EASA PPL Aircraft General Knowledge (P80) — Study Guide

Aircraft General Knowledge is the subject that covers how the aircraft actually works — not the theory of why it flies (that is Principles of Flight), but the practical mechanics of the systems you interact with on every flight: engine, fuel, electrics, instruments, hydraulics, airframe structure, and the limitations that govern them. The P80 paper is typically 20 questions · 30 minutes · 75% (15/20). Connecting each paragraph to what you see on walk-around makes this paper stick.

Airframe and structure

Basic structural categories

Light training aircraft are usually built as semi-monocoque / stressed-skin structures: skin, frames, bulkheads, stringers, and longerons share bending and torsion loads. Pure monocoque relies almost entirely on shell strength — less common in trainers.

Wing structure: spars carry primary bending (spanwise); ribs preserve aerofoil shape; skin transmits torsion and transfers aerodynamic loads into the skeleton. Damage location hints at which load paths are compromised — reason about inspections after bumps or hangar rash.

Load factors and structural limits

Limit and ultimate load factors are certification facts published in the Type Certificate Data Sheet and your AFM/POH. You operate inside placarded speeds and manoeuvre categories — not by guessing margins.

Manoeuvring speed (Va) is the speed at or below which a full, abrupt control input should provoke a stall before limit load factor is exceeded (for the certified flight envelope definition). Above Va, abrupt inputs can generate structural loads beyond limits — tactically important in turbulence.

Va decreases when aircraft mass decreases. Published Va usually applies at maximum gross weight; lighter means less inertia and higher gust/control response — interpret POH carefully if weight-change variants are tabulated.

Fatigue and damage awareness

Every cycle nibbles fatigue life — turbulence, hard touchdowns, repetitive manoeuvres. Report hard landings or unusual stress events so engineering inspection can occur; pilots stay inside limits and observe inspection schedules — they do not self-certify structural repairs.

Piston engines — four-stroke cycle

Most trainers use four-stroke reciprocating engines. Know the strokes in order:

• Induction — piston descends, inlet opens, mixture enters.
• Compression — valves shut, piston rises, temperature/pressure climb.
• Power — spark ignites mixture; piston driven down, torque at crankshaft.
• Exhaust — exhaust valve opens, piston rises, scavenges burnt gas.

Firing order sequences cylinders for smooth torque; a four-cylinder Lycoming/Continental-style layout commonly fires twice per crank revolution overall — tie mental picture to prop pulses you hear on run-up.

Ignition — dual magnetos

Two independent magnetos, each feeding one plug per cylinder, give redundant ignition. Magnetos generate spark energy mechanically — they do not require ship electrical power to keep firing (battery loss still lets engine run). Normal selector position: BOTH for efficient combustion.

Run-up checks switching L/R expect a small RPM drop each side; an excessive drop suggests plugs/leads/magneto trouble on the side being tested; no drop hints the supposedly dead magneto may still be live — a ground fault with the P-lead — treat as fail until maintenance clears it.

OFF must never be treated as “prop safe.” A cracked P-lead can leave a magneto hot — hand-propping or moving the blade can start the engine.

Carburettor vs fuel injection

Carburettors mix fuel and air using a venturi pressure drop to pull fuel. Venturi cooling plus fuel evaporation yields carburettor icing risk in moist air — apply carb heat per POH (partial heat may be appropriate when clearing ice). Carb heat routes generally warmer, unfiltered air ⇒ expect RPM drop even without ice.

Fuel injection removes carb ice failure mode but introduces its own procedures; hot-soak vapour lock and flooding versus starvation behaviours appear in POH cold/hot start sections.

Mixture management

As altitude rises, air density falls — leaving mixture uncorrected tends toward over-rich symptoms (rough running, wasted fuel, weaker power). Lean per POH for cruise; typically full rich for take-off and landing unless AFM states otherwise. Over-lean regimes elevate CHT and invite damage — monitor gauges, not folklore alone.

Temperatures and pressures

• Oil pressure extremely important — low pressure after start or in flight: reduce power, land soon — continued operation risks bearing seizure.
• Oil temperature high with low pressure is doubly ominous.
• CHT / EGT inform mixture and cooling discipline — sustained hot CHT invites detonation margins to collapse.
• MAP (constant-speed props) pairs with RPM — observe POH power charts.

Detonation and pre-ignition

Detonation — end-gas auto-ignition after spark, violent pressure spikes — improper fuel grade, excessive lean, high CHT, or aggressive timing/faults aggravate it.

Pre-ignition — mixture lights before spark timing due to hot spots (carbon, cracked deposits). Both conditions destroy cylinders quickly — mixture/fuel/cooling discipline matters.

Propellers

Fixed pitch

Blade angle fixed at manufacture — compromise between climb (fine / higher RPM capability) and cruise (coarser for efficiency). RPM couples strongly with throttle and airspeed.

Constant speed

Governor adjusts blade angle to hold selected RPM — pilot sets RPM lever and manages throttle (MAP). Increasing power: commonly RPM first, then MAP; decreasing: MAP down before RPM — avoids high MAP / low RPM stressing crank loads per manufacturer guidance.

Fuel systems

Avgas 100LL — typically blue dye; octane resists knock. Wrong grade invites detonation. Mogas STCs are aircraft-specific approvals — verify paperwork.

Jet-A in a piston engine is catastrophic — vigilance at mixed-fuel ramps.

Typical route: wing tanks ⇒ selector ⇒ strainer ⇒ pumps ⇒ metering/injector or carb. Know your aeroplane's LEFT / RIGHT / BOTH / OFF implications and any imbalance/CG notes.

Water sinks — sump drains each flight — cloudy samples mean don't fly until remediated.

Electrical systems

14 V or 28 V DC common — alternator supplies bus with engine running; battery starts, backs brief outages, supplies critical buses per design.

Alternators yield useful charge at lower RPM than older generators — still monitor ammeter / loadmeter: discharge with loads means shedding non-essentials and planning landing.

Reset a popped breaker once; persistent trip ⇒ fault persists — leave open, reduce risk.

Flight instruments

Pitot-static family

ASI — Δ between pitot total and static ⇒ dynamic pressure; altimeter — static vs subscale reference; VSI — rate of static change.

• Pitot blocked + drain open often drives ASI toward zero (no dynamic pressure sensed).
• Pitot blocked + drain blocked can make ASI behave like a defective altimeter — changing with altitude excursions.
• Static blocked freezes altimeter where trapped, VSI to zero-ish trend error, ASI skewed — alternate static (if fitted) restores approximate indications with known errors (often faster trends, slight altitude/ASI bias).

Gyroscopic instruments

Rigidity and precession underpin attitude and heading indications. Electric/vacuum sourcing matters — know failure cues and limitations.

• Attitude indicator — primary artificial horizon for IMC/unusual attitudes.
• Heading indicator / DI — must be periodically synced to compass (drift from friction/apparent forces).
• Turn coordinator / needle & ball — rate hints + coordination via inclinometer.

Magnetic compass reminders

• Turning errors (dip) — Northern hemisphere mnemonic UNOS: undershoot north, overshoot south when rolling out.
• Acceleration errors on east/west — ANDS: accelerate shows north; decelerate shows south (east headings classic trap).

Hydraulics and braking

Many trainers use simple hydraulics for brakes; larger types add gear/flaps.Brake fade follows overheating — control taxi speed with power/parking mindset, not continuous riding.

Airspeed indicator colour coding

• White arc — flap operating band from Vso to Vfe.
• Green arc — normal operating range from Vs1 to Vno.
• Yellow arc — smooth air only caution band above Vno toward Vne.
• Red radial — Vne never exceed.
• Blue radial — best single-engine climb speed on twins (Vyse) — recall for generic exams even if your trainer is single-engine.

Where students lose marks

• Magneto check interpretation — large drop vs zero drop implies different faults; quote POH limits numerically when asked.
• Pitot/static fault matrices — rehearse each instrument symptom rather than rote guessing.
• Va vs weight — lighter ⇒ Va tendency downward versus gross-weight publication — exam loves counter-intuitive framing.

How to prepare

Cluster study by system families visited on pre-flight: sumps, vents, mags, instruments. Tie each failure mode to an observable cockpit cue. Alternate reading with question bursts — AGK rewards mechanics intuition tied to procedures you already perform every flying day.