Module 8.1–8.2 — The atmosphere (ISA) and basic aerodynamics

Where this fits

Module 8 is common to every licence category (A, B1, B2, B2L, B3). Sub-modules 8.1 and 8.2 are examined at level 1 for A and B3 and level 2 for B1, B2 and B2L. Under Regulation (EU) 2023/989 the EASA Module 8 paper is 24 questions in 30 minutes for every category (UK CAA retained papers: 20 questions / 25 minutes). Expect roughly a quarter of the paper on the atmosphere and aerofoil terminology.

8.1 Physics of the atmosphere — the International Standard Atmosphere

The ISA is an agreed "model day" used to calibrate instruments (altimeters, airspeed indicators), compare aircraft and engine performance and define the standard pressure setting. Real days differ; performance is corrected from ISA to actual conditions.

ISA sea-level values

Quantity	ISA mean sea-level value
Temperature	+15 °C (288.15 K; 59 °F)
Pressure	1013.25 hPa (mb) = 29.92 inHg = 760 mmHg = 14.7 psi
Density	1.225 kg/m³ (0.002377 slug/ft³)
Speed of sound	≈ 340 m/s ≈ 661 kt
Temperature lapse rate (troposphere)	6.5 °C per 1 000 m ≈ 1.98 °C (≈ 2 °C) per 1 000 ft
Tropopause	11 000 m (36 089 ft)
Temperature at and above the tropopause (to 20 km)	−56.5 °C, constant (isothermal layer)
Air treated as	dry, perfect gas; gravity constant

Key fact: In the ISA, temperature falls linearly with height to the tropopause (11 km / 36 089 ft) and then stays constant at −56.5 °C in the lower stratosphere. Pressure and density keep falling above the tropopause.

Selected ISA values with altitude

Altitude (ft)	Temperature (°C)	Pressure (hPa)	Density ratio σ (ρ/ρ0)
0	+15.0	1013	1.000
5 000	+5.1	843	0.862
10 000	−4.8	697	0.738
18 000	−20.7	506	0.570
20 000	−24.6	466	0.533
30 000	−44.4	301	0.374
36 089	−56.5	226	0.297
40 000	−56.5	188	0.246

Useful rules of thumb: pressure falls about 1 hPa per 27–30 ft near sea level; pressure is roughly half the sea-level value at about 18 000 ft.

Gas laws applied to the atmosphere

• Pressure falls with height because there is less air above.
• Density (ρ = p / RT) falls with height: falling pressure reduces it, falling temperature partly offsets the fall.
• Temperature rises → density falls (hot day = "high density altitude" = poorer engine, propeller and wing performance).
• Humidity — water vapour is lighter than dry air, so humid air is less dense than dry air at the same pressure and temperature.
• Pressure altitude = height in the ISA corresponding to the measured pressure (altimeter set to 1013.25 hPa). Density altitude = pressure altitude corrected for non-standard temperature.

Key fact: Aircraft performance depends on air density. High, hot and humid conditions all reduce density, increasing take-off run and reducing climb rate.

Worked example — ISA temperature

Find the ISA temperature at 8 000 ft. ISA temperature = 15 − (1.98 × 8) = 15 − 15.8 = −0.8 °C. If the actual outside air temperature is +9 °C, the day is ISA +10 (approximately), so density altitude is higher than pressure altitude.

8.2 Aerodynamics

Airflow around a body

• Free-stream flow — undisturbed air well ahead of the body. Relative airflow — airflow relative to the aircraft; equal and opposite to the flight path.
• Stagnation point — point on the leading edge where local velocity is zero and static pressure is maximum (total pressure).
• Upwash ahead of the wing and downwash behind it; downwash reduces the effective angle of attack of the tailplane.
• Boundary layer — thin layer next to the surface in which velocity rises from zero (at the skin, "no-slip") to free-stream value.
  • Laminar boundary layer: smooth, thin, low skin-friction drag, but easily separates.
  • Turbulent boundary layer: thicker, higher skin friction, but has more energy and resists separation.
  • Transition point — where laminar becomes turbulent; separation point — where flow leaves the surface (moves forward as angle of attack increases).
• Vortices — wing-tip vortices form because high-pressure air under the wing spills around the tip into the low-pressure region above; they are the visible sign of induced drag.

Aerofoil terminology

Term	Meaning
Chord line	Straight line joining leading edge and trailing edge
Mean camber line	Line midway between upper and lower surfaces; its departure from the chord is camber
Camber	Curvature of the aerofoil; a symmetrical aerofoil has zero camber
Mean aerodynamic chord (MAC)	Chord of an equivalent rectangular wing; CG positions are often quoted in % MAC
Angle of attack (α)	Angle between chord line and relative airflow
Angle of incidence	Fixed rigging angle between chord line and aircraft longitudinal axis
Centre of pressure (CP)	Point on the chord through which the resultant aerodynamic force acts; moves forward as α increases (on a cambered aerofoil, up to the stall)
Wash-in / wash-out	Increase / decrease of angle of incidence towards the tip; wash-out makes the root stall first and keeps aileron control
Aspect ratio	span² ÷ area (= span ÷ mean chord for a rectangular wing); high aspect ratio = low induced drag
Taper ratio	tip chord ÷ root chord
Fineness ratio	length ÷ maximum thickness (or diameter) of a streamlined body
Thickness/chord ratio	maximum thickness as % of chord

Lift

Lift acts perpendicular to the relative airflow; drag acts parallel to it. Their resultant is the total reaction (aerodynamic resultant), acting through the centre of pressure.

Lift equation: L = ½ ρ V² S C_L, where ½ρV² is dynamic pressure (q), S wing area and C_L the lift coefficient (depends on aerofoil shape and angle of attack).

• Doubling speed (V) → four times the lift (at the same α).
• C_L rises roughly linearly with angle of attack up to C_L max, at the critical (stalling) angle, typically about 15–16° for a conventional aerofoil.
• Beyond the critical angle the boundary layer separates over most of the upper surface — the stall. A wing always stalls at the same angle of attack, not the same speed.

Drag

Type	Cause	Varies with speed
Parasite (profile) drag — form drag	Shape, pressure difference fore and aft	∝ V²
Parasite — skin friction	Viscosity in boundary layer, surface roughness	∝ V²
Parasite — interference drag	Junctions (wing/fuselage, pylons)	∝ V²
Induced drag	Production of lift; tip vortices tilt the lift vector rearwards	∝ 1/V²

Total drag is a minimum where induced drag = parasite drag; this is the speed for minimum drag and best lift/drag ratio (L/D max). Induced drag is reduced by high aspect ratio, elliptical lift distribution, winglets and tip shaping.

Coefficients and the polar curve

• C_D = D ÷ (q S). The drag polar plots C_L against C_D; a tangent from the origin touches the curve at L/D max.
• The lift curve plots C_L against α; its peak is C_L max (stall).
• Typical best L/D: light aircraft about 10–15, airliners about 15–20, high-performance sailplanes 40+.

Weight, thrust and the aerodynamic resultant

In straight and level unaccelerated flight: lift = weight and thrust = drag. Lift acts through the CP, weight through the CG; the couple between them is balanced by the tailplane load.

Aerofoil contamination

Ice, frost and snow destroy the smooth upper-surface curvature and roughen the boundary layer:

• Lift reduced (C_L max can fall by around 30 % or more), drag increased, stall occurs at a lower angle of attack and higher speed, with possibly no stall warning.
• Even frost with the texture of medium sandpaper is hazardous — aircraft must be clean of frost, ice and snow before take-off ("clean wing concept").
• Contamination also adds weight and may jam control surfaces.

Key fact: Contamination lowers the critical angle of attack, so stall-warning systems calibrated for a clean wing may not warn before an iced wing stalls.

Exam tips

• Questions often swap "angle of attack" (chord to relative airflow) and "angle of incidence" (chord to longitudinal axis) — know which is fixed by rigging.
• Remember the direction of change: altitude up → temperature, pressure and density down (temperature constant above 36 089 ft).
• Induced drag is greatest at low speed / high angle of attack (take-off, landing); parasite drag dominates at high speed.
• Humid air is less dense, not more.