PPL Aircraft General Knowledge common mistakes
PPL Aircraft General Knowledge · Understand the traps and how to avoid them.
When asked which instruments keep working after a total electrical failure, the candidate lists every gauge that has a glass cockpit display, forgetting that turn coordinator, attitude indicator and heading indicator are electrically driven while the standby magnetic compass, airspeed indicator and altimeter are not.
Candidates memorise 'pitot-static = airspeed, altimeter, VSI' but never build the parallel list of 'which instruments need power and from where'. The exam deliberately mixes vacuum/electrically driven gyros with pitot-static instruments, and the candidate has no mental table linking each instrument to its energy source.
What to do instead
Build a two-column table: instrument → power source (pitot-static pressure, engine-driven vacuum pump, or electrical bus). Drill the exception cases: turn coordinator is electric, the attitude and heading indicators are usually vacuum-driven on light twins, and the magnetic compass needs nothing. Rehearse the question 'what fails if the alternator dies and the battery runs down?' from that table, not from memory of the cockpit layout.
The candidate computes mass and balance by adding all masses and checking only that the total is below MTOM, then declares the loading acceptable without checking the centre of gravity against its forward and aft limits.
Total-mass arithmetic is easy and gives a satisfying single number, while the CG calculation needs a moment arm for every item. Under time pressure candidates stop at the weight check, or they compute a CG but forget to interpolate between the envelope's forward-limit lines at the actual mass.
What to do instead
Adopt a fixed procedure: tabulate mass, arm and moment for every item, sum moments, divide by total mass, then plot the point on the envelope using the forward limit at that specific mass (which varies with mass on most envelopes). Practise three loadings that are under MTOM but outside the CG envelope so the weight-only shortcut visibly fails.
On a take-off or landing performance question the candidate reads the chart for the given pressure altitude and temperature, then stops, ignoring the wind, runway slope, runway surface and flap setting corrections printed around the chart margins.
The chart's main grid is visually dominant and yields a number quickly, so the candidate treats the surrounding correction boxes as decoration. Exam charts are built so that the uncorrected value is one of the multiple-choice distractors.
What to do instead
Work the chart in a fixed order every time: pressure altitude from QNH and elevation, then temperature, then the base figure, then apply each correction multiplicatively or additively as the chart specifies (headwind/tailwind, slope, surface, flaps, and for landing the reverse-thrust or braking action). Write the correction list on scratch paper before touching the chart.
The candidate confuses indicated, calibrated, true and groundspeed, and answers a navigation-leg timing question using TAS when the question asked for time overhead a point using groundspeed.
The four speeds are defined in different chapters and the candidate learns the definitions separately without linking them into one chain. The conversion steps (position error, instrument error, density altitude, wind triangle) are forgotten under exam pressure.
What to do instead
Memorise the chain IAS → CAS (position and instrument error) → TAS (density altitude) → groundspeed (wind vector) as a single sequence, and note which speed each instrument or flight-planning calculation actually uses. For leg timing, always force yourself to state the groundspeed explicitly before dividing distance.
When a question describes a constant-speed propeller aircraft and asks what happens to manifold pressure and RPM as the throttle or propeller lever is moved, the candidate answers as if the propeller were fixed-pitch and links RPM directly to throttle.
Most early training is on fixed-pitch aircraft where throttle sets RPM, so the candidate applies that intuition to a constant-speed system. The governor's role and the separate functions of throttle (manifold pressure) and propeller control (RPM) are not firmly separated in the candidate's mind.
What to do instead
Learn the constant-speed propeller as two independent controls: throttle sets manifold pressure (power available) and the propeller lever sets the governed RPM by changing blade angle. Rehearse the standard sequences (increase RPM before manifold pressure when increasing power, reduce manifold pressure before RPM when decreasing) and the failure case where the governor allows an overspeed.
The candidate answers a question about the effect of a blocked pitot tube or static port by describing the wrong instrument behaviour, for example saying the altimeter reads high when the static port is blocked during a climb.
The pitot-static failure cases are symmetrical and easy to invert. Candidates learn 'blocked static = bad altimeter, airspeed and VSI' without working through the pressure actually trapped in the system and how each instrument responds in climb versus descent.
What to do instead
Reason from the trapped pressure rather than memorising outcomes. With the static blocked, the altimeter freezes at the altitude where the blockage occurred, the VSI reads zero, and the airspeed indicator behaves like an altimeter (reads high in climb, low in descent). With the pitot blocked, the airspeed indicator acts as an altimeter. Work each case in both climb and descent until the direction is automatic.
The candidate treats a question about aircraft electrical systems as pure component recall and cannot say what happens to the bus voltage and battery state when the alternator fails in flight with the battery switch left on.
Electrical questions are often skimmed because the candidate feels weak on the topic and hopes for a simple definition. The consequence questions require a small causal model (source, bus, load, battery) that the candidate has not built.
What to do instead
Learn the system as a chain: alternator/generator feeds the bus and charges the battery; on failure the battery supplies the bus and its voltage falls as it discharges. Know the indications (ammeter/loadmeter, low-voltage warning, voltmeter) and the actions (shed load, reduce battery use, land as soon as practicable). Draw the chain once and reuse it for every electrical question.
On a flight-planning question the candidate adds or subtracts fuel and time without distinguishing trip fuel, contingency fuel, alternate fuel, final reserve and taxi fuel, and so computes a landing mass or endurance that is wrong by the reserve amount.
The fuel breakdown is presented as a list in the manual and the candidate collapses it into 'total fuel'. Exam questions deliberately ask for endurance to dry tanks versus endurance to reserve, or landing mass at destination versus at alternate.
What to do instead
Write the fuel ladder explicitly before calculating: taxi + trip + contingency + alternate + final reserve + extra = block fuel. Then answer the specific question asked - endurance to final reserve uses everything except the reserve; landing mass at destination excludes alternate and reserve fuel already burned. Practise converting between the ladder and the question wording.
The candidate answers a structural or control-surface question by naming the component without stating its function or the consequence of its failure - for example identifying a trim tab but not explaining that it relieves control force rather than moving the aircraft.
Airframe questions look like vocabulary tests, so candidates memorise names and skip the 'why it exists' layer. The exam then asks about effect on control forces, flutter, or the consequence of a jammed tab.
What to do instead
For every airframe component, learn three things: what it is, what it does to forces or airflow, and what happens if it fails or jams. Cover trim tabs, balance tabs, servo tabs, stall strips, vortex generators, and the primary versus secondary flight controls with that three-part structure.
The candidate computes a density altitude or performance figure using the wrong units, mixing feet and metres, or applying a QNH correction with the wrong sign when converting between pressure altitude and true altitude.
European PPL material mixes metric and imperial units and the candidate works quickly without tracking units. The sign of the QNH correction (high pressure means true altitude is higher than indicated) is easy to invert.
What to do instead
Carry units through every calculation and write them in the working. Fix the rule 'high to low, look out below' for temperature and pressure: flying from high pressure to low pressure without resetting the altimeter makes the aircraft lower than indicated. Do a set of conversions (hPa to inches, feet to metres, Celsius to Kelvin where needed) until they are automatic.