ATPL Aircraft General Knowledge cheat sheet

Limit load is the maximum load expected in service; ultimate load is limit load multiplied by the factor of 1.5, and the structure must withstand ultimate load without failure for 3 seconds.

Fail-safe (damage-tolerant) design allows a cracked member to shed load to redundant structure until the defect is found; safe-life design retires a component after a fixed fatigue life.

Shear webs and skin carry shear and torsion; stringers and spar caps carry bending; a monocoque skin carries all loads, a semi-monocoque uses frames and stringers to stabilise thin skin.

Hydraulic pressure P = Force/Area, so a 20 cm² piston at 200 bar (20 MPa) delivers 40 kN; pump flow (not pressure) sets actuator speed.

A pressure relief valve protects the system against overpressure and returns fluid to return; a pressure regulator (unloading valve) cuts pump delivery to zero at system pressure.

Selectors are typically open-centre (flow returns to reservoir at neutral, pump unloads) or closed-centre (flow blocked at neutral, requires a pressure regulator or variable-displacement pump).

Hydraulic fluid colour codes: MIL-H-5606/mineral red, Skydrol (phosphate ester) light purple/violet, and phosphate-ester fluid attacks natural rubber seals, so use butyl or EPR seals only.

Accumulators store energy for peak demand and damp pressure surges; precharge with dry nitrogen, never oxygen or air, and check precharge with the hydraulic system depressurised.

Air-cycle (bootstrap) cooling: hot engine bleed passes through a primary heat exchanger, compressor, secondary heat exchanger, then expansion through a turbine that drops temperature and pressure before delivery to the cabin.

Cabin pressure altitude is the equivalent altitude corresponding to cabin pressure; maximum differential pressure (ΔP = Pcabin − Pambient) is limited by the outflow valve and positive-pressure relief valve.

Cabin rate of climb/descent is controlled by the outflow valve; typical scheduled rates are about 500 ft/min climb and 300 ft/min descent, with 8,000 ft cabin altitude a common limit.

The outflow valve modulates cabin pressure; if it fails open the cabin altitude rises toward aircraft altitude; if it fails closed, ΔP rises and the positive relief valve protects the structure.

A negative pressure relief valve opens inward to prevent ambient pressure exceeding cabin pressure during rapid descent, avoiding inward structural loading.

Pressurisation is normally not applied above a cabin altitude of about 8,000 ft without oxygen; crew use quick-donning masks with demand regulators delivering 100% oxygen under pressure.

Oxygen systems: gaseous (high-pressure cylinders, reducing valve to about 300–400 psi), liquid (LOX converter, about 1 litre liquid yields roughly 800 litres gas), and chemical (sodium chlorate candles, about 2–3 minutes to full flow).

Continuous-flow oxygen systems suit passengers; diluter-demand regulators supply air/oxygen mixture to crew up to about 33,000–40,000 ft, above which 100% oxygen with pressure breathing is required.

Pneumatic de-icing boots cycle inflation to break ice; they remove ice rather than prevent it, and boot cycling must not be left on continuously because ice can bridge over an inflated boot.

Thermal anti-ice uses engine bleed air through piccolo tubes on wing leading edges and engine inlets; it prevents ice formation and is used before ice accretion, whereas de-icing removes accumulated ice.

Electro-thermal systems use heater mats on propeller blades, pitot heads, static plates and windshield; pitot heat is switched on before flight in visible moisture and checked by current draw.

Freezing rain and supercooled large droplets (SLD) can cause ice beyond protected areas (runback ice); ice-contaminated tails can stall at lower angles with little or no warning.

Holdover time (HOT) is the estimated period a de/anti-icing fluid protects against ice accretion; it depends on fluid type, precipitation, temperature and wind, and must be checked before takeoff.

Type I fluid is a heated, low-viscosity de-icing fluid with short holdover; Types II, III and IV are thickened anti-icing fluids with longer holdover, and Type IV has the longest.

Windshield rain removal uses wipers, permanent hydrophobic coatings, or pneumatic rain repellent; windshield heating prevents fogging and ice and is interlocked to avoid overheating on the ground.

Air data probes: pitot tubes must be heated; static ports are usually not heated but must be clear, and blocked static ports cause airspeed and altitude errors that persist after level-off.

Fire detection zones: engine fire loops use resistance or thermocouple (rate-of-rise) sensing; cargo and lavatory use smoke detectors; wheel wells use overheat detectors.

A continuous-loop detector (e.g. Kidde) uses a resistance wire whose resistance falls with temperature, triggering the fire warning; a shorted or open loop usually produces a fault indication.

Two-loop engine fire detection gives redundancy: both loops must detect (AND logic) for a warning in some designs, while one-loop operation can be selected after a loop failure.

Fire extinguishing agents: Halon 1301 (bromotrifluoromethane) is the standard for engines and APU; it inhibits the flame reaction and is effective at low concentration, about 5–10%.

Engine fire bottles are usually discharged by an explosive squib; a single bottle may be wired to two discharge heads so the crew has a choice of one or two shots.

Cargo compartment fire suppression uses bottles with metered (slow) discharge to hold concentration after the initial knock-down; Class C cargo compartments have built-in fire suppression.

Fire warning test is done before flight; a red fire warning requires the memory items: thrust lever off, fire handle pull, agent discharge, then land as soon as possible.

APU fire detection and extinguishing is automatic on the ground in many types, and a fire handle closes the fuel shut-off valve, bleeds and electrical supply before agent release.

DC generator/alternator output is regulated by a voltage regulator; a reverse-current relay or diode prevents battery discharge through a failed generator.

A starter-generator combines starting and generating functions; on turbine engines a CSD (constant-speed drive) or integrated drive generator (IDG) keeps AC frequency at 400 Hz.

AC power is typically 115 V AC, 400 Hz, three-phase; a transformer-rectifier (TRU) converts AC to 28 V DC for battery-charging and DC buses.

Batteries: lead-acid (24 V, 20–25 Ah typical) or NiCd (nickel-cadmium); NiCd cells have lower internal resistance and can supply high current but suffer thermal runaway if overcharged.

Bus bar systems: a battery bus is powered directly from the battery and feeds essential loads; an essential bus can be fed by the battery through a static inverter if all generators fail.

Circuit protection: fuses are one-shot; circuit breakers are resettable, and a tripped breaker should not be reset more than once without finding the fault.

Electrical bonding and shielding reduce radio interference and static discharge; static dischargers on trailing edges bleed accumulated charge.

Ohm's law V = I × R and power P = V × I = I²R; a 28 V, 5 A load dissipates 140 W and has 5.6 Ω resistance.

Four-stroke cycle: induction, compression, power (combustion), exhaust; in a horizontally opposed engine the firing order and valve timing are set by the camshaft at half crankshaft speed.

Detonation is uncontrolled explosive combustion causing high peak pressures; pre-ignition is ignition before the spark. Both raise cylinder head temperature and can damage the engine.

Mixture control: best power mixture is about 50–100 °F rich of peak EGT; best economy is lean of peak, but lean mixtures raise cylinder temperature and risk detonation if too lean at high power.

Turbocharging uses exhaust gas energy to drive a turbine which drives a compressor; a wastegate controls boost, and an overboost can exceed the manifold pressure limit.

Turbine engine: air is compressed, fuel added in the combustor, and expansion through the turbine drives the compressor; a free-turbine (turboprop) has no mechanical link between gas generator and propeller.

Engine pressure ratio (EPR) = turbine discharge pressure / compressor inlet pressure and is a thrust parameter on turbojet/turbofan engines; N1 (fan speed) is used on high-bypass engines.

Turbine engine start sequence: starter rotates N1/N2, ignition on, fuel on at a minimum N2, light-up indicated by EGT rise; a hot start (EGT exceeding limit) requires fuel and ignition off and continued motoring.

A hung start is failure to accelerate to idle due to insufficient starter or fuel; a wet start is fuel without ignition, and residual fuel must be cleared by motoring before another start attempt.

Propeller pitch is the angle between the blade chord line and the plane of rotation; coarse pitch suits high speed, fine pitch suits takeoff and high power at low speed.

Blade angle plus angle of attack equals the helix (pitch) angle; as forward speed increases at constant RPM the angle of attack decreases unless pitch is increased.

Constant-speed propellers use a governor with flyweights sensing RPM; a drop in RPM moves the pilot valve to decrease pitch and restore RPM.

Propeller efficiency = thrust horsepower / shaft horsepower; it peaks at a specific advance ratio (V/nD) and falls off at very high or very low speeds.

Feathering aligns the blades with the relative airflow to minimise drag and stop windmilling; unfeathering uses accumulator pressure or starter to return blades to fine pitch.

Reverse thrust is achieved by rotating blades through flat (zero) pitch to a negative angle; it is used on the ground only and is limited by engine and propeller limits.

Propeller synchronisation matches RPM of multiple engines; synchrophasing sets a fixed blade phase angle between propellers to reduce noise and vibration.

Propeller ice protection is usually electro-thermal on the blades; ice increases drag, reduces thrust and can cause severe vibration and imbalance.

Fuel system components: tanks, boost/engine-driven pumps, selector valves, filters, fuel/air or fuel/oil heat exchangers, and a fuel flow transmitter to the indicator.

Fuel tank vents prevent vacuum or overpressure during climb and descent; a blocked vent can cause fuel starvation or tank structural damage during refuelling.

Fuel quantity is measured by float, capacitance or ultrasonic systems; capacitance gauges measure dielectric change and are affected by fuel density, so they read mass only when calibrated.

Water in fuel collects at sumps and drains; check drains before flight, since water causes rough running and can freeze in lines at altitude.

Lubrication: pressure pump supplies oil to bearings, a scavenge pump returns it to the tank, and a pressure relief valve limits pressure; oil cools, cleans, seals and protects against corrosion.

Oil viscosity grade e.g. SAE 40 or 15W-50; multigrade oils keep viscosity more constant with temperature; too high viscosity raises pressure and can bypass the relief valve.

Engine indications: EGT reflects exhaust gas temperature and mixture; CHT reflects cylinder head temperature; TIT (turbine inlet temperature) is the limiting turbine parameter.

On turbine engines, oil pressure low and oil temperature high together suggest a failing pump or blocked cooler; oil consumption rising sharply can indicate a seal or breather problem.

Primary controls: ailerons (roll), elevators (pitch), rudder (yaw); secondary controls: flaps (lift/drag), trim tabs (balance control forces), spoilers (lift dump and roll assist).

Differential ailerons give more up-deflection than down-deflection to reduce adverse yaw; aileron reversal can occur at high speed if wing torsional stiffness is insufficient.

Static stability: positive static stability means a disturbance produces a restoring force; the horizontal stabiliser provides pitch stability, and the CG position sets the static margin.

A trim tab moves opposite to the control surface; an anti-balance (anti-servo) tab moves in the same direction to increase control feel on stabilators.

Autopilot modes: pitch (altitude hold, vertical speed, glide slope), roll (heading, VOR/LOC, approach); a servo drives the control surface through a clutch that can be overridden by the pilot.

Autopilot disengagement must be possible by a quick-release (control wheel) button, a disconnect switch, and automatic disengagement under excessive attitude or servo failure.

Landing gear: oleo (shock absorber) strut uses compressed nitrogen and hydraulic oil; the torque links keep the wheel aligned and transmit braking and side loads.

Gear retraction uses hydraulic or electric actuators with uplocks; a squat switch (weight-on-wheels) prevents gear retraction on the ground and inhibits other systems such as pressurisation.

Limit load is the maximum expected in service; ultimate load is 1.5 × limit load, and the structure must withstand it for 3 seconds without failure.

Fail-safe design allows a cracked member to shed load to redundant structure; safe-life design retires a component at a fixed fatigue life.

Semi-monocoque construction: skin carries shear and torsion, stringers and spar caps carry bending, frames stabilise the skin.

Hydraulic pressure P = F/A; pump flow, not pressure, sets actuator speed, and a relief valve protects against overpressure.

Open-centre selector returns flow to the reservoir at neutral (pump unloads); closed-centre blocks flow and needs a pressure regulator or variable-displacement pump.

Accumulators store energy for peak demand and damp surges; precharge with dry nitrogen only, with the hydraulic system depressurised.

Phosphate-ester hydraulic fluid (Skydrol) requires butyl or EPR seals; mineral fluid (MIL-H-5606) is red and uses natural rubber seals.

A pressure regulator (unloading valve) cuts pump delivery to zero at system pressure, while a relief valve opens only as a backup against overpressure.

Air-cycle cooling: bleed air passes through a primary heat exchanger, compressor, secondary heat exchanger, and expansion turbine before entering the cabin.

Cabin pressure altitude is the equivalent altitude of cabin pressure; maximum differential pressure ΔP = Pcabin − Pambient is limited by the outflow valve and positive relief valve.

The outflow valve controls cabin pressure and rate of change; typical limits are about 500 ft/min climb and 300 ft/min descent, with 8,000 ft cabin altitude a common maximum.

If the outflow valve fails open, cabin altitude rises toward aircraft altitude; if it fails closed, ΔP rises and the positive pressure relief valve protects the structure.

A negative pressure relief valve opens inward to prevent ambient pressure exceeding cabin pressure during rapid descent.

Continuous-flow oxygen suits passengers; diluter-demand regulators supply air/oxygen to crew, and above about 33,000–40,000 ft 100% oxygen with pressure breathing is required.

Chemical oxygen generators (sodium chlorate candles) produce oxygen for about 2–3 minutes after activation and are used for passenger drop-down masks.

LOX converters store liquid oxygen, about 1 litre yielding roughly 800 litres of gas; high-pressure gaseous systems use a reducing valve to about 300–400 psi.

Pneumatic de-icing boots remove accumulated ice by cycling inflation; they do not prevent ice, and continuous inflation lets ice bridge over the boot.

Thermal anti-ice uses engine bleed air through piccolo tubes on wing leading edges and engine inlets; it prevents ice before accretion.

Electro-thermal heating protects pitot heads, static plates, propeller blades and windshields; pitot heat is switched on before flight in visible moisture.

Supercooled large droplets and freezing rain can produce runback ice beyond protected areas, and an ice-contaminated tail can stall with little or no warning.

Holdover time is the period a de/anti-icing fluid protects against ice accretion; it depends on fluid type, precipitation, temperature and wind.

Type I fluid is heated, low-viscosity de-icing fluid with short holdover; Types II, III and IV are thickened anti-icing fluids, with Type IV giving the longest holdover.

Windshield rain removal uses wipers, hydrophobic coatings or pneumatic rain repellent; windshield heating prevents fogging and ice and is interlocked against ground overheating.

A blocked static port causes airspeed and altitude errors that persist after level-off; pitot tubes must be heated, static ports must be clear.

Engine fire loops use resistance or rate-of-rise thermocouple sensing; cargo and lavatory compartments use smoke detectors; wheel wells use overheat detectors.

A continuous-loop detector uses a resistance wire whose resistance falls with temperature; a shorted or open loop usually produces a fault indication.

Halon 1301 is the standard engine and APU extinguishing agent; it inhibits the flame reaction and is effective at about 5–10% concentration.

Engine fire bottles are discharged by an explosive squib; a single bottle can be wired to two discharge heads, giving the crew one or two shots.

Cargo fire suppression uses an initial knock-down followed by metered (slow) discharge to hold agent concentration.

A red fire warning requires memory items: thrust lever off, fire handle pull, agent discharge, then land as soon as possible.

The fire handle closes the fuel shut-off valve, bleeds and electrical supply before agent release; the APU fire system is often automatic on the ground.

Two-loop engine fire detection gives redundancy; some designs require both loops to detect (AND logic), and one-loop operation can be selected after a loop failure.

DC generator/alternator output is regulated by a voltage regulator; a reverse-current relay or diode prevents battery discharge through a failed generator.

AC power is typically 115 V, 400 Hz, three-phase; a transformer-rectifier (TRU) converts AC to 28 V DC for battery charging and DC buses.

A CSD (constant-speed drive) or integrated drive generator (IDG) keeps AC frequency at 400 Hz regardless of engine speed.

NiCd batteries have low internal resistance and high current capability but can suffer thermal runaway if overcharged; lead-acid batteries are 24 V, about 20–25 Ah.

The battery bus is fed directly from the battery and supplies essential loads; an essential bus can be fed through a static inverter if all generators fail.

Fuses are one-shot; circuit breakers are resettable, and a tripped breaker should not be reset more than once without finding the fault.

Bonding and shielding reduce radio interference and static discharge; static dischargers on trailing edges bleed accumulated charge.

Ohm's law V = I × R and power P = V × I = I²R; a 28 V, 5 A load dissipates 140 W and has 5.6 Ω resistance.

Four-stroke cycle: induction, compression, power, exhaust; the camshaft turns at half crankshaft speed and sets valve timing.

Detonation is uncontrolled explosive combustion with high peak pressures; pre-ignition is ignition before the spark; both raise cylinder head temperature.

Best power mixture is about 50–100 °F rich of peak EGT; best economy is lean of peak, but too lean a mixture at high power risks detonation.

A turbocharger uses exhaust energy to drive a compressor; the wastegate controls boost, and overboost can exceed the manifold pressure limit.

Turbine engine: air compressed, fuel added in the combustor, expansion through the turbine drives the compressor; a free-turbine turboprop has no mechanical link between gas generator and propeller.

EPR = turbine discharge pressure / compressor inlet pressure and is a thrust parameter on turbojets; N1 fan speed is used on high-bypass turbofans.

Start sequence: starter rotates N2, ignition on, fuel on at minimum N2, light-up shown by EGT rise; a hot start requires fuel and ignition off and continued motoring.

A hung start is failure to accelerate to idle; a wet start is fuel without ignition, and residual fuel must be cleared by motoring before another attempt.

Blade angle is the angle between the blade chord line and the plane of rotation; coarse pitch suits high speed, fine pitch suits takeoff.

Blade angle plus angle of attack equals the helix angle; as forward speed rises at constant RPM the angle of attack falls unless pitch is increased.

A constant-speed governor uses flyweights to sense RPM; a drop in RPM moves the pilot valve to decrease pitch and restore RPM.

Propeller efficiency = thrust horsepower / shaft horsepower; it peaks at a specific advance ratio (V/nD).

Feathering aligns the blades with the relative airflow to minimise drag and stop windmilling; unfeathering uses accumulator pressure or the starter.

Reverse thrust rotates the blades through flat (zero) pitch to a negative angle; it is used on the ground only and is limited by engine and propeller limits.

Synchronisation matches RPM of multiple engines; synchrophasing sets a fixed blade phase angle between propellers to reduce noise and vibration.

Electro-thermal propeller ice protection adds drag and imbalance if ice accumulates; ice reduces thrust and causes severe vibration.

Fuel system components: tanks, boost and engine-driven pumps, selector valves, filters, heat exchangers and a fuel flow transmitter.

Tank vents prevent vacuum or overpressure in climb and descent; a blocked vent can cause starvation or tank structural damage during refuelling.

Capacitance fuel gauges measure dielectric change and are affected by fuel density; they read mass only when calibrated.

Water in fuel collects at sumps and drains; check drains before flight, because water causes rough running and can freeze in lines at altitude.

Lubrication: a pressure pump supplies oil to bearings, a scavenge pump returns it to the tank, and a pressure relief valve limits pressure.

Oil viscosity grade e.g. SAE 40 or 15W-50; multigrade oils keep viscosity more constant with temperature, and too high viscosity can bypass the relief valve.

EGT reflects exhaust gas temperature and mixture; CHT reflects cylinder head temperature; TIT is the limiting turbine parameter.

Low oil pressure with high oil temperature suggests a failing pump or blocked cooler; a sharp rise in oil consumption can indicate a seal or breather problem.

Primary controls: ailerons (roll), elevators (pitch), rudder (yaw); secondary: flaps, trim tabs and spoilers.

Differential ailerons give more up-deflection than down-deflection to reduce adverse yaw; aileron reversal can occur at high speed if wing torsional stiffness is insufficient.

Positive static stability means a disturbance produces a restoring force; the horizontal stabiliser provides pitch stability and CG position sets the static margin.

A trim tab moves opposite to the control surface; an anti-balance (anti-servo) tab moves in the same direction to increase control feel.

Autopilot modes: pitch (altitude hold, vertical speed, glide slope) and roll (heading, VOR/LOC, approach); servos drive the surfaces through an overridable clutch.

Autopilot disengagement must be possible by a control-wheel quick-release button, a disconnect switch, and automatically under excessive attitude or servo failure.

An oleo strut uses compressed nitrogen and hydraulic oil; torque links keep the wheel aligned and transmit braking and side loads.

A squat switch (weight-on-wheels) prevents gear retraction on the ground and inhibits systems such as pressurisation.