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ATPL principles of flight course: 14 illustrated chapters

The ATPL principles of flight course follows the syllabus in 14 chapters: 136 key points explained in a few paragraphs, often with a diagram, what to remember and, for many, the classic exam trap. The first chapter, « Basics: physical laws, air and airspeeds », can be read in full on this page.

Read chapter 1The 1,335 questions of this subject

Contents

Each chapter explains, point by point, what the questions of the bank ask, with a « Remember » box and often the classic exam trap. The first chapter is free, below and in the app; the next ones are for ATPL subscribers.

  1. 1.Basics: physical laws, air and airspeeds

    Free

    Units, Newton's laws, axes and angles, atmosphere, Bernoulli, continuity and the airspeeds IAS, CAS, EAS, TAS and Mach.

    10 key points · 102 questions of the bank

  2. 2.The aerofoil, the wing and lift

    Subscribers

    Aerofoil and wing geometry, flow around the aerofoil, pressure distribution, CL-α, centre of pressure and aerodynamic centre.

    11 key points · 170 questions of the bank

  3. 3.The boundary layer and separation

    Subscribers

    Laminar and turbulent boundary layers, transition point, adverse pressure gradient, separation and aerofoil stall.

    6 key points · 28 questions of the bank

  4. 4.Drag, the polar curve and ground effect

    Subscribers

    Parasite drag and induced drag, wingtip vortices, aspect ratio, drag curve and VMD, polar curve, lift-to-drag ratio, ground effect and wake turbulence.

    13 key points · 207 questions of the bank

  5. 5.High-lift devices, spoilers and speed brakes

    Subscribers

    Trailing edge flaps, leading edge slats and flaps, vortex generators, spoilers and speed brakes: effects on Cz, drag, critical angle of attack and pitch.

    10 key points · 115 questions of the bank

  6. 6.The stall and stall protection

    Subscribers

    Stall speed and the factors that change it, spanwise stall progression, swept wing, stall warning, stick shaker and stick pusher, deep stall, spin and icing.

    11 key points · 192 questions of the bank

  7. 7.High-speed aerodynamics

    Subscribers

    Speed of sound and Mach number, propagation of disturbances, shock and expansion waves, critical Mach number, transonic effects, tuck under, sweep, area rule and supercritical aerofoil.

    12 key points · 165 questions of the bank

  8. 8.Flight envelope at altitude and buffeting

    Subscribers

    Lift expressed in terms of Mach number, collapse of the polar, low- and high-speed stalls, buffet onset diagram, coffin corner, g margins and ceilings.

    6 key points · 28 questions of the bank

  9. 9.Longitudinal stability

    Subscribers

    Static and dynamic stability, pitch equilibrium, neutral point and static margin, Cm-α curve, contributions, CG limits, stick forces, stick force per g, phugoid and short period.

    10 key points · 118 questions of the bank

  10. 10.Directional stability and lateral stability

    Subscribers

    Sideslip, directional stability and its factors, lateral stability and the dihedral effect, roll-yaw coupling, Dutch roll and spiral instability.

    7 key points · 74 questions of the bank

  11. 11.Flight controls and trimming

    Subscribers

    Control surfaces and axes, elevator and all-moving tailplane, ailerons and adverse yaw, aileron reversal, hinge moment, balancing, tabs, trim devices, trimmable horizontal stabiliser (PHR), servo controls and flutter.

    10 key points · 121 questions of the bank

  12. 12.Load factor, limitations and gusts

    Subscribers

    Load factor, limit and ultimate loads, CS-23 and CS-25 categories, manoeuvring envelope, VA, gusts, VB and VRA, limiting speeds and airspeed indicator.

    8 key points · 66 questions of the bank

  13. 13.Propellers

    Subscribers

    Blade angles, geometric and effective pitch, twist, fixed-pitch and constant-speed propellers, efficiency and thrust coefficient, operating modes, feathering and secondary effects.

    10 key points · 95 questions of the bank

  14. 14.Flight mechanics

    Subscribers

    Level flight, power required, control regimes and thrust ceiling, cruise, climb and gradient calculations, Vx and Vy, descent and glide, turn, balanced flight and asymmetric flight.

    12 key points · 113 questions of the bank

Chapter 1: Basics: physical laws, air and airspeeds

Units, Newton's laws, axes and angles, atmosphere, Bernoulli, continuity and the airspeeds IAS, CAS, EAS, TAS and Mach.

Quantities and units

← Faites glisser le schéma →

SI units used in flight mechanics

© ForAllPilots

The exam checks that you can handle International System (SI) units.

  • a force (lift, weight, thrust) is expressed in newtons: 1 N = 1 kg·m/s². Weight is W = m·g, with g ≈ 9.81 m/s² (10 m/s² in exam calculations);
  • a pressure is a force per unit area: pascal (N/m²), hPa or psi. Wing loading (weight / area) and dynamic pressure therefore have the same unit, N/m²;
  • density ρ is expressed in kg/m³;
  • power is work per unit of time: watt = N·m/s (P = F × V).

The magnitude of a force is its size; a force also has a direction and a sense.

Remember

N = kg·m/s²; Pa = N/m²; W = N·m/s; ρ in kg/m³. 1 kt ≈ 1.15 mph: 100 mph ≈ 87 kt, and 75 kt ≈ 75 / 0.87 ≈ 86 mph.

Exam trap

Do not confuse the joule (energy, N·m) with the watt (power, N·m/s). A moment is also expressed in N·m, but with no displacement: the distance is only a lever arm.

Newton's laws and moments

← Faites glisser le schéma →

Moment of a force about the centre of gravity

© ForAllPilots

Newton's three laws govern the whole course:

  • inertia: if the sum of the forces is zero, the body stays at rest or keeps a constant speed in magnitude and direction. In steady level flight, the opposing forces are therefore equal;
  • dynamics: F = m × a. Acceleration is proportional to the force and inversely proportional to the mass;
  • action-reaction: the wing pushes the air downwards, the air pushes the wing upwards.

For rotations, what counts is the sum of the moments: if it is not zero, the aeroplane undergoes an angular acceleration about the axis concerned, and it always pivots about its centre of gravity.

Remember

Sum of forces zero: uniform straight-line motion. Sum of moments not zero: angular acceleration about the CG.

The aeroplane's three axes

← Faites glisser le schéma →

Longitudinal, lateral and normal axes, originating at the centre of gravity

© ForAllPilots

The reference axes all pass through the centre of gravity (CG):

  • the longitudinal axis (nose to tail): roll rotation, controlled by the ailerons (lateral control);
  • the lateral axis or transverse axis (wingtip to wingtip): pitch rotation, controlled by the elevator (longitudinal stability and control);
  • the normal axis (perpendicular to the other two): yaw rotation, controlled by the rudder (directional stability and control).

In the aeroplane frame, X points forwards, Y towards the right wing and Z downwards. In straight level flight, the normal axis is roughly vertical, even at high angle of attack and low speed.

Exam trap

Pitch is a rotation about the lateral axis, not the longitudinal axis. "Longitudinal" stability concerns pitch, hence the lateral axis.

Pitch attitude, flight path angle and angle of attack

← Faites glisser le schéma →

Pitch attitude = flight path angle + angle of attack

© ForAllPilots

Three angles describe the aeroplane's position in the vertical plane:

  • the pitch attitude θ: angle between the longitudinal axis and the horizontal;
  • the flight path angle γ: angle between the flight path (airspeed vector) and the horizontal;
  • the angle of attack α: angle between the reference axis (for an aerofoil, the chord) and the direction of the undisturbed flow (the relative airflow).

In symmetric flight with no vertical wind: θ = γ + α. The relative airflow is parallel to the flight path and in the opposite direction, with the pressure, temperature and speed of the undisturbed air.

The sideslip angle β is the angle between the plane of symmetry and the airflow, measured in the aeroplane's horizontal plane; it is positive when the air strikes from the right.

Remember

θ = γ + α. Angle of attack = chord / relative airflow. Angle of incidence (rigging angle) = chord / longitudinal axis (fixed construction angle).

Exam trap

Do not confuse angle of attack (aerodynamic angle, variable in flight) with angle of incidence (the mounting angle of the wing on the fuselage, fixed).

Air and the standard atmosphere

← Faites glisser le schéma →

Temperature and pressure in the standard atmosphere

© ForAllPilots

Air behaves as a perfect gas: P = ρ·r·T (r = 287 J/kg·K, T in kelvin).

  • at constant temperature, if pressure increases, ρ increases;
  • at constant pressure, if temperature increases, ρ decreases;
  • humid air is less dense than dry air (water vapour is lighter than the air it displaces);
  • with altitude, P and T decrease, but the effect of pressure dominates: ρ decreases.

Standard atmosphere (ISA) at sea level: 1013.25 hPa, 15 °C (288 K), 1.225 kg/m³. Temperature falls by 6.5 °C per 1,000 m (about 2 °C per 1,000 ft) up to the tropopause (11 km, 36,089 ft), then stays at −56.5 °C.

Remember

ISA: 1013.25 hPa, +15 °C, 1.225 kg/m³, −2 °C/1,000 ft, tropopause 36,089 ft at −56.5 °C. Near the ground, 1 hPa ≈ 28 ft (the exam rounds to 30 ft).

Exam trap

A descent at 500 ft/min from 5,000 ft to the ground changes the outside pressure faster than the same descent between 15,000 and 10,000 ft: pressure changes faster per foot in the lower layer.

Static, dynamic and total pressure

← Faites glisser le schéma →

Measuring dynamic pressure with a pitot tube and a static port

© ForAllPilots

Three pressures must be distinguished:

  • the static pressure Ps: the pressure of the fluid, which acts in all directions; it is measured through an opening parallel to the flow (static port, located on the side of the fuselage between the nose and the wing, where the local pressure is almost that of the ambient air);
  • the dynamic pressure q = ½ρV²: kinetic energy per unit volume, proportional to ρ and to the square of the speed;
  • the total pressure Pt = Ps + q: pressure measured where the air is brought to rest, in a pitot tube open facing the flow or at the stagnation point of an aerofoil.

Dynamic pressure is the difference between the pressure of a flow brought to rest and its static pressure: q = Pt − Ps.

Remember

q = ½ρV² (Pa); Pt = Ps + q. Speed doubled: q × 4. At constant TAS, q is higher at sea level than at altitude.

Continuity and Bernoulli's theorem

← Faites glisser le schéma →

Venturi in subsonic incompressible flow

© ForAllPilots

Continuity: mass flow rate is conserved, ρ·S·V = constant. At low speeds air is considered incompressible (ρ constant, a valid assumption up to about Mach 0.3; the textbook and the exam take "above 300 km/h" as the point where compressibility matters): S·V = constant. When the streamlines converge, the speed increases.

Bernoulli (perfect fluid, steady flow): the sum of static pressure + dynamic pressure remains constant, Ps + ½ρV² = Pt. Speed and static pressure vary in opposite directions.

In a subsonic venturi: at the throat, V increases, Ps decreases, q increases, Pt remains constant. In the divergent section that follows, V decreases and Ps rises again. A perfect fluid is a fluid with no viscosity (and here incompressible).

Remember

Subsonic convergent: V ↑, Ps ↓, q ↑, Pt constant, ρ constant. Divergent: the reverse.

Exam trap

In supersonic flow it is the opposite: speed decreases in a convergent section and increases in a divergent one (see the high-speed chapter).

IAS, CAS, EAS and TAS

← Faites glisser le schéma →

From indicated airspeed to true airspeed

© ForAllPilots

  • IAS (indicated airspeed): read from the airspeed indicator;
  • CAS (calibrated airspeed): IAS corrected for position and instrument errors;
  • EAS (equivalent airspeed): CAS corrected for compressibility (EAS ≤ CAS at high speeds and altitudes);
  • TAS (true airspeed): EAS corrected for density alone, EAS = TAS·√(ρ/ρ0).

Aerodynamic forces depend on dynamic pressure, and therefore on EAS. Two aeroplanes of the same mass at the same EAS (or roughly the same IAS) fly at the same angle of attack, whatever the altitude. TAS is equal to ground speed only in still air.

Remember

IAS → (position, instrument) → CAS → (compressibility) → EAS → (density) → TAS. The gap between IAS and TAS decreases as you descend.

Exam trap

The stall speed in IAS does not depend on temperature: it is the stall TAS that changes. On take-off from a high-elevation airfield, the IAS is the same, but TAS and ground speed are higher.

Airspeeds in climb and descent

← Faites glisser le schéma →

Change in airspeeds in the climb (schematic)

© ForAllPilots

Climb at constant IAS: density decreases, TAS increases; temperature also falls, so Mach number increases sharply. EAS decreases slightly above 10,000 ft (compressibility).

Climb at constant Mach: TAS = M × a, and a decreases with temperature. Below the tropopause, TAS decreases; above it (constant temperature in ISA), TAS remains constant.

Airliners climb at constant IAS then switch to a constant Mach at the crossover altitude. Consequences for the limits: in a climb at VMO you can exceed MMO; in a descent at constant Mach the IAS increases and you can exceed VMO.

At constant IAS, if the OAT varies in level flight, the Mach number does not change (same dynamic pressure and same pressure altitude).

Remember

Constant IAS in climb: TAS ↑, Mach ↑. Constant Mach in climb: TAS ↓ below the tropopause. Descent at constant M: IAS ↑ → VMO.

Exam trap

Descending at constant Mach makes TAS increase below the tropopause (temperature rises) and IAS too: VMO becomes the limit. Climb at constant IAS and mass: it is MMO that may be exceeded.

Energy: trading speed for height

← Faites glisser le schéma →

Conservation of mechanical energy

© ForAllPilots

The aeroplane's mechanical energy is the sum of kinetic energy ½mV² and potential energy mgh. With the thrust/drag balance unchanged, it is conserved to a first approximation: an aeroplane that loses height gains speed, and vice versa.

For small variations: Δh ≈ V·ΔV/g (V in m/s).

On a flight path inclined at γ, the component of weight along the flight path is W·sin γ. If you level off without changing thrust, this component disappears: the aeroplane decelerates by about g·sin γ (for small angles, sin γ ≈ gradient in %).

Remember

Δh ≈ V·ΔV/g. Deceleration on levelling off ≈ g × gradient: 5% → 0.5 m/s² → about 10 kt in 10 s.

The course goes on with chapter 2, « The aerofoil, the wing and lift ». It is read in the Course tab of the app with a ATPL subscription, and each chapter ends with the questions of the bank about it. See pricing

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