DGCA CPL Tech Specific key terms

The angle between the wing chord line and the relative airflow approaching the wing. Increasing it raises lift up to the critical angle, beyond which the airflow separates and the wing stalls.

The angle of attack at which airflow separation begins and the wing stalls. It is a fixed value for a given aerofoil and is reached at any speed, weight or attitude if the angle is exceeded.

Drag produced as a by-product of lift when high pressure below the wing spills around the tips into low pressure above, creating vortices. It is greatest at low speed and high angle of attack.

Drag from skin friction and form resistance of the airframe and protuberances, plus interference between components. It rises with the square of airspeed and is independent of lift production.

The ratio of total lift generated to the aircraft's weight, expressed in g. In a level turn it equals one divided by the cosine of the bank angle, raising stall speed accordingly.

The lowest indicated airspeed at which the aircraft can be flown without exceeding the critical angle of attack in a given configuration and weight. It increases with weight, load factor and forward centre of gravity.

A helicopter condition in powered descent where recirculating tip vortices engulf the rotor disc, causing loss of lift, vibration and poor control response. Recovery is by forward cyclic and reducing collective.

The extra lift a rotor system produces once forward speed lets clean air pass through the disc instead of recirculated downwash. It is most pronounced around translational speed and improves efficiency markedly.

The point through which the aircraft's total weight acts. Its position along the chord affects stability and control; it must stay within the certified forward and aft limits for every loading state.

The mass of the aircraft with unusable fuel, full operating fluids, optional equipment and trapped oil, but without crew, passengers, baggage or usable fuel. It is the datum from which all loading calculations start.

The mass of the aircraft excluding usable fuel, comprising basic empty mass plus crew, passengers, baggage and payload. It is checked against its limit before fuel is added for the flight.

Pressure altitude corrected for temperature deviation from ISA. High density altitude reduces engine power, propeller thrust and wing lift, lengthening take-off and landing runs and reducing climb performance.

The altitude indicated when the altimeter subscale is set to 1013.25 hPa. It is the reference for flight levels, performance charts and true altitude computations using temperature correction.

The highest density altitude at which an aircraft can still climb at a specified minimum rate, typically 100 feet per minute for piston types. It defines the practical upper limit of operation.

The runway length needed so that the distance to continue a take-off after an engine failure equals the distance to stop. It determines the maximum permissible take-off mass on a given runway.

The main longitudinal structural member of a wing that carries bending and shear loads from lift to the fuselage. Spars may be built as solid beams, box structures or machined integral sections.

A chordwise wing structural member that maintains the aerofoil shape, supports the skin and transfers loads to the spar. Ribs are spaced along the span and often carry fuel tank boundaries.

A longitudinal stiffening member riveted to the skin or frames that carries axial and bending loads and prevents skin buckling. Stringers run spanwise in wings and lengthwise along the fuselage.

A structural method where the skin carries most of the loads, with little or no internal framework. It is light but weak against bending, so most aircraft use semi-monocoque with frames and stringers.

A heavy transverse structural member in the fuselage that carries concentrated loads such as engine mounts, wing attachments or landing gear forces and maintains the cross-section shape.

A self-contained engine-driven ignition unit that generates high-tension pulses for the spark plugs. Two independent magnetos are fitted so that a single failure cannot stop the engine.

Ice formation in the carburettor venturi caused by fuel vaporisation and pressure drop cooling, occurring at ambient temperatures well above freezing in humid air. It causes rough running and power loss, countered by carburettor heat.

The cockpit control that sets the fuel-to-air ratio delivered to the engine. It must be leaned as altitude increases and enriched for high power, with full rich used for take-off and climb.

An exhaust-driven compressor that raises intake manifold pressure to maintain engine power as altitude increases. It requires careful wastegate and mixture management to avoid overboost and detonation.

Explosive, uncontrolled combustion of the fuel-air charge after normal ignition, producing high cylinder pressures, knocking and possible engine damage. It is caused by excessive temperature, low octane fuel or over-lean mixtures.

A propeller whose blade angle is varied by a governor to hold a selected RPM as power or airspeed changes. It allows efficient operation across the flight envelope and prevents overspeeding in descents.

The absolute pressure in the engine intake manifold, indicated in inches of mercury, used with RPM to set power on constant-speed propeller aircraft. It falls as altitude increases unless turbocharged.

The pressure system supplying ram air from the pitot tube and static air from the static ports to the airspeed indicator, altimeter and vertical speed indicator. Blockages cause specific and often misleading instrument errors.

Deviations in indicated airspeed from position error, installation error and compressibility, plus density error that makes true airspeed exceed indicated airspeed at altitude. Blocked pitot or static sources produce characteristic false readings.

The actual speed of the aircraft through the air mass, obtained by correcting indicated airspeed for position error, compressibility and density altitude. It is used for navigation and flight planning.

The pressure reference set on the altimeter: QNH for altitude above mean sea level, QFE for height above a datum, or 1013.25 hPa for flight levels. Incorrect setting is a major cause of altitude deviation.

The property of a spinning rotor to maintain its axis fixed in space unless acted on by an external force. It is the basis of the attitude indicator and heading indicator.

The apparent movement of a spinning gyroscope's axis at right angles to an applied force. It explains turn indicator behaviour and the need for erection and latitude corrections in some instruments.

Apparent error in a gyroscopic heading indicator caused by real precession, friction and the earth's rotation. It must be reset periodically against the magnetic compass in level, straight flight.

The angular difference between true north and magnetic north at a given place, shown on charts as easterly or westerly. It is applied when converting between true and magnetic directions.

The error in a magnetic compass caused by the aircraft's own magnetic fields, varying with heading and electrical load. It is recorded on a compass swing card and applied to magnetic headings.

A line on an aeronautical chart joining points of equal magnetic variation. Pilots use it to obtain the local variation for converting true bearings to magnetic and vice versa.

A magnetic compass error during turns in the northern hemisphere where the compass under-reads on northerly headings and over-reads on southerly headings, with the opposite in the south. It is largest near the poles and must be anticipated.

A magnetic bearing from a VOR station along which the aircraft is located, selected with the omni-bearing selector. Correct interpretation of the course deviation indicator depends on the selected radial and TO/FROM flag.

An instrument approach using satellite navigation, typically with lateral guidance and possibly vertical guidance from augmented systems. It requires database currency, RAIM prediction and adherence to published minima.

The four-digit octal code set on the transponder to identify the aircraft to air traffic control secondary radar. Specific codes are assigned for emergencies, radio failure and unlawful interference.

The operating setting of the SSR transponder, such as mode A for identity only or mode C adding pressure altitude. Mode S adds unique aircraft addressing and data link capability.

The international radiotelephony distress signal spoken three times to indicate grave and imminent danger requiring immediate assistance. It has absolute priority over all other transmissions.

The international radiotelephony urgency signal spoken three times to report a condition concerning safety but not immediate distress. It ranks below Mayday but above all routine traffic.

The repetition by a pilot of ATC clearances and instructions containing safety-critical items such as altitudes, headings, speeds and frequencies. It confirms the clearance was received and understood correctly.