Brunei DCA CPL(A) Principles of Flight common mistakes
Brunei DCA CPL(A) Principles of Flight · Understand the traps and how to avoid them.
When asked what happens to the indicated stall speed as bank angle increases in a level turn, the candidate applies the square-root-of-load-factor relationship to the indicated (airspeed-indicator) stall speed instead of the true stall speed, and concludes the IAS stall speed rises by √n.
Textbook derivations are usually done in true airspeed, and candidates memorise 'stall speed increases as the square root of load factor' without linking it to which speed is being scaled. The IAS–TAS relationship through density and position error is not revisited, so the distinction is invisible to them at the moment of answering.
What to do instead
Before answering any stall-speed question, write down the reference speed being used. Work the load-factor scaling in TAS, then convert to IAS through the density ratio at the stated altitude. Practise a set of turns at 30, 45 and 60 degrees of bank at one altitude and one at a high-altitude case, computing both TAS and IAS stall speeds each time so the difference becomes automatic.
The candidate states that induced drag is highest at low angle of attack and falls as the aircraft slows, or states that induced drag is constant with speed.
Induced drag is often confused with parasite drag because both are plotted on the same total-drag curve, and the candidate remembers only the shape of the total curve rather than the two components. The lift-coefficient term in the induced-drag expression is not linked to speed in their mental model.
What to do instead
Rebuild the relationship from the definition: induced drag depends on lift coefficient squared divided by aspect ratio, and lift coefficient rises as speed falls at constant weight. Sketch the induced, parasite and total curves from first principles for one weight, then repeat for a heavier weight and for a lower aspect ratio, labelling the minimum-drag speed each time.
When asked about a wing in a steady climb, the candidate says lift equals weight, or when asked about a level turn, says lift equals weight divided by the cosine of the bank angle but then uses weight instead of lift in the load-factor calculation for the stall-speed question that follows.
The 'lift equals weight in level flight' simplification is learned early and never qualified. Candidates do not separate the vertical and horizontal force balances, so they cannot move between the climb, turn and level cases without contradiction.
What to do instead
Draw the force diagram every time: weight always vertical, lift perpendicular to the flight path, thrust along the path, drag opposing it. Resolve vertically and horizontally for level flight, climb at angle gamma, and level turn at bank phi. Then state load factor as lift over weight and check it against the vertical resolution before using it in any stall-speed or structural calculation.
The candidate treats the critical angle of attack as a fixed angle in space or as a function of speed, saying for example that the aircraft stalls at a particular airspeed regardless of weight, load factor or configuration.
Stall speed is the most frequently quoted number in training, so it becomes the mental definition of a stall. The underlying invariance of the critical angle of attack is stated in ground school but not exercised in numerical problems with varying weight, g-load, power and flap setting.
What to do instead
Answer every stall question in two steps: first state that the stall occurs at the critical angle of attack, then show how the 1g stall speed at a given weight and configuration is only one manifestation of it. Work problems where weight, load factor, flap setting and CG position each change, computing the new 1g stall speed and explaining which of them shifts the critical angle of attack and which does not.
The candidate answers questions on longitudinal static stability by referring only to the horizontal tail or only to the CG position, and cannot explain why an aft CG reduces both static margin and stick-fixed stability.
Stability is taught as a list of factors rather than as a moment balance. The candidate has not internalised that the tail provides a restoring moment whose magnitude depends on the distance from the CG, so the CG and tail contributions are learned as separate facts.
What to do instead
Construct the pitching-moment diagram for a complete aircraft: wing and fuselage contribution with its slope, tail contribution with its slope, and the sum. Move the CG aft and redraw, noting the reduced negative slope of the total curve and the reduced stick-fixed margin. Do the same with a different tail volume and with a destabilising fuselage alone, then state the static margin as the distance between CG and neutral point.
The candidate confuses static and dynamic stability, for example describing an aircraft as dynamically stable because it tends to return to trim, or failing to distinguish the short-period and phugoid modes when asked about longitudinal dynamic behaviour.
Both terms are defined with similar wording in many study notes, and the distinction between the tendency to return (static) and the nature of the motion over time (dynamic) is not tied to a concrete example. The two longitudinal modes are rarely plotted.
What to do instead
Use the pendulum analogy explicitly: static stability is whether the initial tendency is restoring, dynamic stability is what the subsequent motion does. Then classify the three dynamic cases (divergent, neutral, damped) for each mode, and sketch a phugoid (long period, small angle-of-attack change, approximately constant angle of attack with speed and height exchanging) against a short-period oscillation (short period, large angle-of-attack change, usually heavily damped).
The candidate answers asymmetric-flight questions by stating that the rudder alone balances the yawing moment, ignoring the fin's contribution and the resulting sideslip, or ignores the effect of bank angle on the required rudder and the associated increase in VMC.
The balance of forces in the asymmetric case involves thrust, fin sideforce, rudder sideforce and the resulting sideslip, which is a multi-body problem. Candidates reduce it to a single 'rudder balances the yaw' statement and then cannot answer follow-on questions about VMC being quoted in terms of rudder force and bank.
What to do instead
Draw the plan view with both thrust lines, the CG, the fin and the rudder, and mark the yawing moments. Show that the fin generates a sideslip-dependent sideforce and that a small bank towards the live engine adds a useful horizontal component of weight. Then relate the certification conditions for VMC to rudder force limits, bank angle, and the effects of altitude, temperature and power on the available rudder authority.
The candidate treats thrust and power as interchangeable when answering questions about climb performance, saying for example that maximum rate of climb occurs at maximum thrust or that propeller aircraft climb best at the same speed as jet aircraft.
The two quantities differ by speed, and the climb-gradient and climb-rate relationships are derived from them in different ways. Candidates who memorise 'excess thrust gives climb' and 'excess power gives rate of climb' without linking each to the speed axis cannot select the correct speed for best angle or best rate.
What to do instead
Plot thrust required and thrust available against speed, then plot power required and power available against speed on the same axes. Identify the speed for maximum excess thrust (best angle) and maximum excess power (best rate), and repeat for a jet where thrust available is approximately constant with speed. Note how each optimum shifts with weight, altitude and configuration.
The candidate answers Mach-effect questions by saying that the critical Mach number is a fixed property of the aerofoil, or that sweep increases the critical Mach number because it reduces the chord, without explaining the mechanism through the component of velocity normal to the leading edge.
Critical Mach number is quoted as a single figure for a type, and sweep is taught as a rule ('sweep helps at high speed') without the velocity-resolution argument. The candidate therefore cannot handle questions where thickness, camber, lift coefficient or sweep angle change the answer.
What to do instead
Derive the critical Mach number from the local velocity peak on the upper surface: the higher the peak suction, the lower the free-stream Mach number at which the local flow reaches Mach one. Then show that sweep reduces the effective normal component of the free-stream velocity, lowering the peak. Work through how thickness ratio, lift coefficient, camber and sweep each shift the critical Mach number and the drag-rise Mach number.
The candidate describes a spin as a high-speed spiral dive or as a stall of the tailplane, and cannot state the conditions for a steady spin or explain why aileron input can worsen the autorotation in some aircraft.
Spin entries are usually taught as a procedure rather than as an aerodynamic state, so the candidate has no model of the stalled wing with unequal lift and drag on the two sides. The distinction between autorotation and spiral dive is not tied to angle of attack and airspeed.
What to do instead
Build the spin from the stalled condition: one wing more deeply stalled than the other, giving differential lift and drag and hence a yawing and rolling moment that sustains autorotation. State the two conditions for a steady spin (stalled, and yawing/rolling moment balance maintained) and contrast the airspeed, angle of attack and control response of a spin with those of a spiral dive. Then explain the effect of aileron against the spin direction and of mass distribution along the fuselage.