CPL Aircraft General Knowledge common mistakes
CPL Aircraft General Knowledge · Understand the traps and how to avoid them.
Treating a constant-speed propeller as if blade angle stayed fixed, and answering that RPM rises when the aircraft accelerates in level flight at fixed power.
Candidates learn the fixed-pitch picture first and carry it over. In a constant-speed system the governor increases blade angle to absorb the extra power as speed rises, so RPM stays essentially constant while manifold pressure and thrust change.
What to do instead
Rehearse the governor as a closed loop: RPM is the controlled variable, blade angle is the output. For any 'what happens to RPM' question, first ask whether the governor is on-speed and has authority. Only if the propeller is at fine-pitch stop, or the governor fails to coarse pitch, does RPM move with airspeed.
Confusing series and parallel behaviour of hydraulic or electrical systems, for example stating that a failed pump in a parallel hydraulic system halves system pressure.
Candidates memorise 'parallel gives redundancy' without linking it to the physical quantity held constant. In a parallel system pressure is common and flow adds, so a lost pump reduces available flow rate, not pressure, until demand exceeds remaining supply.
What to do instead
Anchor each architecture to its held-constant variable: parallel holds pressure and sums flow; series holds flow and sums pressure. Practise predicting the first observable effect of a single failure (pressure drop versus slow actuation) for each layout.
Describing a turbine engine start sequence in the wrong order, for example igniting before the starter has established adequate N2, or shutting off the starter before light-off.
The sequence is learned as a list rather than as a reason. Ignition needs sufficient core airflow to avoid a hot or hung start, and the starter must keep motoring until the engine is self-sustaining.
What to do instead
Learn the sequence with the 'why' attached: crank to a minimum N2 for airflow, then fuel, then ignition, then monitor EGT and N1 for light-off, then release the starter once self-sustaining. Attach the failure consequence (hot start, hung start, wet fire) to each out-of-order step.
Answering ice-protection questions as if all surfaces use the same method, for example claiming wing leading edges are routinely anti-iced with bleed air on transport jets.
Candidates merge anti-ice and de-ice and assume one technology covers the airframe. In practice leading edges are usually thermal anti-ice (bleed air or electrical) while airframe protection may be pneumatic de-ice boots, and some surfaces are never protected.
What to do instead
Build a two-way table: anti-ice (prevent formation) versus de-ice (remove after formation), and thermal versus pneumatic versus fluid. Then place each protected zone (engine intakes, wing leading edges, tail, props, pitot, static ports) in a cell, noting which are not protected at all.
Mixing up which instrument is fed by which pitot or static source, and therefore misdiagnosing a blocked pitot or static port from the resulting indications.
The symptom pattern is memorised as a phrase rather than derived. Airspeed depends on pitot (total) pressure and static pressure; the altimeter and VSI depend on static pressure only; the ASI behaves like an altimeter if the static line is blocked.
What to do instead
Derive each symptom from first principles. Blocked pitot with drain open: ASI reads like an altimeter. Blocked static: ASI over-reads in climb and under-reads in descent, altimeter freezes, VSI reads zero. Write the derivation once and re-derive it under exam timing.
Treating structural terms such as stress, strain, limit load and ultimate load as interchangeable, and selecting the wrong factor when asked about certification loads.
The words are used loosely in everyday language. In certification, limit load is the maximum expected in service and ultimate load is limit load multiplied by the factor of 1.5, at which the structure must not fail though permanent deformation is permitted.
What to do instead
Fix the definitions with their units and consequences: stress in force per area, strain as deformation ratio, limit load as no permanent deformation, ultimate load as no failure. Memorise the 1.5 factor with its meaning rather than as a bare number.
Assuming a control surface always moves in the same direction as the aircraft response, or forgetting that trim tabs and servo tabs move opposite to the surface they assist.
Candidates picture the surface and the aircraft together, but tabs work by generating a moment on the parent surface, so a trim tab deflects opposite to the surface to hold it in position.
What to do instead
For every tab question, draw the parent surface, the tab, and the resulting hinge moment. State the rule: trim tab moves opposite the surface; servo tab is aerodynamically geared and also opposes the surface; anti-balance tab moves with the surface to increase feel.
Answering fuel-system questions by naming components without tracking where fuel actually goes, for example ignoring the role of the vent or the return line, or assuming all tanks feed all engines.
System diagrams are memorised as labels. Fuel feed architecture determines what happens after a pump failure, a blocked vent, or transfer between tanks, and many types feed engines from separate collector tanks.
What to do instead
Trace fuel as a path from tank to engine for the specific architecture in the question, naming each valve and pump in order, then state the consequence of failure at each point. Note that a blocked vent prevents transfer and can cause tank collapse or feed starvation.
Reading performance limitations as advisory and answering 'yes, legal' when a limitation is a hard limit, such as maximum operating altitude, maximum differential pressure, or a takeoff mass above the field-limited value.
Candidates conflate performance (what the aircraft can do) with limitations (what it is certified to do). Limitations are airworthiness limits that cannot be exceeded regardless of conditions.
What to do instead
Classify each figure in the flight manual as limitation, performance, or advisory before answering. If a question asks whether an operation is permitted, check the limitations section first and treat any exceedance as a no regardless of demonstrated capability.
Describing a warning system as simply 'it lights up', without distinguishing the sensing element, the logic, and the annunciation, so failure-mode questions about false warnings or missed warnings cannot be answered.
Warning systems are learned as panel pictures. Questions test the chain from sensor to comparator to alert, and a failure anywhere in that chain produces a specific symptom such as a nuisance warning or a silent failure.
What to do instead
For each system (stall warning, overspeed, fire, configuration, master caution), write the sensor, the logic condition, and the output, then ask what a sensor fault and a logic fault each look like. Note which warnings are tested automatically at power-up and which need a manual test.