Every airframe is built to withstand set flight and ground loads without permanent deformation or failure. The CAA groups light aircraft into strength categories, and understanding these limits keeps you flying within the safe envelope.
Most light aircraft use a four-stroke Otto cycle engine: induction, compression, power, exhaust.
Avgas 100LL is the standard fuel for most piston trainers, with a Research Octane Number around 100 and low lead content.
Each cylinder normally has two spark plugs, fired by two separate magnetos, for redundancy and better, more even combustion.
As altitude increases, air density falls, so the fuel-air mixture becomes richer than ideal unless leaned.
Leaning the mixture at altitude restores the correct fuel-air ratio, improves economy and prevents fouling.
Full rich is normal for take-off, climb and landing at low level, where excess fuel also helps cool the cylinders.
Running too lean at high power can cause detonation and overheating, so lean only as the engine flight manual permits.
Carb icing can form even in warm, humid conditions, typically between 0C and 30C outside air temperature, because fuel vaporisation and pressure drop in the venturi cause sharp local cooling.
Symptoms are a gradual loss of RPM (fixed-pitch prop) or manifold pressure (constant-speed prop), with rough running.
Applying carb heat should be done early and fully, not partially, since partial heat can worsen icing in some conditions.
Carb heat richens the mixture and reduces power, so expect a small RPM drop when applying it in cruise.
A fixed-pitch propeller has one efficient speed and RPM combination; a constant-speed propeller uses a governor to hold a selected RPM across a range of power and airspeed settings by changing blade pitch.
On a constant-speed unit, throttle controls manifold pressure (power) and the propeller lever controls RPM; the standard rule of thumb is to avoid high manifold pressure with low RPM, which over-stresses the engine.
Before take-off, cycling the prop lever a few times during the power check warms and circulates the oil in the propeller governor.
Propeller blade angle of attack is affected by both aircraft airspeed and rotational speed, changing along the blade from root to tip; the blade is therefore twisted, with a finer angle at the tip.
Most light singles use gravity-fed fuel from wing tanks, though low-wing types (or high-power aircraft) need engine-driven and electric boost pumps to guarantee pressure at all attitudes.
Always check fuel quality before flight: drain sumps at the lowest point of each tank, the fuel strainer/gascolator, and check for water, sediment, and correct colour. Water is heavier than fuel and sits at the bottom, which is exactly why sumps are drained there.
AVGAS 100LL is dyed blue and is the standard piston fuel; UL91 is dyed a different colour (light blue/green depending on brand) and is unleaded, only usable in engines placarded for it. Never mix fuel grades without checking the aircraft's approved fuel schedule - using the wrong grade can cause detonation or damage.
Carb ice can form even in warm, humid conditions (up to around 25-30C) because fuel vaporisation and pressure drop across the venturi cause a sharp temperature drop - often 20-30C - inside the carburettor. It is most likely at low power settings (descent, glide, taxi) because the throttle butterfly restricts airflow further.
Symptoms: rough running, loss of RPM (fixed-pitch prop) or loss of manifold pressure (constant-speed prop), then engine roughness as ice breaks off. Cure: apply full carb heat, which routes unfiltered warm air from around the exhaust manifold into the carb - this may cause a further temporary power loss before it clears. Use full carb heat, not partial, when checking or clearing ice, since partial heat can be worse than none by melting ice into a re-freezing slush.
As altitude increases, air density drops, so the fuel/air mixture becomes too rich unless leaned. Leaning restores the correct ratio, improves fuel economy, and prevents rough running or fouling at altitude. Always return to full rich before descent, landing, and go-around, since a lean mixture at low altitude and high power risks overheating and detonation.
Engine oil lubricates, cools, cleans (carries away contaminants to the filter), and seals piston rings. Check the oil level and colour on every pre-flight - low oil pressure combined with high oil temperature is a strong warning sign of an internal problem (e.g. a failing pump or a leak) and calls for a precautionary landing.
Most light aircraft engines are air-cooled, using cowl flaps and baffling to direct airflow over the cylinders. Closing cowl flaps in the climb is wrong - open them in the climb (low airspeed, high power = less cooling airflow) and close them in the cruise or descent to avoid over-cooling and shock-cooling the cylinders.
Students often confuse carb heat (always full ON, reduces power when selected due to less dense warm air) with mixture leaning (gradual, altitude-related). Also remember carb heat should be off for takeoff (it reduces available power and lets unfiltered air in), but on during the pre-landing checks and during any prolonged low-power descent.
Most light aircraft use a 12V or 24V lead-acid battery to start the engine and power systems when the engine isn't running.
The master switch (battery master) connects the battery to the main bus - always check it's ON before starting and OFF (with mags OFF and mixture cut) after shutdown to prevent a flat battery.
Some aircraft split the master into BAT and ALT switches - BAT alone lets you check systems without the alternator loading things up.
Once the engine is running, a generator or (more commonly on modern types) an alternator supplies electrical power and recharges the battery.
Alternators produce AC internally but rectify it to DC for aircraft use, and they generate usable output even at low engine RPM, unlike older generators.
A low-voltage or 'AMMETER/ALT' warning light means the alternator has failed - the battery alone is now carrying the load, so shed non-essential electrics and plan a precautionary landing.
Piston aero-engines almost always use two independent magneto systems, not the battery, to fire the spark plugs - this is why the engine can run with the master switch off.
Each cylinder normally has two spark plugs, each fed by a different magneto (dual ignition), giving redundancy and a more complete, efficient burn.
The key/ignition switch has positions OFF - R - L - BOTH - START; a mag check during the pre-takeoff power check should show a small, roughly equal RPM drop on each single-mag position (typically under 150 RPM drop and under 50 RPM difference between mags per most POHs) - a dead-cut on OFF or excessive drop signals a fault.
Because magnetos are 'live' any time the propeller turns, always treat the prop as if the ignition is on, even with the key out.
'Avionics' covers radios, navigation instruments (VOR, ADF, GPS), and the transponder, all drawing power from the electrical bus, usually through an avionics master switch.
Switch the avionics master OFF before engine start/stop to protect sensitive equipment from voltage spikes.
Circuit breakers protect individual circuits from overload - a popped breaker can be reset once (if it pops again, leave it and land).
Three of your four classic flight instruments run off pitot and static pressure: the Airspeed Indicator (ASI), the Altimeter, and the Vertical Speed Indicator (VSI). All three need clean, unblocked pitot and static sources - blockages are the classic exam trap.
The ASI compares pitot (dynamic + static) pressure against static pressure alone, so it displays dynamic pressure as a speed. If the pitot tube blocks but its drain hole stays clear, the ASI reads zero regardless of speed. If both pitot inlet and drain block (trapped pressure), the ASI acts like an altimeter - reading increases in a climb, decreases in a descent, and airspeed changes have no effect. A blocked static source alone makes the ASI under-read in a climb and over-read in a descent, because trapped static pressure no longer falls or rises with altitude.
Measures static pressure and displays it as height using a sub-scale set to a reference pressure (QNH, QFE, or 1013.25 hPa for QNE/flight levels). A blocked static source freezes the altimeter at the last correct reading - it will not change even as true altitude changes. Remember: 1 hPa change is roughly 30 ft near sea level, and pressure falls as you climb, so a low QNH setting left in (flying from high pressure into low pressure, or forgetting to update) makes you LOWER than indicated - the memory aid is 'high to low, look out below'.
Uses static pressure through a calibrated leak into a capsule, showing the rate of pressure change as a rate of climb/descent in feet per minute. It lags reality by 6-9 seconds, so it is a trend instrument, not instantaneous. A blocked static source freezes the VSI at zero.
The Attitude Indicator (AI), Heading Indicator (HI/DI), and Turn Coordinator all rely on gyroscopic properties: rigidity in space (a spinning gyro resists tilting) and precession (a force applied to a spinning gyro produces an effect 90 degrees later in the direction of rotation). Traditionally the AI and HI are vacuum/suction driven (via an engine-driven pump or venturi), while the Turn Coordinator is electrically driven - this split protects you if one power source fails.
The magnetic compass is simple but flawed. Two families of error matter for the exam: deviation (caused by the aircraft's own magnetism) and the turning/acceleration errors that only show up in flight (caused by magnetic dip).