CAAS PPL Aircraft General Knowledge (AGK) Study Guide

Aircraft General Knowledge (AGK) tests whether you understand the machine you are flying well enough to operate it safely and to recognise when something is wrong. The starting point is the airframe itself — the load-bearing skeleton on which engines, controls, fuel, instruments and avionics are mounted. A modern light aircraft like a Cessna 172 or Piper Warrior uses a semi-monocoquefuselage construction: a stressed skin riveted to internal formers (bulkheads) and longitudinal stringers, so that loads are shared between the skin and the underlying framework.

Five kinds of stress act on an airframe and you should be able to point to an example of each:

• Tension — fibres pulled apart. Example: lower wing-skin during positive-g flight.
• Compression — fibres pushed together. Example: upper wing-skin in positive-g flight.
• Shear — adjacent fibres sliding past each other. Example: the rivets joining wing-skin to spar.
• Torsion — twisting forces. Example: a propeller shaft transmitting engine torque.
• Bending — a combination of tension on one side and compression on the other. Example: a wing under aerodynamic load.

Wings carry the largest single load on a light aircraft. The main spar runs spanwise and is designed to take bending loads; ribs maintain the aerofoil shape; and the skin contributes to torsional stiffness. In a high-wing aircraft, struts transfer some wing bending load directly into the fuselage. In a low-wing aircraft, the spar typically passes through the cabin floor as a carry-through structure.

The three axes of an aeroplane are pitch (lateral axis through the wings), roll (longitudinal axis nose-to-tail) and yaw (vertical axis). Each axis is controlled by a primary control surface:

• Elevator — pitch, controlled by fore-and-aft movement of the control column.
• Ailerons — roll, controlled by left-and-right movement of the control column.
• Rudder — yaw, controlled by the rudder pedals.

Secondary controls modify how the aircraft performs rather than directly steering it. They include flaps (increase camber and, depending on type, area — giving more lift and drag for slower approach speeds), trim tabs (relieve the pilot of holding a constant control force), slats and slots (delay the stall by re-energising airflow over the wing) and spoilers (disrupt lift, mostly on heavier aircraft).

A subtlety often tested is adverse yaw: when ailerons deflect, the down-going aileron produces more induced drag than the up-going aileron, so the nose initially yaws away from the intended turn. The remedy is coordinated rudder. Some aircraft fit differential ailerons(the up-going aileron deflects further) or Frise ailerons (the leading edge of the up-going aileron protrudes into the airflow below the wing) to counteract this effect aerodynamically.

Most PPL trainers are powered by a four-stroke, air-cooled, horizontally-opposed piston engine — typically a Lycoming O-320, O-360 or Continental O-200/O-300 series. The four-stroke cycle is the single most-tested topic in AGK and the mnemonic "Suck — Squeeze — Bang — Blow" gives you the order:

1. Intake (Suck) — inlet valve open, exhaust valve closed. Piston moves down, drawing fuel-air mixture into the cylinder.
2. Compression (Squeeze) — both valves closed. Piston moves up, compressing the mixture. Compression ratios for aviation engines are typically in the 7:1 to 9:1 range.
3. Power (Bang) — both valves remain closed. Spark plugs ignite the mixture; expanding gases force the piston down. This is the only stroke that produces useful work.
4. Exhaust (Blow) — exhaust valve open, inlet valve closed. Piston moves up, pushing the burnt gases out.

The crankshaft completes two revolutions per cycle, while each valve opens once and each spark plug fires once. Ignition occurs slightly before top dead centre (typically 25° BTDC, but this varies by engine) to allow the mixture to develop peak pressure just after TDC for maximum work.

Aviation engines use magnetos rather than battery-fed ignition coils. A magneto is a self-contained generator: a rotating magnet inside a coil produces a high-voltage spark whenever the engine is turning, independent of the aircraft battery or alternator. This independence is the safety rationale — even with a total electrical failure the engine keeps running.

Two reasons drive the dual ignition design (two magnetos, two spark plugs per cylinder):

1. Redundancy. If one magneto fails, the other continues to fire its plugs and the engine keeps running, albeit with a small power loss.
2. More complete combustion. Two flame fronts initiated from opposite sides of the cylinder burn the mixture faster and more uniformly than one, increasing power and reducing the chance of detonation.

The magneto check before take-off (the "run-up") tests this redundancy. With the engine at a specified RPM, you switch from BOTH to LEFT and watch for a small RPM drop, return to BOTH, then switch to RIGHT for the same check. A drop that is excessive, or that exceeds the maximum difference between magnetos specified in the POH, indicates a fouled plug or a magneto problem. A complete loss of RPM on one side means that magneto is producing no spark at all and the flight must be cancelled.

Engines burn a fuel-air mixture. The carburettor or fuel-injection system meters fuel into the incoming air at a target ratio — for piston aviation engines this is around 15:1 by massfor best power, slightly leaner for best economy, and richer (around 11:1 to 12:1) for cooling at full power. As you climb, the air becomes less dense but the fuel-metering system continues to deliver the same mass of fuel for a given throttle setting. The mixture progressively enriches with altitude, hurting performance and fouling spark plugs.

The mixture control (the red knob) lets the pilot reduce fuel flow to compensate. Procedure varies by aircraft, but the principle in a non-turbocharged trainer is to lean above about 3,000 ft AGL or whenever the engine is operating below 75% power. You typically lean to peak exhaust gas temperature (EGT), then enrich slightly for cylinder cooling, watching for rough running which indicates the mixture is too lean.

Two errors to avoid: running too lean at high power can cause detonation (uncontrolled combustion that hammers pistons and overheats cylinders); running too rich on the ground wastes fuel and fouls plugs. Always set full rich for take-off from a low-elevation airfield like Seletar, and re-lean in the climb if the POH calls for it.

A float-type carburettor draws air through a venturi, where the pressure drop accelerates the air and cools it. Fuel is introduced into this low-pressure region and evaporates, cooling the mixture further. The combined cooling effect can drop the temperature inside the carburettor by 20-30°C compared to ambient. In humid conditions, the moisture in the air can freeze on the throttle butterfly and venturi walls, gradually choking the engine.

The dangerous truth is that carburettor icing can form in warm, humid weather — even at outside air temperatures up to about +30°C with high humidity. Singapore's climate is therefore a textbook environment for carburettor icing, especially during descent at reduced throttle. Carb-icing charts published by the manufacturer or in the AIP show the worst risk zone is roughly +5°C to +20°C ambient with relative humidity above about 50%.

Symptoms in a fixed-pitch propeller aircraft are a gradual loss of RPM (engine power decreases but throttle position is unchanged), rough running, and sometimes a richer-smelling exhaust. In a constant-speed propeller aircraft the manifold pressure drops while RPM stays constant. The remedy is carburettor heat: pulling the carb-heat knob diverts engine-warmed air into the intake, melting any ice. The expected initial reaction when carb heat is applied with ice present is a further drop in RPM (because hot air is less dense) followed by a recovery as the ice clears, often accompanied by rough running while water passes through the engine.

The propeller converts the rotational power of the engine into thrust. Each blade is a small rotating aerofoil, and just like a wing it produces a forward-acting force when it meets the air at a useful angle of attack. The defining feature of a propeller is that the tip travels much faster than the root, so to keep a sensible angle of attack along the whole blade the manufacturer builds in blade twist: a coarse (large) pitch angle near the hub and a fine (small) pitch angle near the tip. Without this twist the inner part of the blade would be stalled while the tip was barely working.

Two terms are easy to confuse and routinely tested. Geometric pitch is the distance the propeller would advance in one revolution if it were screwing through a solid; effective pitch is the distance it actually advances through the air. The difference between the two is propeller slip, and it exists because air is not solid and the blade must have an angle of attack to generate thrust. The angle of attack of a blade depends on two things working together — the rotational speed (engine RPM) and the forward speed of the aircraft. This is why a fixed-pitch propeller is an unavoidable compromise.

• A climb (fine) propeller has a small blade angle. It gives excellent take-off and climb performance because the engine can reach full RPM, but it is inefficient and over-revs in the cruise.
• A cruise (coarse) propeller has a larger blade angle. It is efficient at speed but sluggish on take-off and may not allow the engine to reach rated RPM on the ground.
• A constant-speed (variable-pitch) propeller resolves the compromise. A governor automatically adjusts blade pitch to hold a selected RPM: fine pitch for take-off and climb, coarse pitch for an efficient cruise. The pilot sets RPM with the propeller (blue) lever and power with the throttle, reading manifold pressure on a separate gauge.

A useful airmanship rule for a constant-speed unit is to avoid a high manifold pressure with low RPM combination, which over-stresses the engine — broadly, increase RPM before manifold pressure when adding power, and reduce manifold pressure before RPM when reducing power. On a fixed-pitch trainer none of this applies: the throttle is your only power control and RPM is simply read off the tachometer.

Most exam questions about oil and cooling are really questions about recognising a developing failure from the cockpit gauges, so it pays to understand what the systems actually do. Engine oil performs four jobs that you should be able to list: it lubricates moving surfaces; it cools by carrying heat away from pistons and bearings to the oil cooler; it seals the small gap between piston rings and cylinder walls; and it cleans by carrying combustion by-products and metal particles to the filter. A pressure pump forces oil through galleries, and a pressure-relief valve caps the maximum pressure to protect seals and the cooler.

Read the two oil gauges together — their combination tells you far more than either alone:

The classic post-start check is that oil pressure must register within a specified time (often quoted as about thirty seconds) after starting — if it does not, the engine must be shut down immediately to avoid running dry. Most light trainers are air-cooled: cooling air is ducted over the cylinder fins by baffles and a cowling that force air to flow where it is needed rather than simply blowing past. This is why prolonged ground running, a long full-power climb at low airspeed, or a steep climb on a hot day can all overheat the engine — the airflow over the fins falls just when heat production is highest. The remedies are to lower the nose for more airflow, enrich the mixture for extra cooling, and reduce power.

Aviation gasoline (AVGAS) is differentiated from automotive gasoline by its higher octane rating, tighter purity standards and the inclusion of a small amount of tetraethyl lead. The most common grade for light aircraft is AVGAS 100LL — "low lead" — coloured blue. Grade 100/130 is green, grade 80/87 is red (rare now), and turbine fuel (JET A-1) is straw-coloured. Filling a piston-engine aircraft with Jet A-1 is a textbook accident and is the reason fuel-truck nozzles and aircraft fuel ports are differently sized.

The pitot-static system feeds three instruments: the airspeed indicator (ASI), the altimeter, and the vertical speed indicator (VSI). The pitot tube measures total pressure(static + dynamic). Static ports measure ambient static pressure. The instruments combine them as follows:

• ASI compares pitot total pressure with static pressure — the difference is dynamic pressure, calibrated to airspeed.
• Altimeter uses only static pressure, calibrated to a standard pressure-altitude relationship.
• VSI measures the rate of change of static pressure.

Blockage failures are a classic exam question. If the pitot tube blocks (e.g., by ice or an insect): the ASI behaves like an altimeter — reading higher as you climb and lower as you descend. If the static port blocks: the altimeter freezes at the current altitude, the VSI reads zero, and the ASI under-reads in a climb and over-reads in a descent. The pilot response is to select pitot heat (if fitted) and use the alternate static source, typically in the cabin — but because cabin static is slightly lower than ambient, the altimeter will over-read and the ASI will over-read in the cruise.

Three of the six standard instruments are gyroscopic: the Attitude Indicator (AI), the Heading Indicator (HI, sometimes called the directional gyro), and the Turn Coordinator (TC). Gyroscopes have two useful properties — rigidity in space (the spin axis tends to maintain its orientation) and precession (a force applied to a spinning gyro produces a reaction 90° later in the direction of spin).

Splitting the drive between vacuum and electric power gives redundancy — a vacuum failure leaves you with the turn coordinator; an electrical failure leaves you with the vacuum AI and HI. The HI suffers mechanical drift and earth-rotation drift, so it must be aligned with the magnetic compass at intervals (typically every 10-15 minutes) during steady, straight, level flight.

The electrical system in a typical light trainer is 14 V or 28 V direct current, with an alternator as the primary generator and a battery for starting and emergency reserves. An alternator differs from an older-style generator in that it produces alternating current internally and rectifies it to DC. The advantages: an alternator delivers full output at lower engine RPM (so battery charging continues at idle), it weighs less, and it is more reliable. A voltage regulator keeps output near 14 V (or 28 V), and an ammeter or load meter on the panel shows whether the battery is charging or discharging. If the ammeter shows a discharge in flight, suspect alternator failure: shed non-essential loads, switch off avionics until needed, and divert if necessary.

The magnetic compass is the only direction instrument that needs no electrical or vacuum power, so it is the ultimate fallback — and the exam loves its errors precisely because they are non-intuitive. A pivoted magnet aligns itself with the Earth's magnetic field, but that field is not a clean horizontal line. Near the magnetic poles the field dips steeply into the ground, and this magnetic dip is the root cause of the two dynamic errors below.

First, the compass points to magnetic north, not true north, and the difference at a given location is variation. It is also influenced by the aircraft's own metal and electrical fields, an error called deviation, which is minimised by compass swinging and recorded on a correction card near the instrument. A handy memory aid for applying corrections is "variation east, magnetic least; variation west, magnetic best".

The two errors that catch candidates out appear only while turning or changing speed in the northern or southern hemisphere:

• Turning (acceleration of the magnet) error is greatest on northerly and southerly headings and zero on east/west. In the northern hemisphere the compass lags when turning through north (it under-reads the turn) and leads when turning through south. The mnemonic UNOS — "Undershoot North, Overshoot South" — captures it: roll out early when turning towards south and late (past the heading) when turning towards north.
• Acceleration error appears on east/west headings. The mnemonic is ANDS — "Accelerate North, Decelerate South": on an easterly or westerly heading the compass swings towards north when you accelerate and towards south when you decelerate.

The practical lesson is that the compass is only reliable in steady, straight, level, unaccelerated flight. That is exactly why the heading indicator exists and why you align the HI to the compass only when wings-level and at constant speed — try to do it in a turn and you will set the wrong heading.

The undercarriage seems mundane until you realise how many accidents involve it. Two layouts dominate training. A tricycle (nose-wheel) undercarriage places the main wheels behind the centre of gravity and a steerable nose-wheel ahead of it; it is stable on the ground, gives good forward visibility and is forgiving of small directional errors because the CG tends to keep the aircraft tracking straight. A tailwheel (conventional) undercarriage places the CG behind the main wheels, which makes it directionally unstable on the ground — any swing tends to worsen into a ground loop unless caught promptly with rudder. This is why tailwheel flying demands more footwork.

Most trainers have fixed gear, but you should understand the principle of retractable gear: it reduces drag in the cruise at the cost of weight, complexity and the ever-present risk of a gear-up landing. Hence the universal checklist discipline of confirming three greensbefore landing.

Tyres and brakes generate several reliable exam points:

• Creep marks are paint lines across the tyre and wheel rim. If they no longer line up, the tyre has crept on the rim, which can shear the inflation valve — the tyre must be inspected.
• Sidewall damage, exposed cords, or a deep cut into the carcass grounds the aircraft, because the sidewall carries the structural load and a failure there is far more dangerous than tread wear.
• Most light-aircraft brakes are hydraulically operated disc brakes. They work by converting the aircraft's kinetic energy into heat through friction, which is why heavy or repeated braking can cause brake fade and overheating.
• On a wet or contaminated runway, locking the wheels can cause aquaplaning (hydroplaning), where a film of water lifts the tyre off the surface and braking and steering are lost. Anti-skid systems on larger aircraft release pressure momentarily to keep the wheel rolling and preserve grip.

A small but high-yield habit: on the take-off and landing roll, keep directional control with the rudder (and nose-wheel steering) and use the brakes gently and symmetrically. Differential braking that locks one main wheel can pull the aircraft sharply off the centreline.

Most marks are lost in AGK not because the underlying science is hard but because a small number of counter-intuitive points are misremembered under exam pressure. Watch for these:

• Confusing the pitot and static blockage symptoms. Remember the shortcut: a blocked pitot tube makes the ASI behave like an altimeter (reads high in a climb); a blocked static port freezes the altimeter and makes the ASI under-read in a climb.
• Believing carburettor icing only happens in the cold. The highest risk is in warm, humid air at part throttle — exactly Singapore conditions. It can form well above freezing ambient temperatures.
• Expecting RPM to rise when carb heat is selected. With ice present the first effect is a further small drop in RPM (hot air is less dense), then a recovery as the ice clears. Do not panic and push the knob back in.
• Thinking a magneto drop should be zero. A small drop on each magneto is normal and expected; an excessive drop, an excessive difference between magnetos, or a complete dead-cut is the fault.
• Getting the mixture-with-altitude effect backwards. As you climb the air thins, so for a fixed fuel flow the mixture becomes richer, not leaner — which is why you lean as you climb.
• Forgetting the compass turning rules. Memorise UNOS (Undershoot North, Overshoot South) and ANDS (Accelerate North, Decelerate South), and remember the compass is trustworthy only in steady level flight.
• Treating "rigidity" and "precession" as interchangeable. The attitude and heading indicators rely on rigidity in space; the turn coordinator relies on precession.
• Reading the oil pressure gauge in isolation. Always interpret oil pressure together with oil temperature — the pair distinguishes a faulty sender from a genuine and urgent oil-loss emergency.

AGK is by far the largest bank in this app — 366 questions covering both the airframe-and-systems side of "Aircraft General Knowledge" and a generous amount of aerodynamics that other syllabuses would file under Principles of Flight. Expect a roughly even split between systems (engine, electrical, fuel, instruments, undercarriage) and aerodynamic theory (lift, drag, stall, stability, controls). Many questions ask you to identify symptoms in a one-line scenario — a fluctuating oil pressure, a rising oil temperature, an asymmetric flap — so make sure you can map symptoms to systems quickly. The clusters below are by sheer question count the most rewarding to revise.

• Stall behaviour and recovery: a very large cluster. Know that the critical angle of attack is a fixed property of the wing, that the first symptom is buffet, that recovery starts with reducing angle of attack, why top rudder is used on a wing-drop stall, and how weight, load factor and ice on the leading edge affect stall speed.
• Lift, drag and the L/D ratio: lift acts perpendicular to the relative airflow through the centre of pressure; drag acts parallel to it; the best L/D is at minimum-drag speed; induced drag dominates at low speed, parasite drag at high speed; tapered wings and aspect ratio change span efficiency.
• Carburettor, mixture and induction system: the four-stroke cycle, the back-suction (idle-cut-off) mixture system, why partial carb-heat is dangerous without a CAT gauge, why applying full carb heat to a fouled engine briefly worsens running, and how engine power is enriched at high power for cooling.
• Magnetos and ignition: that aviation engines use self-contained magnetos (current AC type), the function of the impulse coupling at start, the redundancy and combustion advantages of dual ignition, and why a dead-cut on one magneto is unflightworthy.
• Pitot-static system and ASI errors: which instruments use pitot and which use static, what the ASI does when the pitot tube is blocked on the ground, what an alternate static source does to indicated altitude, and the colour-coded arcs on the ASI (white = flap operating, green = normal, yellow = caution, red = V_NE).
• Gyroscopic instruments: the difference between rigidity and precession, which instruments use which property, vacuum vs electric drive, and the typical limit on heading-indicator drift between resets.
• Oil, cooling and engine indications: the three functions of engine oil (lubrication, cooling, sealing), the role of the pressure-relief valve, why baffles and cowlings matter for cooling, how to interpret combined oil-pressure and oil-temperature anomalies, and what to do if no oil pressure rises after start.
• Propeller effects and torque on take-off: torque reaction, P-factor, slipstream and gyroscopic precession all act together; the direction depends on prop rotation. A test favourite is an anti-clockwise-from-the-cockpit propeller — the aircraft yaws and rolls right at high power, and the pilot inputs the opposite rudder to what a typical Western trainer needs.
• Electrical system and AC vs DC: alternator vs generator, what an ammeter shows, how AC sine-wave voltage is described, parallel-circuit power capacity, why AC is preferred on large aircraft, and what a load-relay or commutator does.
• Undercarriage and tyres: tricycle vs tailwheel handling differences, why creep marks matter, why a deep sidewall cut grounds the aircraft, and the principle of anti-skid braking (kinetic energy converted to heat through friction).
• AVGAS and fuel system: the colour and grade conventions (100LL blue), why low-grade fuel causes detonation, water as the most common contaminant, why you select a fuel boost pump on before changing tanks, and the gravity-feed limitations.
• CG, weight and stability: aft CG reduces stall speed but compromises stability; forward CG increases stall speed; the CG is the point through which weight acts; dynamic stability is built on static stability; the V-n diagram limits load factor.

Because the bank is so large, the best strategy is to drill it in multiple sessions of fifty questions rather than trying to absorb it in one sitting. Group your study by cluster — spend one session on aerodynamics, another on the engine, another on instruments — and re-attempt the same fifty until you are clearing them at ninety per cent before moving on. The Lycoming/Continental engine sections of any standard PPL textbook plus a careful reading of your aircraft's POH will close most of the systems gaps; the aerodynamics half is best supported by a textbook with diagrams of boundary-layer separation and downwash, since the quiz frequently asks about phenomena that are easier to recognise visually than from prose alone.