CPL Helicopter key terms

The extra thrust produced when airflow through the rotor disc becomes more horizontal as forward speed increases, reducing induced flow and improving rotor efficiency. It explains the improved performance and lower power required in forward flight compared with the hover.

The downward velocity of air through the rotor disc caused by the rotor producing thrust. Higher induced flow means a larger induced power requirement, so the helicopter needs more power to hover than to fly at moderate forward speed.

The speed at which the rotor works entirely in undisturbed air and translational lift is fully realised, typically around 15 to 20 knots depending on type. Performance and control response change noticeably at this point during takeoff and approach.

The reduction in induced flow and power required when hovering close to a surface, because the surface restricts the downward airflow. It is lost abruptly as height increases or when hovering over uneven or obstructed ground.

A condition in which a helicopter descends through its own downwash at low forward speed, so the rotor recirculates its wake and loses thrust. It causes vibration, poor control response and a high rate of descent, and is recovered by gaining forward speed.

A flight condition in which the rotor is driven by upward airflow from the helicopter's descent rather than by the engine. The pilot controls rotor rpm and rate of descent with collective and cyclic to achieve a safe landing after power loss.

The control that changes the pitch angle of all main rotor blades equally and simultaneously. Raising it increases lift and drag, so it is used with the throttle or governor to manage power, rotor rpm and rate of climb or descent.

The control that varies blade pitch cyclically around the rotor disc to tilt the thrust vector, producing forward, aft or lateral movement. It also provides the primary attitude control and is used to manage airspeed and flight path.

The unequal lift between the advancing and retreating sides of the rotor disc in forward flight, caused by different airflow velocities over the blades. Blade flapping and cyclic feathering equalise lift and prevent excessive rolling moments.

Loss of lift on the retreating blade at high forward speed and high blade loading, because its low relative airflow requires a large angle of attack. It causes vibration, pitch-up and roll, and limits maximum airspeed.

A hinge allowing each rotor blade to move up and down relative to the hub, equalising lift between advancing and retreating sides and reducing bending loads at the root. It is a key feature of articulated rotor heads.

The upward bending of rotor blades into a shallow cone shape under the combination of lift and centrifugal force. Excessive coning indicates high blade loading or low rotor rpm and increases stress on the rotor system.

The rotational speed of the main rotor, normally kept within a narrow approved range by the engine governor or by pilot throttle control. Low or excessive rpm affects lift, control authority and structural limits.

The ratio of helicopter gross weight to the total area of the main rotor blades. Higher blade loading increases the angle of attack needed for a given lift, raising the risk of retreating blade stall and reducing autorotative performance.

The ratio of helicopter gross weight to the area of the rotor disc. It influences hover performance, downwash strength and autorotative descent characteristics, with higher values generally requiring more power to hover.

The power required to maintain a steady hover, made up mainly of induced, profile and parasite power. It increases with weight, altitude and temperature and determines hover ceilings and available payload.

The highest density altitude at which the helicopter can hover without ground effect at a given weight. It is a critical performance limitation for operations such as winching, confined-area approaches and external load work.

Pressure altitude corrected for non-standard temperature, representing the altitude at which the air density would produce the same performance. High density altitude reduces lift, engine power and tail rotor authority, especially in hot or high conditions.

The altitude indicated on an altimeter set to the standard pressure of 1013.25 hPa. It is used for flight levels above the transition altitude and as the basis for computing density altitude and aircraft performance.

The altitude above which pilots set the altimeter to standard pressure and fly by flight levels. Below it, altimeters are set to local pressure so that terrain clearance is referenced to mean sea level.

The altimeter setting that makes the altimeter indicate elevation above mean sea level when the aircraft is on the ground. It is used for approach, landing and en-route flight below the transition altitude.

The altimeter setting that makes the altimeter read zero when the aircraft is on the ground at a specified reference point, usually an aerodrome. It indicates height above that point rather than altitude above mean sea level.

Controlled airspace in which both IFR and VFR flights are permitted, all flights receive air traffic control service, and VFR flights require a clearance. It is typically established around aerodromes with an operating control tower.

Controlled airspace extending upwards from the surface around an aerodrome, normally to a specified upper limit, established to protect approach and departure paths. Flight within it requires an air traffic control clearance.

An authorisation from air traffic control for a flight or portion of it to proceed under specified conditions. It does not relieve the pilot in command of responsibility for terrain clearance, separation from obstacles or safe conduct of the flight.

The pilot's repetition to air traffic control of safety-related elements of a clearance or instruction, confirming correct receipt. It is required for route clearances, runway and hold instructions, altimeter settings and similar items.

The four-digit code selected on the aircraft transponder to identify the flight to air traffic control radar. Codes are assigned by controllers or taken from published procedures and must be set accurately.

An airborne radio device that replies to ground radar interrogations with the selected code and, in Mode S, with aircraft identification and altitude information. It supports radar identification and traffic avoidance systems.

A radio message giving the aircraft's location, altitude, time and next position to air traffic control or on a common frequency. It supports separation and search and rescue in areas without radar coverage.

The spoken distress signal used when an aircraft is threatened by serious or imminent danger and requires immediate assistance. It is transmitted three times, followed by the call sign and the nature of the problem.

The spoken urgency signal used when an aircraft is in a condition concerning safety but not requiring immediate assistance, such as a minor technical problem. It is repeated three times and followed by details of the situation.

The international VHF frequency 121.5 MHz used for distress and urgency communications and for direction finding by rescue services. Pilots may use it when unable to establish contact on assigned frequencies.

A device that automatically or manually transmits a distress signal on the 406 MHz satellite band and often a homing signal on 121.5 MHz after a crash. It aids search and rescue in locating the aircraft.

A vertical descent at low forward speed in which the helicopter descends into its own downwash and needs more power than is available. Recovery requires lowering collective and gaining forward speed or entering autorotation.

An uncommanded yaw that can occur in certain wind and power conditions when the tail rotor produces less thrust than required. It is countered by reducing power, lowering collective and gaining airspeed.

The anti-torque rotor that counteracts main rotor torque and provides directional control. Its thrust is affected by wind direction, translational lift and the main rotor wake, which can lead to loss of effectiveness.

A certification category defining the performance and operating rules for a helicopter, such as Class 1, 2 or 3, based on the ability to continue or reject a takeoff safely after an engine failure. It determines permitted operating conditions.

The point during takeoff or approach at which a decision must be made to continue or to reject the manoeuvre after an engine failure. It is defined by the helicopter's performance class and guarantees a safe outcome.