NCAA PPL (Helicopter) Theory Exam key terms

The sideways drift of a helicopter in the direction of tail rotor thrust, caused because the tail rotor pushes the fuselage laterally as well as counteracting main rotor torque. Pilots correct it by holding a small cyclic input opposite the drift.

The unequal lift produced across the rotor disc in forward flight because the advancing blade meets the air faster than the retreating blade. Blade flapping and cyclic feathering equalise the lift so the disc stays balanced.

The up-and-down movement of each rotor blade about its flapping hinge, allowing the advancing blade to rise and the retreating blade to descend. This movement balances lift across the disc and reduces bending loads at the hub.

The tendency of a rotating blade to accelerate as its radius of rotation shortens, and to decelerate as it lengthens. Flapping changes blade radius, so designers use lead-lag hinges or dampers to absorb the resulting in-plane forces.

A hinge at the rotor hub that lets a blade move slightly fore and aft within the disc plane. It relieves the in-plane stresses created when flapping alters the blade's effective radius of rotation.

Loss of lift on the retreating blade when high forward speed or high disc loading makes its airspeed too low. It produces vibration, pitch-up and roll toward the retreating side, and it sets a practical forward-speed limit.

The reduction in induced downwash and induced drag when a helicopter hovers close to a surface, roughly within one rotor diameter. Less power is needed to hover, and the effect fades as height increases.

The extra lift gained once forward airspeed lets the rotor work in undisturbed air rather than recirculating its own downwash. It explains the improved performance and reduced power needed above translational speed, typically around 15 to 20 knots.

The airspeed at which the rotor disc fully escapes its own downwash and enters clean airflow, giving a marked gain in lift and performance. It is commonly reached around 15 to 20 knots in still air.

A vertical descent in which the helicopter sinks into its own downwash while power is applied, causing buffeting and loss of control effectiveness. Recovery requires forward cyclic to gain airspeed and escape the vortex ring state.

A condition in which the rotor recirculates its own tip vortices during a powered descent, producing turbulence, poor control response and a high sink rate. Adding power worsens it; the remedy is to lower collective and move forward.

A descent with the engine disengaged or failed, in which upward airflow through the rotor keeps the blades turning and generates lift. The pilot controls rate of descent with collective and airspeed with cyclic.

A one-way clutch in the transmission that lets the rotor turn faster than the engine but never slower. It disconnects the engine automatically during autorotation and re-engages when engine speed recovers.

The rotational speed of the main rotor, usually displayed as a percentage of a nominal value. It must stay within the approved green range because both lift and control authority depend on it.

A system that automatically adjusts engine power to hold rotor RPM near the selected value as collective pitch or load changes. It reduces pilot workload but does not replace manual throttle control after a failure.

The control that changes blade pitch equally on all blades, increasing or decreasing total lift and drag together. Raising it demands more power; lowering it reduces lift and is the primary means of controlling descent in autorotation.

The control that varies blade pitch cyclically around the disc to tilt the rotor thrust vector and move the helicopter in the desired direction. It also sets airspeed and compensates for translating tendency.

The lateral force produced by tail rotor thrust that pushes the helicopter sideways, requiring a cyclic input to counter. It is closely related to translating tendency and varies with power and wind.

Pressure altitude corrected for temperature, representing the altitude at which the aircraft 'feels' it is flying. High density altitude reduces engine power, rotor thrust and therefore hover and climb performance.

The altitude indicated when the altimeter subscale is set to standard pressure, 1013.25 hPa. It is the basis for flight levels and for calculating density altitude and aircraft performance.

Hovering at a height where the surface no longer boosts rotor efficiency, generally above one rotor diameter. It requires more power than in-ground-effect hover and defines the limiting mass for many operations.

The forward and aft boundaries within which the loaded helicopter's centre of gravity must lie for safe control and performance. Exceeding them can cause controllability problems, especially in hover and autorotation.

The mass of the helicopter with standard equipment, unusable fuel and full operating fluids, but without crew, passengers or usable fuel. It is the starting figure for every mass and balance calculation.

The difference between maximum certificated mass and basic empty mass, covering crew, passengers, baggage and usable fuel. It sets the payload available for a given fuel load.

The altitude above which pilots set the altimeter to standard pressure and fly by flight level rather than altitude. It is published for each aerodrome or area and marks the change from QNH to 1013.25 hPa.

The altimeter setting that makes the instrument read elevation above mean sea level when on the ground at the reporting station. It is used below transition altitude for terrain clearance.

The altimeter setting that makes the instrument read zero on the ground at the reference point, giving height above that point. It is useful for traffic pattern work but not for terrain clearance away from the aerodrome.

Uncontrolled airspace where air traffic control does not provide separation to IFR or VFR traffic. Pilots remain responsible for see-and-avoid, and specific visibility and cloud clearance minima apply.

The minimum flight visibility and distance from cloud that must be maintained when flying under visual flight rules. The values vary with airspace class and altitude and are published in the applicable regulations.

A clearance allowing flight under visual flight rules in controlled airspace when weather is below normal VFR minima. It requires an ATC clearance, specified ground visibility and clearance from cloud.

Airspace of defined dimensions within which air traffic control service is provided to all flights, such as Classes A to E. Entry normally requires a clearance and, for IFR flights, separation is provided.

A shortage of oxygen reaching body tissues, causing impaired judgement, euphoria and reduced vision before the pilot notices symptoms. It develops progressively with altitude and is a key reason for supplemental oxygen above certain levels.

Excessive breathing rate or depth that lowers blood carbon dioxide, producing dizziness, tingling and confusion. Its symptoms resemble hypoxia, so the pilot must distinguish them and control breathing deliberately.

A mistaken perception of the aircraft's attitude or motion caused by misleading vestibular and visual cues, especially without a visible horizon. Pilots must trust flight instruments rather than sensations.

The inner-ear organs that sense acceleration and rotation, giving the pilot a sense of balance and motion. It can produce illusions in manoeuvres such as a prolonged turn or rapid acceleration, so instrument reference is essential.

A change in wind speed or direction over a short distance, often associated with fronts, thunderstorms or inversions. It can cause sudden airspeed and lift changes close to the ground, especially dangerous in the hover and approach.

A strong, localised downdraught spreading out at the surface, typically from a thunderstorm. It creates severe low-level wind shear and a sudden loss of performance on approach or departure.

The mandatory repetition of ATC instructions such as clearances, runway assignments and hold-short instructions to confirm correct reception. It forms part of standard radiotelephony discipline and reduces misunderstanding.

The standard ICAO spelling alphabet used to pronounce letters clearly over the radio, for example Alpha, Bravo, Charlie. It prevents confusion between similar-sounding letters in callsigns and instructions.

The four-digit code selected on the transponder to identify the aircraft to air traffic control radar. It is assigned by ATC or set according to published procedures for emergencies or radio failure.