GCAA ATPL Theory key terms

The speed by which a failure of the critical engine must be recognised so the pilot can either continue the take-off or stop within the remaining runway. Below V1 a stop is normally possible; above it, take-off must continue.

The minimum speed that must be attained by the end of the take-off distance and maintained with one engine inoperative, giving adequate climb gradient and control. It is a scheduled performance speed, not a handling speed.

The runway length at which the accelerate-stop distance equals the take-off distance required with one engine failing at V1. Adjusting V1 shifts the balance point; a longer field than balanced is not limiting.

The total distance needed to accelerate from a standing start to V1 with all engines, recognise an engine failure, and bring the aircraft to a full stop using brakes and any other stopping means.

The ratio of height gained to horizontal distance travelled, usually expressed as a percentage. It differs from rate of climb, which depends on speed, and is the value used for obstacle clearance in take-off and approach procedures.

The pressure altitude at which an aircraft's maximum rate of climb falls to a specified low value, typically 100 feet per minute for piston and turboprop aeroplanes. Above this altitude climb performance becomes practically negligible.

Pressure altitude corrected for temperature deviation from standard. High temperature, low pressure or high humidity raises density altitude, reducing engine power and lift and increasing take-off and landing distances.

The altitude indicated when the altimeter subscale is set to 1013.25 hPa. It is the reference used for flight levels and for aircraft performance calculations, independent of local weather.

The altitude at or below which the vertical position of an aircraft is expressed in altitudes on the local altimeter setting, and above which flight levels are used. It is published for each aerodrome or area.

The lowest flight level available for use above the transition altitude. It is established so that adequate vertical separation exists between aircraft at the transition altitude and those at the transition level.

The point through which the total weight of the aircraft is considered to act. Its position must stay within a certified forward and aft range so that control, stability and performance remain acceptable.

The horizontal distance from the datum to a point where a mass acts. Multiplying a mass by its arm gives its moment, and summing moments allows the loaded centre of gravity to be calculated.

The mass of the aircraft with structure, powerplant, systems, unusable fuel and other specified operating equipment, excluding crew, passengers, payload and usable fuel. It includes optional equipment fitted at delivery.

The mass of the aircraft including crew, passengers and payload but excluding usable fuel. It must not exceed a certified limit that protects wing bending and structural loads.

The angle between the wing chord line and the relative airflow. Lift increases with angle of attack until the critical angle, beyond which the airflow separates and the wing stalls.

The angle of attack at which airflow separation begins and maximum lift is produced. A wing always stalls at this angle, regardless of speed, weight, altitude or bank angle.

Drag caused by the generation of lift, arising from the rearward tilt of the lift vector and from wingtip vortices. It increases with angle of attack and decreases with increasing speed.

The ratio of total lift to weight, expressed in g. It increases in turns and pull-ups, raising stall speed and structural load, and must remain within the certified positive and negative limits.

The tendency of a wing with upward-angled panels to roll away from a sideslip. It contributes lateral stability, so a disturbance producing sideslip generates a rolling moment that restores the wings level.

The initial tendency of an aircraft, after a disturbance from equilibrium, to return toward, move away from, or remain at the original condition. Positive static stability is a prerequisite for satisfactory dynamic behaviour.

The ratio of true airspeed to the local speed of sound. It governs compressibility effects such as shock-wave formation and is displayed to pilots as a primary limit at high altitude.

The free-stream Mach number at which airflow over some part of the airframe first reaches the local speed of sound. Beyond it, shock waves and drag rise begin.

The speed of the aircraft relative to the air mass, corrected for air density and temperature. It is used for navigation and performance, and is higher than indicated airspeed at altitude.

The speed shown on the airspeed indicator, derived from dynamic pressure. It relates directly to aerodynamic forces and is the value compared with published operating speeds and stall margins.

The system of pitot tubes and static ports that supplies total and static pressure to the airspeed indicator, altimeter and vertical speed indicator. Blockages cause characteristic and misleading indications.

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

The apparent movement of a spinning rotor's axis at right angles to an applied force. It explains real precession errors in gyro instruments and is exploited in rate gyros.

The altimeter subscale setting which, when applied, causes the altimeter to indicate altitude above mean sea level at the aerodrome. It is used for terrain clearance and aerodrome-level operations.

The altimeter subscale setting which, when applied at an aerodrome, causes the altimeter to read zero on the ground. It indicates height above the aerodrome reference point.

The standard altimeter setting of 1013.25 hPa used above the transition altitude. With this setting the altimeter displays pressure altitude, which is expressed as flight levels.

A magnetic bearing from a VOR station, expressed in degrees, along which an aircraft is positioned. It is unaffected by wind and is used to fix position and track to or from the station.

The bearing of an aircraft from a non-directional beacon, measured by the aircraft's automatic direction finder. It is a relative bearing that, combined with heading, gives a magnetic bearing to the station.

The line-of-sight distance measured by distance measuring equipment, which is greater than the ground distance. The error is small at long range but significant when directly overhead a station.

A radio signal, normally at 3 degrees, defining the vertical descent path to a runway for a precision approach. It is paired with a localiser providing lateral guidance.

The altitude on a precision approach at which a missed approach must be initiated if the required visual reference has not been established. It is referenced to the altimeter, unlike decision height.

A deep convective cloud with strong vertical development, often producing thunderstorms, severe turbulence, icing and heavy precipitation. It should be avoided by a wide margin in flight.

A change in wind speed or direction over a short distance, horizontally or vertically. Low-level shear can cause sudden airspeed and lift changes close to the ground and is a major hazard on approach.

The accumulation of ice on airframe surfaces, most likely in supercooled cloud droplets. It degrades lift, increases drag and weight, and can block pitot and static sources.

A condition in which the body's tissues receive insufficient oxygen. In aviation it commonly results from high altitude without supplementary oxygen and impairs judgement before the pilot notices the symptoms.

The period after exposure to a given altitude during which a person can still take effective corrective action before incapacitation from hypoxia. It shortens rapidly with increasing altitude.

Abnormally rapid or deep breathing that lowers blood carbon dioxide. Its symptoms resemble hypoxia, and treatment involves slowing the breathing rate and, if appropriate, checking oxygen supply.

A mistaken perception of the aircraft's position or motion relative to the ground, caused by misleading vestibular and visual cues. It is countered by relying on instruments and avoiding sudden head movements.

A false sensation of pitching up during rapid acceleration, or pitching down during deceleration, caused by linear acceleration stimulating the otolith organs. It is a hazard during take-off and go-around.