continuous loads, does not change with time.

• Cyclic - loads of similar amplitude that occur regularly during normal operation.

Hydraulics

• Hydraulic Demand Pump = Variable Swash Plate Pump.

• Constant Delivery Hydraulic Pump = Fixed Volume = Fixed Displacement Pump. (it turns at a fixed speed, no control of output).

  • Combined with Automatic Cut-Out Valve (ACOV) = fixed pressure.

  • You can Demand volume, or you get Delivered fixed.

• Check Valve = Non Return Valve.

• Active (always on) vs. Open (on demand)

Flight Controls

• Primary (REAR):

  • Rudder.

  • Elevators.

  • Ailerons.

  • Roll spoilers.

Moving Sidestick →

• PFC (Primary Flight Computer) converts movement into electrical output >

• ACE (Actuator Control Electronics) converts electrical signal into analogue >

• PCU (Power Control Unit) takes analogue values to control the hydraulic valves (open/closed)

Pneumatics

• Isolation Valve (Crossfeed): separates the 2 pneumatic systems + APU.

  • Open when system is off, for the APU.

• Bootstrap/Air Cycle/Packs: Primary Heat Exchanger > Compressor > Secondary Heat Exchanger > Expansion Turbine.

• Piston source: compressor (or supercharger, usually not turbocharger!).

Fuel

• Density:

  • AVGAS: C-172 = 0.72.

  • Jet A/A1: A380 = 0.80 (Jet A/A1).

    • Range: B777, 0.77 + 0.07 = 0.84.

  • Jet B: 0.7B = 0.78.

• Freezing Point:

  • Watch the letters and numbers:

    • "A" is like a 4 = -40.

    • A1, "1" is like a 7 = -47, (747).

    • "B" is like a 6 = -60.

• Diesel = Jet A1, Petrol = AVGAS.

• Colours: RGB - 80 - 100 - 100LL.

Fire

• Smoke Detectors: not in the Cockpit, Engine and Fuel-Tank.

• 2 discharges per designated fire zone.

  • Usually the Nacelle.

Electricity: general

• Torque = (V I) / RPM, or Power = Torque RPM.

  • V * I == Power!

• U = I*R, R = U/I, I = V/R.

  • The term on top of the fraction is directly proportional, the term on the bottom is inversely proportional

  • Usually EASA: Current prop. to Voltage.

• Power (W) = E / t or V I or I^2 R.

• 2 bus systems:

  • Split: each generator feeds its own bus. Automatically switched to working generator.

  • Parallel: several generators power all the buses simultaneously. Other generators keep working.

Fuses & Logic Gates

• Current (I, Amp.) rating:

  • "Rated Amperage" = carry continuously without trouble.

  • "Nominal Amperage" = more about the circuit itself, what you're expecting.

• AND = flat end.

• OR = rounded end, like an O.

• NOT = Triangle.

• Negative = O at the end.

Generator/CSD/IDG

• Disconnect when: low oil pressure, high oil temperature.

• IDG runs in its own oil.

• Voltage Regulator will increase the DC current (excitation current) in the coils.

• Generator Control Unit (GCU): checks for faults in the AC generator.

  • Over-excitation or Overvoltage & all commands pass through GCU (except CSD disconnect), and records failure indications.

Electrical Motors

• Seisure (slowing down): Current in Strator/Rotor and Torque Increases.

  • Because "Back EMF" (natural force against rotation/current flow) reduces (to 0).

  • Applied EMF = Net EMF.

• Runaway: Current in Strator/Rotor and Torque Decreases.

  • No Load (0), Applied EMF = Back EMF.

    • The motor keeps accelerating until Back EMF increases enough to balance.

• "Seize the net and run back".

• Torque = (U x I) / RPM.

  • Also proportional to Magnetic Field.

Engines, general

• SFC = Fuel Flow / Power == Kg/hr/power.

• Compression (=increase in static pressure), a process where heat is released. Pressure rises, temperature also.

Magnetos

• High Voltage, Low Amperage current, distributed to the Spark Plugs.

  • Primary Circuit gets opened/closed to get high voltage pulses in the Secondary Circuit.

  • The high-voltage pulse is sent to the distributor, which sends it to the Spark Plugs.

  • Thus, Secondary Current to the Sparking Plugs.

• Impulse Magnetos: retard the spark during engine start, provide stronger spark on TDC.

Piston Engines

• Intermittent process, at a constant Volume, variable Pressure.

• Compression Ratio: total vs. clearance volume.

  • Higher ratio = more detonation = higher octane needed.

• Crank Assembly = Crankshaft, Connecting Rods, Pistons (CCP).

• Carburettor Diffuser: maintains constant mixture across power settings.

• Ground-boosted = ground and altitude.

Oil

• Wet: pressure pump, integrated reservoir (WIP).

• Dry: scavenger pump, separate reservoir (DSS).

• Low Pressure = less oil for cooing = Higher Temperature.

• Auxiliary gear box = Engine oil. Supplementary & IDG = own oil.

• Fuel/Oil Heat Exchange Leak: flows from HP to LP.

FADEC

• Ignition timing (with RPM), mixture ratio (based on EGT, MAP, RPM).

• Has 2 separate ECUs (digital channels/computers).

• In manual mode, TLA directly send to the ECU, which adjusts FF for commanded N1.

• Total Failure = Engine Failure (EASA).

Propeller

• Slipstream: low TAS, high Power.

• Alpha & Beta Range:

  • Via the POWER LEVER!

  • Alpha: power lever between flight idle, and maximum power. RPM lever controls the angle.

    • Feather via the RPM/Condition lever.

  • Beta: between flight idle and maximum reverse.

    • Beta-range/Reverse: via the POWER/THROTTLE lever.

• RPM-lever is brave! Goes in first, out last.

• CentriFugal twist = Fine pitch direction.

• AerOdynamic twist = cOarse pitch.

Turbines (Jets)

• Continuous combustion at constant Pressure (minimum loss), variable Volume.

• Continuous = Low-intense Spark (High and 'Wet'):

  • Continuous Automatically: engine anti-ice on, Stick Shaker, throttle idle.

• Ground Idle:

  • Short = core, Period = prevent rotor seizure.

• Always pick the combination of "expansion" and "velocity".

• Measurement:

  • Fuel: between control unit and engine burners.

  • Pressure: between booster pump and engine.

Turbine Problems

• Stall (imbalance of air) = N1/N2 still rotating, but fluctuating.

  • Stagnates in the Rear, Stall in the Front.

  • VIGVs/VGVs/Bleed Valves reduce it.

• Surge (too much fuel) = fluctuating/no RPM. High EGT.

• FOD = No RPM, No FF, Low Oil Pressure, High EGT, High Vibration.

• Dry Start: Fuel fails to flow to the engine.

• Hung Start: The engine lights up (EGT>), but fails to accelerate properly (low N2 RPM).

• Hot Start: Light-up (at low RPM) is followed by a very rapid rise in EGT.

• Wet Start: The engine fails to light up, RPM stabilises at a low value.

Thrust

• Thrust = M x (V_jet – V_flight) + A (p_nozzle – P_ambient)

  • Little thrust variation with speed!

• EPR: turbine out / engine inlet (compressor or fan pressure).

• Air from the compressor, via the diffuser. Fuel directly added as a constant spray.

  • Diffuser is not Swirl Vane!

    • Diffuser reduces V (high speed from the compressor), increases P.

      • For optimal combustion.

    • Swirl Vane reduces axial Fuel Flow speed, and swirls the mixture in the flame-zone.

      • Stabilises the flame.

Impulse vs Reaction Turbine

• Pressure:

  • IDC RCD - "I Don't Care about Remote Control Devices".

    • Vanes, Rotor Blades.

    • So, RCD - rotors are D, which means that they are convergent.

• Impulse = nozzle-guide-vane is the jet, Reaction = turbine-rotor is the jet.

  • Impulse rotor spins because air pushes against it.

  • Reaction rotor spins because it is generating a force.