CAA NZ CPL Theory - Principles of Flight & Aircraft Performance common mistakes
CAA NZ CPL Theory - Principles of Flight & Aircraft Performance · Understand the traps and how to avoid them.
Treating the four forces as independent quantities and concluding that in a steady climb thrust equals drag, or that in a level turn lift equals weight.
Candidates memorise 'lift opposes weight, thrust opposes drag' from straight-and-level flight and never re-draw the force diagram for other flight paths. They forget that in a climb the weight component along the flight path adds to the drag that thrust must overcome, and that in a banked turn only the vertical component of lift balances weight, so lift must exceed weight.
What to do instead
Before answering any force question, sketch the aircraft with the flight path and resolve weight into components parallel and perpendicular to the flight path. State the two equilibrium equations explicitly (along path: T = D + W sin γ; perpendicular: L = W cos γ for a climb; L cos φ = W in a level turn). Practise expressing load factor as 1/cos φ and check that your answers give L > W in a turn and T > D in a climb.
Claiming that an aircraft stalls at a fixed airspeed regardless of weight, bank angle or load factor, and using the 1g stall speed in all circumstances.
The 1g stall speed is the only figure candidates see in the flight manual, so it becomes 'the' stall speed. The relationship Vs = Vs1g × √n is a standard examinable point, but under pressure candidates revert to the memorised single number and ignore that the stall is an angle-of-attack phenomenon.
What to do instead
Anchor every stall answer to the critical angle of attack first, then convert to speed using Vs = Vs1g × √n. Work numerical examples for 30°, 45° and 60° bank (n = 1.15, 1.41 and 2.0) and for a 10% weight increase, and note that the stall angle of attack does not change with weight even though the stall speed does.
Assuming that lowering flap always increases the maximum lift coefficient and therefore shortens the take-off ground roll, without considering the change in drag and the effect on the climb gradient.
Flap questions are answered from a single mental rule ('more flap = more lift = better take-off'), ignoring that the extra drag from large flap settings reduces acceleration and can degrade the second-segment climb gradient, and that the take-off flap setting in the flight manual is a compromise chosen for the obstacle clearance requirement.
What to do instead
Separate the effects: lift coefficient rises with flap up to the optimum, drag rises steeply with deflection, and the published take-off flap setting is the one that satisfies the certification climb gradient, not the one with the shortest ground roll. Learn the difference between take-off and landing flap detents and be able to explain why a reduced take-off flap setting is used when obstacle clearance, not field length, is limiting.
Reading performance charts once, at the intersection, and reporting the value without applying the associated corrections for wind, slope, temperature or pressure, or interpolating in the wrong direction.
Chart questions are time-pressured and candidates treat the graph as a single lookup rather than a sequence of steps. They also mix units (feet versus metres, knots versus km/h, °C versus °F) and misread which axis is the entry and which is the correction, so a headwind correction is added instead of subtracted.
What to do instead
Use a fixed written procedure: list every input with its unit, identify the entry axis and the reference line for each correction, and write the correction with its sign before calculating. State out loud whether the correction makes the result more or less favourable and check it against physical sense (tailwind lengthens the roll, uphill slope lengthens it, higher temperature lengthens it). Do at least ten chart exercises with mixed units under timed conditions.
Calculating the centre of gravity using the basic empty weight and moment only, or omitting an item such as unusable fuel, oil or crew, and then declaring the aircraft within limits.
Candidates copy the empty weight and moment from the front of the weight and balance schedule and start adding payload, forgetting that the empty weight already excludes some items and that unusable fuel and oil are part of the empty weight in some formats but not others. The result is a CG that looks acceptable because the error is small at light loads.
What to do instead
Build the load sheet from a standard template that lists every item in the order given in the aircraft's weight and balance schedule, with a column for weight, arm and moment, and a running total. Check the total against the maximum all-up weight and the zero-fuel weight separately, then plot the final CG on the envelope rather than comparing a single number. Verify the empty weight and moment against the current schedule, not an older one.
Confusing the forward and aft CG limits in terms of their aerodynamic consequences, for example stating that an aft CG increases the stall speed or that a forward CG reduces stability.
The two effects are easy to reverse in memory: the forward limit is associated with 'nose heavy' and the aft limit with 'tail heavy', but candidates do not connect these to elevator authority, stall speed, cruise speed and longitudinal stability. The forward limit is a controllability and landing-flare limit; the aft limit is a stability limit.
What to do instead
Learn the two limits as different problems. Forward limit: the elevator may not have enough authority to flare or to hold the nose up at low speed, stick force and stall speed increase, cruise speed falls. Aft limit: static margin shrinks, longitudinal stability and stall recovery degrade, and stall speed falls slightly. Write a one-line justification for each limit and test yourself by predicting what happens to range and handling if the CG moves in each direction.
Planning fuel as trip fuel plus a fixed percentage, or treating the fixed reserve and contingency fuel as interchangeable, and failing to check the fuel required against the tank capacity at the planned temperature.
Candidates learn a single reserve figure and apply it to every flight, ignoring the structure of the fuel plan (taxi, trip, contingency, alternate, final reserve, extra) and the difference between a fixed reserve and a contingency allowance. They also forget that fuel density changes with temperature, so a volume figure in litres does not equal a fixed mass of fuel.
What to do instead
Write the fuel plan as a vertical sum in the order used in the flight plan, labelling each line with its regulatory basis, and total it before comparing with usable capacity. Convert between mass and volume using the density at the expected fuel temperature and show the conversion explicitly. Check that the landing fuel never falls below the final reserve and that the departure fuel plus taxi fuel does not exceed usable capacity.
Using still-air figures for a cross-country leg and then applying wind as a simple addition or subtraction to true airspeed, rather than solving the wind triangle.
Candidates are comfortable with headwind and tailwind components along track and forget that a crosswind component changes both groundspeed and track, so the heading must be adjusted to make good the planned track. The error grows with wind strength and is largest when the wind is at about 45° to track.
What to do instead
Always resolve the wind into along-track and cross-track components, calculate the wind correction angle from the crosswind component and true airspeed, then derive groundspeed from the along-track component and the cosine of the drift angle. Practise the triangle of velocities with a flight computer and check the answer with a sketch: drift should be towards the wind and groundspeed should be less than TAS whenever there is any headwind component.
Treating the published take-off and landing distances as guaranteed figures and ignoring that they are measured under specific conditions (dry runway, sea level, ISA, maximum weight, specific flap and bleed configuration).
The numbers in the flight manual look absolute, and candidates do not read the conditions printed at the foot of the chart. They therefore compare a wet, uphill, high-altitude, high-temperature runway distance directly with the available runway and conclude the flight is legal.
What to do instead
For every performance question, write the certification basis beside the figure: dry or wet, sea level or pressure altitude, ISA or actual temperature, maximum weight, flap setting, air conditioning and bleed status. Apply each correction in the order the manufacturer specifies and check the result against the available distance with the required margin. If any condition differs from the chart basis, state which correction you applied and why.
Ignoring the effect of density altitude on both engine power and aerodynamic performance, and using indicated airspeed as a proxy for the true speed over the ground when assessing take-off and landing distance.
Candidates learn that indicated airspeed governs the stall and handling, which is true, and then over-apply it to distance calculations. High density altitude reduces power output and thrust while the true airspeed for a given indicated airspeed increases, so the ground roll and the distance to 50 feet both grow, even though the aircraft feels normal on the airspeed indicator.
What to do instead
For any performance question, calculate pressure altitude and density altitude first and state them. Explain the two separate effects: reduced engine power and propeller thrust, and increased true airspeed for the same indicated airspeed, both increasing ground distance. Work a comparison at the same mass and indicated airspeed at sea level and at 8,000 feet density altitude, and note that the same reasoning applies to the climb gradient and to obstacle clearance.