CAA NZ CPL Aircraft Technical Knowledge common mistakes
CAA NZ CPL Aircraft Technical Knowledge · Understand the traps and how to avoid them.
On drag questions, states that induced drag rises with increasing speed and parasite drag rises as speed falls, then picks the minimum-drag speed as the point of minimum power required.
The two curves are memorised as textbook pictures rather than as relationships, so the 1/V² and V² behaviour reverses under exam pressure.
What to do instead
Rebuild the curve from the formulas every time: induced drag ∝ 1/V² (falls as speed rises), parasite drag ∝ V² (rises with speed), total is lowest where the two curves cross, which is L/Dmax. Then learn that minimum power required occurs at the slower speed, roughly 0.76 × V_imd, so either version of the question can be answered.
Treats the up-going aileron as the one with less travel, says adverse yaw is corrected by the rudder rather than caused by the ailerons, or says balance weights exist to reduce control forces.
Flight-control items test the mechanism, but candidates learn only the effect ('ailerons roll the aircraft') and never trace which surface moves where, or why.
What to do instead
Sketch a roll to the left: left aileron up, right aileron down. The down-going aileron lifts the right wing and adds induced drag, yawing the nose right, away from the turn - that is adverse yaw - and differential linkage gives the up-going aileron more travel to reduce it. Say explicitly that mass balance weights sit ahead of the hinge line to prevent flutter, not to lighten the controls.
Answers that the skin carries the bending loads in a semi-monocoque fuselage, or says ultimate load is limit load divided by 1.5.
Monocoque versus semi-monocoque descriptions get compressed into 'the skin takes everything', and limit, proof and ultimate load factors are memorised without the direction of the relationship.
What to do instead
Assign a job to each element: skin takes shear and pressurisation tension, stringers and longerons take bending, frames and bulkheads hold shape and carry concentrated loads. Fix the factor: ultimate = 1.5 × limit, limit load must be sustained without permanent deformation, and proof load is the test load between them.
Uses 'detonation' and 'pre-ignition' interchangeably, then selects the wrong remedy - for example leaning the mixture, or increasing power, as a cure for detonation.
Both present as rough running with high CHT, so the symptoms are learned from a list without the underlying physical cause, and the words sound similar.
What to do instead
Separate the causes: detonation is explosive burning of the end gas after normal spark ignition, corrected by a richer mixture, less power, lower CHT and higher octane fuel; pre-ignition is ignition before the spark, usually from a hot spot such as a glowing plug or carbon deposit, and mixture changes will not stop it. Practise symptom-plus-prompt questions where you must name the condition before choosing the action.
Says a coarse (high) blade angle gives the best take-off acceleration, and uses blade angle, geometric pitch and angle of attack as if they were the same thing.
Car-like gearing intuition ('higher gear for more speed') is applied in reverse, and the three propeller terms are never defined separately.
What to do instead
Define each: blade angle is the angle between the blade chord line and the plane of rotation; geometric pitch is the theoretical forward distance in one revolution; angle of attack is blade angle minus helix angle, so it falls as forward speed rises. Fine pitch for take-off (high RPM, high thrust), coarse for cruise, and the constant-speed unit coarsens the blades as speed builds to hold the selected RPM.
Explains vapour lock as simply a hot-day, high-power problem cured by leaning, and describes the electric boost pump as a starting aid only.
Fuel-system components are memorised as a component list, so the pressure–temperature reasoning behind vapour formation never gets attached to the pump roles.
What to do instead
State the condition fully: vapour forms when fuel pressure falls towards vapour pressure - high altitude, hot fuel, low pump pressure during climb or after a hot soak. The cure is boost pump on, cooler fuel and a richer mixture, not leaner. Learn which pump runs when: engine-driven pump in normal operation, electric pump for start, take-off and landing, and as backup if the engine pump fails.
Treats carburettor heat as airframe anti-ice, expects carburettor icing only in winter, and assumes pitot heat de-ices the wing.
The word 'ice' merges three separate systems, and carburettor icing is remembered as cold-weather folklore rather than as cooling from fuel vaporisation and venturi depression.
What to do instead
Keep the systems apart: carburettor heat supplies heated air to the carburettor and can be needed at outside air temperatures up to about 20–25 °C in humid air, causing an initial RPM drop as the mixture richens; pitot heat protects the pitot head and static source only; inflatable boots or fluid systems protect the leading edges. Answer by system, not by season.
For a blocked pitot with static clear in a climb, answers that the ASI falls; for a blocked static, answers that the ASI is unaffected.
The two blockages are learned as two unconnected lists of symptoms instead of one rule about which pressure is trapped.
What to do instead
Apply one rule: the ASI indicates the difference between pitot (total) pressure and static pressure. With pitot trapped and static still venting, the ASI behaves like an altimeter and climbs as you climb. With static trapped, the ASI over-reads in a climb and under-reads in a descent, the altimeter freezes at the trapped altitude and the VSI sits at zero. Add that the alternate static source normally causes a slight over-read.
Adds every moment regardless of whether the arm lies ahead of the datum, or starts the calculation from basic empty weight without the empty-weight moment and forgets unusable fuel and oil.
The arithmetic looks easy, so it is done quickly from memory, and negative arms plus the empty-weight moment are where the method breaks down.
What to do instead
Always begin with the licensed empty weight and moment from the flight manual, convert every arm to the same datum and the same units, keep the signs (arms ahead of the datum are negative), and total each column twice. Then check maximum take-off, maximum landing and maximum zero-fuel weight, and confirm the CG sits inside the envelope at zero fuel as well as at take-off, because fuel burn moves it.
Quotes a climb gradient in feet per minute, uses rate of climb for obstacle clearance, and enters performance charts at aerodrome elevation without converting to pressure and density altitude.
Performance charts deliver several similar-looking outputs and the elevation-to-pressure-to-density-altitude chain is skipped when time is short.
What to do instead
Keep units attached: rate of climb is ft/min, gradient is ft/NM (or percent), and they convert only through groundspeed - 500 ft/min at 90 kt groundspeed is about 333 ft/NM. Enter charts with pressure altitude (elevation plus roughly 30 ft per hPa that QNH is below 1013), then correct for temperature to density altitude, and apply wind and slope corrections to ground roll separately from the airborne distance.